feat: add function type to midas syntax

feat(types): add type params to extend statement
refactor(types): extract TypeParams
2026-06-09 23:48:06 +02:00 · 2026-06-09 23:40:57 +02:00 · 2026-06-09 15:30:45 +02:00 · 2026-06-09 12:59:36 +02:00 · 2026-06-09 08:06:46 +02:00 · 2026-06-09 08:05:31 +02:00
38 changed files with 3919 additions and 989 deletions
--- a/examples/01_simple_type_checking/04_complex_types.midas
+++ b/examples/01_simple_type_checking/04_complex_types.midas
@@ -0,0 +1,11 @@
+type Meter = float
+
+extend Meter {
+    op __add__(Meter) -> Meter
+    op __sub__(Meter) -> Meter
+}
+
+type Coordinate = {
+    x: Meter
+    y: Meter
+}
--- a/examples/01_simple_type_checking/04_complex_types.py
+++ b/examples/01_simple_type_checking/04_complex_types.py
@@ -0,0 +1,14 @@
+# type: ignore
+# ruff: disable [F821]
+p1: Coordinate
+p2: Coordinate
+
+diff_x = p2.x - p1.x
+diff_y = p2.y - p1.y
+
+dist = diff_x + diff_y
+
+p2.x += cast(Meter, 1)
+p2.y = True
+p2.z = 3
+p2.x.a = 3
--- a/gen/gen.py
+++ b/gen/gen.py
@@ -30,6 +30,7 @@ from __future__ import annotations

 T = TypeVar("T")

+{preamble}
 {sections}
 """

@@ -57,6 +58,11 @@ IMPORTS_REGEX = re.compile(
    re.MULTILINE | re.DOTALL,
 )

+PREAMBLE_REGEX = re.compile(
+    r"^###>\s*Preamble\s*?\n(?P<body>.*?)\n###<$",
+    re.MULTILINE | re.DOTALL,
+)
+

 def snake_case(text: str) -> str:
    return re.sub(r"[A-Z]", lambda c: "_" + c.group().lower(), text).lower().strip("_")
@@ -88,13 +94,14 @@ def make_banner(text: str) -> str:


 def make_section(full_name: str, base: str, param: str, body: str) -> str:
+    print(f"    Generating {full_name}")
    visitor_methods: list[str] = []
    classes: list[str] = []
    definitions: list[str] = body.strip("\n").split("\n\n\n")
    for cls in definitions:
        cls = cls.strip("\n")
        name: str = re.match("class (.*?):", cls).group(1)  # type: ignore
-        print(f"Processing {name}")
+        print(f"        Processing {name}")
        visitor_methods.append(make_visitor_method(name, param))
        classes.append(make_class(name, cls, base))

@@ -107,6 +114,7 @@ def make_section(full_name: str, base: str, param: str, body: str) -> str:


 def generate(definitions_path: Path, out_path: Path):
+    print(f"Processing generating {out_path} from {definitions_path}")
    root_dir: Path = Path(__file__).parent.parent
    rel_path: Path = definitions_path.relative_to(root_dir)
    src: str = definitions_path.read_text()
@@ -116,6 +124,10 @@ def generate(definitions_path: Path, out_path: Path):
    if m := IMPORTS_REGEX.search(src):
        imports = m.group("body").strip("\n")

+    preamble: str = ""
+    if m := PREAMBLE_REGEX.search(src):
+        preamble = m.group("body")
+
    for section_m in SECTION_REGEX.finditer(src):
        full_name: str = section_m.group("name")
        base: str = section_m.group("base")
@@ -129,6 +141,7 @@ def generate(definitions_path: Path, out_path: Path):
            gen_path=Path(__file__).relative_to(root_dir),
        ),
        imports=imports,
+        preamble=preamble,
        sections="\n\n\n".join(sections),
    )
    out_path.write_text(result)
--- a/gen/midas.py
+++ b/gen/midas.py
@@ -12,18 +12,23 @@ from midas.lexer.token import Token
 ###<


+###> Preamble
+@dataclass(frozen=True, kw_only=True)
+class TypeParam:
+    location: Location
+    name: Token
+    bound: Optional[Type]
+
+
+###<
+
+
 ###> Stmt | Statements
 class TypeStmt:
    name: Token
-    params: list[Param]
+    params: list[TypeParam]
    type: Type

-    @dataclass(frozen=True, kw_only=True)
-    class Param:
-        location: Location
-        name: Token
-        bound: Optional[Type]
-

 class PropertyStmt:
    name: Token
@@ -31,6 +36,7 @@ class PropertyStmt:


 class ExtendStmt:
+    params: list[TypeParam]
    type: Type
    operations: list[OpStmt]

@@ -103,7 +109,7 @@ class NamedType:

 class GenericType:
    type: Type
-    params: list[Type]
+    args: list[Type]


 class ConstraintType:
@@ -115,4 +121,17 @@ class ComplexType:
    properties: list[PropertyStmt]


+class FunctionType:
+    pos_args: list[Argument]
+    kw_args: list[Argument]
+    returns: Type
+
+    @dataclass(frozen=True, kw_only=True)
+    class Argument:
+        location: Optional[Location] = None
+        name: Optional[Token]
+        type: Type
+        required: bool
+
+
 ###<
--- a/gen/python.py
+++ b/gen/python.py
@@ -128,12 +128,6 @@ class LogicalExpr:
    right: Expr


-class SetExpr:
-    object: Expr
-    name: str
-    value: Expr
-
-
 class CastExpr:
    type: MidasType
    expr: Expr
@@ -145,4 +139,8 @@ class TernaryExpr:
    if_false: Expr


+class ListExpr:
+    items: list[Expr]
+
+
 ###<
--- a/midas/ast/midas.py
+++ b/midas/ast/midas.py
@@ -14,6 +14,13 @@ from midas.lexer.token import Token

 T = TypeVar("T")

+@dataclass(frozen=True, kw_only=True)
+class TypeParam:
+    location: Location
+    name: Token
+    bound: Optional[Type]
+
+
 ##############
 # Statements #
 ##############
@@ -46,15 +53,9 @@ class Stmt(ABC):
@dataclass(frozen=True)
 class TypeStmt(Stmt):
    name: Token
-    params: list[Param]
+    params: list[TypeParam]
    type: Type

-    @dataclass(frozen=True, kw_only=True)
-    class Param:
-        location: Location
-        name: Token
-        bound: Optional[Type]
-
    def accept(self, visitor: Stmt.Visitor[T]) -> T:
        return visitor.visit_type_stmt(self)

@@ -70,6 +71,7 @@ class PropertyStmt(Stmt):

@dataclass(frozen=True)
 class ExtendStmt(Stmt):
+    params: list[TypeParam]
    type: Type
    operations: list[OpStmt]

@@ -231,6 +233,9 @@ class Type(ABC):
        @abstractmethod
        def visit_complex_type(self, type: ComplexType) -> T: ...

+        @abstractmethod
+        def visit_function_type(self, type: FunctionType) -> T: ...
+

@dataclass(frozen=True)
 class NamedType(Type):
@@ -243,7 +248,7 @@ class NamedType(Type):
@dataclass(frozen=True)
 class GenericType(Type):
    type: Type
-    params: list[Type]
+    args: list[Type]

    def accept(self, visitor: Type.Visitor[T]) -> T:
        return visitor.visit_generic_type(self)
@@ -264,3 +269,20 @@ class ComplexType(Type):

    def accept(self, visitor: Type.Visitor[T]) -> T:
        return visitor.visit_complex_type(self)
+
+
+@dataclass(frozen=True)
+class FunctionType(Type):
+    pos_args: list[Argument]
+    kw_args: list[Argument]
+    returns: Type
+
+    @dataclass(frozen=True, kw_only=True)
+    class Argument:
+        location: Optional[Location] = None
+        name: Optional[Token]
+        type: Type
+        required: bool
+
+    def accept(self, visitor: Type.Visitor[T]) -> T:
+        return visitor.visit_function_type(self)
--- a/midas/ast/printer.py
+++ b/midas/ast/printer.py
@@ -100,12 +100,12 @@ class MidasAstPrinter(
                    self._idx = i
                    if i == len(stmt.params) - 1:
                        self._mark_last()
-                    self._print_type_stmt_param(param)
+                    self._print_type_param(param)
            self._write_line("type", last=True)
            with self._child_level(single=True):
                stmt.type.accept(self)

-    def _print_type_stmt_param(self, param: m.TypeStmt.Param) -> None:
+    def _print_type_param(self, param: m.TypeParam) -> None:
        self._write_line("Param")
        with self._child_level():
            self._write_line(f'name: "{param.name.lexeme}"')
@@ -122,6 +122,13 @@ class MidasAstPrinter(
    def visit_extend_stmt(self, stmt: m.ExtendStmt) -> None:
        self._write_line("ExtendStmt")
        with self._child_level():
+            self._write_line("params")
+            with self._child_level():
+                for i, param in enumerate(stmt.params):
+                    self._idx = i
+                    if i == len(stmt.params) - 1:
+                        self._mark_last()
+                    self._print_type_param(param)
            self._write_line("type")
            with self._child_level(single=True):
                stmt.type.accept(self)
@@ -234,11 +241,11 @@ class MidasAstPrinter(
            self._write_line("type")
            with self._child_level():
                type.type.accept(self)
-            self._write_line("params", last=True)
+            self._write_line("args", last=True)
            with self._child_level():
-                for i, param in enumerate(type.params):
+                for i, param in enumerate(type.args):
                    self._idx = i
-                    if i == len(type.params) - 1:
+                    if i == len(type.args) - 1:
                        self._mark_last()
                    param.accept(self)

@@ -263,6 +270,41 @@ class MidasAstPrinter(
                        self._mark_last()
                    prop.accept(self)

+    def visit_function_type(self, type: m.FunctionType) -> None:
+        self._write_line("FunctionType")
+        with self._child_level():
+            self._write_line("pos_args")
+            with self._child_level():
+                for i, arg in enumerate(type.pos_args):
+                    self._idx = i
+                    if i == len(type.pos_args) - 1:
+                        self._mark_last()
+                    self._print_function_arg(arg)
+
+            self._write_line("kw_args")
+            with self._child_level():
+                for i, arg in enumerate(type.kw_args):
+                    self._idx = i
+                    if i == len(type.kw_args) - 1:
+                        self._mark_last()
+                    self._print_function_arg(arg)
+
+            self._write_line("returns", last=True)
+            with self._child_level(single=True):
+                type.returns.accept(self)
+
+    def _print_function_arg(self, arg: m.FunctionType.Argument) -> None:
+        self._write_line("Argument")
+        with self._child_level():
+            name: str = "None"
+            if arg.name is not None:
+                name = f'"{arg.name.lexeme}"'
+            self._write_line(f"name: {name}")
+            self._write_line("type")
+            with self._child_level(single=True):
+                arg.type.accept(self)
+            self._write_line(f"required: {arg.required}", last=True)
+

 class MidasPrinter(m.Expr.Visitor[str], m.Stmt.Visitor[str], m.Type.Visitor[str]):
    def __init__(self, indent: int = 4):
@@ -279,14 +321,12 @@ class MidasPrinter(m.Expr.Visitor[str], m.Stmt.Visitor[str], m.Type.Visitor[str]
    def visit_type_stmt(self, stmt: m.TypeStmt) -> str:
        template: str = ""
        if len(stmt.params) != 0:
-            params: list[str] = [
-                self._print_type_template_param(param) for param in stmt.params
-            ]
+            params: list[str] = [self._print_type_param(param) for param in stmt.params]
            template = f"[{', '.join(params)}]"
        res: str = f"type {stmt.name.lexeme}{template} = {stmt.type.accept(self)}"
        return self.indented(res)

-    def _print_type_template_param(self, param: m.TypeStmt.Param) -> str:
+    def _print_type_param(self, param: m.TypeParam) -> str:
        res: str = param.name.lexeme
        if param.bound is not None:
            res += "<:" + param.bound.accept(self)
@@ -358,9 +398,9 @@ class MidasPrinter(m.Expr.Visitor[str], m.Stmt.Visitor[str], m.Type.Visitor[str]

    def visit_generic_type(self, type: m.GenericType) -> str:
        res: str = type.type.accept(self)
-        if len(type.params) != 0:
-            params: list[str] = [param.accept(self) for param in type.params]
-            res += f"[{', '.join(params)}]"
+        if len(type.args) != 0:
+            args: list[str] = [param.accept(self) for param in type.args]
+            res += f"[{', '.join(args)}]"
        return res

    def visit_constraint_type(self, type: m.ConstraintType) -> str:
@@ -378,6 +418,29 @@ class MidasPrinter(m.Expr.Visitor[str], m.Stmt.Visitor[str], m.Type.Visitor[str]
        res += self.indented("}")
        return res

+    def visit_function_type(self, type: m.FunctionType) -> str:
+        pos_args: list[str] = [self._print_arg(arg) for arg in type.pos_args]
+        kw_args: list[str] = [self._print_arg(arg) for arg in type.pos_args]
+        args: list[str] = pos_args
+
+        if len(pos_args) != 0:
+            args.append("/")
+        if len(kw_args) != 0:
+            args.append("*")
+        args += kw_args
+
+        return f"({', '.join(args)}) -> {type.returns.accept(self)}"
+
+    def _print_arg(self, arg: m.FunctionType.Argument) -> str:
+        res: str = ""
+        if arg.name is not None:
+            res += arg.name.lexeme
+            res += ": "
+        res += arg.type.accept(self)
+        if not arg.required:
+            res += "?"
+        return res
+

 class PythonAstPrinter(
    AstPrinter,
@@ -602,17 +665,6 @@ class PythonAstPrinter(
            with self._child_level(single=True):
                expr.right.accept(self)

-    def visit_set_expr(self, expr: p.SetExpr) -> None:
-        self._write_line("SetExpr")
-        with self._child_level():
-            self._write_line("object")
-            with self._child_level(single=True):
-                expr.object.accept(self)
-            self._write_line(f"name: {expr.name}")
-            self._write_line("value", last=True)
-            with self._child_level(single=True):
-                expr.value.accept(self)
-
    def visit_cast_expr(self, expr: p.CastExpr) -> None:
        self._write_line("CastExpr")
        with self._child_level():
@@ -637,3 +689,14 @@ class PythonAstPrinter(
            self._write_line("if_false", last=True)
            with self._child_level(single=True):
                expr.if_false.accept(self)
+
+    def visit_list_expr(self, expr: p.ListExpr) -> None:
+        self._write_line("ListExpr")
+        with self._child_level():
+            self._write_line("items", last=True)
+            with self._child_level():
+                for i, item in enumerate(expr.items):
+                    self._idx = i
+                    if i == len(expr.items) - 1:
+                        self._mark_last()
+                    item.accept(self)
--- a/midas/ast/python.py
+++ b/midas/ast/python.py
@@ -14,6 +14,7 @@ from midas.ast.location import Location

 T = TypeVar("T")

+
 ####################
 # Type annotations #
 ####################
@@ -214,15 +215,15 @@ class Expr(ABC):
        @abstractmethod
        def visit_logical_expr(self, expr: LogicalExpr) -> T: ...

-        @abstractmethod
-        def visit_set_expr(self, expr: SetExpr) -> T: ...
-
        @abstractmethod
        def visit_cast_expr(self, expr: CastExpr) -> T: ...

        @abstractmethod
        def visit_ternary_expr(self, expr: TernaryExpr) -> T: ...

+        @abstractmethod
+        def visit_list_expr(self, expr: ListExpr) -> T: ...
+

@dataclass(frozen=True)
 class BinaryExpr(Expr):
@@ -298,16 +299,6 @@ class LogicalExpr(Expr):
        return visitor.visit_logical_expr(self)


-@dataclass(frozen=True)
-class SetExpr(Expr):
-    object: Expr
-    name: str
-    value: Expr
-
-    def accept(self, visitor: Expr.Visitor[T]) -> T:
-        return visitor.visit_set_expr(self)
-
-
@dataclass(frozen=True)
 class CastExpr(Expr):
    type: MidasType
@@ -325,3 +316,11 @@ class TernaryExpr(Expr):

    def accept(self, visitor: Expr.Visitor[T]) -> T:
        return visitor.visit_ternary_expr(self)
+
+
+@dataclass(frozen=True)
+class ListExpr(Expr):
+    items: list[Expr]
+
+    def accept(self, visitor: Expr.Visitor[T]) -> T:
+        return visitor.visit_list_expr(self)
--- a/midas/checker/builtins.py
+++ b/midas/checker/builtins.py
@@ -0,0 +1,112 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from midas.checker.types import (
+    BaseType,
+    ComplexType,
+    Function,
+    GenericType,
+    Type,
+    TypeVar,
+    UnitType,
+)
+
+if TYPE_CHECKING:
+    from midas.checker.registry import TypesRegistry
+
+
+BUILTIN_SUBTYPES: dict[str, set[str]] = {
+    "float": {"int"},
+    "int": {"bool"},
+}
+
+
+def op(reg: TypesRegistry, t1: Type, operator: str, t2: Type, t3: Type):
+    reg.define_operation(
+        left=t1,
+        operator=operator,
+        right=t2,
+        result=t3,
+    )
+
+
+def basic_op(reg: TypesRegistry, type: Type, op: str):
+    reg.define_operation(
+        left=type,
+        operator=op,
+        right=type,
+        result=type,
+    )
+
+
+def define_builtins(reg: TypesRegistry):
+    """Define builtin types and operations"""
+    unit = reg.define_type("None", UnitType())
+    bool = reg.define_type("bool", BaseType(name="bool"))
+    int = reg.define_type("int", BaseType(name="int"))
+    float = reg.define_type("float", BaseType(name="float"))
+    str = reg.define_type("str", BaseType(name="str"))
+
+    basic_op(reg, int, "__add__")  # int + int = int
+    basic_op(reg, int, "__sub__")  # int - int = int
+    basic_op(reg, int, "__mul__")  # int * int = int
+    basic_op(reg, int, "__pow__")  # int ** int = int
+    basic_op(reg, int, "__mod__")  # int % int = int
+    basic_op(reg, int, "__and__")  # int & int = int
+    basic_op(reg, int, "__or__")  # int | int = int
+    basic_op(reg, int, "__xor__")  # int ^ int = int
+    op(reg, int, "__lt__", int, bool)  # int < int = bool
+    op(reg, int, "__gt__", int, bool)  # int > int = bool
+    op(reg, int, "__le__", int, bool)  # int <= int = bool
+    op(reg, int, "__ge__", int, bool)  # int >= int = bool
+    op(reg, int, "__eq__", int, bool)  # int == int = bool
+    basic_op(reg, float, "__add__")  # float + float = float
+    basic_op(reg, float, "__sub__")  # float - float = float
+    basic_op(reg, float, "__mul__")  # float * float = float
+    basic_op(reg, float, "__truediv__")  # float / float = float
+    op(reg, float, "__lt__", float, bool)  # float < float = bool
+    op(reg, float, "__gt__", float, bool)  # float > float = bool
+    op(reg, float, "__le__", float, bool)  # float <= float = bool
+    op(reg, float, "__ge__", float, bool)  # float >= float = bool
+    op(reg, float, "__eq__", float, bool)  # float == float = bool
+    basic_op(reg, str, "__add__")  # str + str = str
+    op(reg, str, "__eq__", str, bool)  # str == str = bool
+
+    op(reg, int, "__lt__", float, bool)  # int < float = bool
+    op(reg, int, "__gt__", float, bool)  # int > float = bool
+    op(reg, int, "__le__", float, bool)  # int <= float = bool
+    op(reg, int, "__ge__", float, bool)  # int >= float = bool
+    op(reg, int, "__eq__", float, bool)  # int == float = bool
+
+    op(reg, float, "__lt__", int, bool)  # float < int = bool
+    op(reg, float, "__gt__", int, bool)  # float > int = bool
+    op(reg, float, "__le__", int, bool)  # float <= int = bool
+    op(reg, float, "__ge__", int, bool)  # float >= int = bool
+    op(reg, float, "__eq__", int, bool)  # float == int = bool
+
+    list = reg.define_type(
+        "list",
+        GenericType(
+            name="list",
+            params=[TypeVar(name="T", bound=None)],
+            body=ComplexType(
+                properties={
+                    "append": Function(
+                        name="append",
+                        pos_args=[
+                            Function.Argument(
+                                pos=0,
+                                name="object",
+                                type=TypeVar(name="T", bound=None),
+                                required=True,
+                            )
+                        ],
+                        args=[],
+                        kw_args=[],
+                        returns=UnitType(),
+                    )
+                }
+            ),
+        ),
+    )
--- a/midas/checker/checker.py
+++ b/midas/checker/checker.py
@@ -1,540 +1,35 @@
-import logging
-from dataclasses import dataclass
 from pathlib import Path
 from typing import Optional

-import midas.ast.midas as m
-import midas.ast.python as p
-from midas.ast.location import Location
-from midas.checker.diagnostic import Diagnostic, DiagnosticType
-from midas.checker.environment import Environment
-from midas.checker.operators import COMPARATOR_METHODS, OPERATOR_METHODS
-from midas.checker.types import Function, Type, UnitType, UnknownType
-from midas.lexer.midas import MidasLexer
-from midas.lexer.token import Token
-from midas.parser.midas import MidasParser
-from midas.resolver.midas import MidasResolver
+from midas.checker.diagnostic import Diagnostic
+from midas.checker.midas import MidasTyper
+from midas.checker.python import PythonTyper
+from midas.checker.registry import TypesRegistry
+from midas.checker.reporter import Reporter


-class ReturnException(Exception):
-    pass
+class TypeChecker:
+    def __init__(self):
+        self.types: TypesRegistry = TypesRegistry()
+        self.reporter: Reporter = Reporter()

+        self.midas_typer = MidasTyper(self.types, self.reporter)
+        self.python_typer = PythonTyper(self.types, self.reporter)

-@dataclass(frozen=True, kw_only=True)
-class MappedArgument:
-    expr: p.Expr
-    type: Type
-    argument: Function.Argument
+    def import_midas(self, path: Path):
+        source: str = path.read_text()
+        return self.import_midas_source(source, path=str(path))

+    def import_midas_source(self, source: str, path: Optional[str] = None):
+        self.midas_typer.process(source, path)

-class Checker(
-    p.Stmt.Visitor[None],
-    p.Expr.Visitor[Type],
-    p.MidasType.Visitor[Type],
-):
-    """A type checker which can use custom type definitions"""
+    def type_check(self, path: Path):
+        source: str = path.read_text()
+        return self.type_check_source(source, path=str(path))

-    def __init__(
-        self,
-        locals: dict[p.Expr, int],
-        source_path: Path,
-        types_paths: list[Path],
-    ):
-        self.logger: logging.Logger = logging.getLogger("Checker")
-        self.source_path: Path = source_path
-        self.types_paths: list[Path] = types_paths
-        self.ctx: MidasResolver = MidasResolver()
-        self.global_env: Environment = Environment()
-        self.env: Environment = self.global_env
-        self.locals: dict[p.Expr, int] = locals
-        self.diagnostics: list[Diagnostic] = []
+    def type_check_source(self, source: str, path: Optional[str] = None):
+        self.python_typer.process(source, path)

-    def diagnostic(self, type: DiagnosticType, location: Location, message: str):
-        self.diagnostics.append(
-            Diagnostic(
-                file_path=self.source_path,
-                location=location,
-                type=type,
-                message=message,
-            )
-        )
-
-    def error(self, location: Location, message: str):
-        self.diagnostic(
-            type=DiagnosticType.ERROR,
-            location=location,
-            message=message,
-        )
-
-    def warning(self, location: Location, message: str):
-        self.diagnostic(
-            type=DiagnosticType.WARNING,
-            location=location,
-            message=message,
-        )
-
-    def info(self, location: Location, message: str):
-        self.diagnostic(
-            type=DiagnosticType.INFO,
-            location=location,
-            message=message,
-        )
-
-    def type_of(self, expr: p.Expr) -> Type:
-        """Evaluate the type of an expression
-
-        Args:
-            expr (p.Expr): the expression to evaluate
-
-        Returns:
-            Type: the type of the given expression
-        """
-        return expr.accept(self)
-
-    def process_block(self, block: list[p.Stmt], env: Environment) -> bool:
-        """Evaluate a sequence of statements
-
-        Args:
-            block (list[p.Stmt]): the statements to evaluate
-            env (Environment): the environment in which to evaluate
-
-        Returns:
-            bool: whether a return statement is present in the block
-        """
-        previous_env: Environment = self.env
-        self.env = env
-        returned: bool = False
-        for i, stmt in enumerate(block):
-            try:
-                stmt.accept(self)
-            except ReturnException:
-                returned = True
-                if i < len(block) - 1:
-                    self.warning(block[i + 1].location, "Unreachable statement")
-                break
-        self.env = previous_env
-        return returned
-
-    def check(self, statements: list[p.Stmt]) -> list[Diagnostic]:
-        """Type check a sequence of statements and returns diagnostics
-
-        Args:
-            statements (list[p.Stmt]): the statements to evaluate and check
-
-        Returns:
-            list[Diagnostic]: the list of diagnostics (errors, warning, etc.)
-        """
-        self.diagnostics = []
-
-        for path in self.types_paths:
-            self.import_midas(path)
-        self.logger.debug(f"Midas types: {self.ctx._types}")
-        self.logger.debug(f"Midas operations: {self.ctx._operations}")
-
-        for stmt in statements:
-            stmt.accept(self)
-
-        self.logger.debug(f"Final environment: {self.env.flat_dict()}")
-        return self.diagnostics
-
-    def look_up_variable(self, name: str, expr: p.Expr) -> Optional[Type]:
-        """Look up a variable in the environment it was declared
-
-        Args:
-            name (str): the name of the variable
-            expr (p.Expr): the variable expression, used to lookup the scope distance
-
-        Returns:
-            Optional[Type]: the type of the variable, or None if it was not found
-        """
-        distance: Optional[int] = self.locals.get(expr)
-        if distance is not None:
-            return self.env.get_at(distance, name)
-        return self.global_env.get(name)
-
-    def import_midas(self, path: Path) -> None:
-        """Import Midas definitions from a path
-
-        Args:
-            path (Path): the import path
-        """
-        self.logger.debug(f"Importing type definitions from {path}")
-        lexer: MidasLexer = MidasLexer(path.read_text())
-        tokens: list[Token] = lexer.process()
-        parser: MidasParser = MidasParser(tokens)
-        stmts: list[m.Stmt] = parser.parse()
-        self.ctx.resolve(stmts)
-
-    def visit_expression_stmt(self, stmt: p.ExpressionStmt) -> None:
-        self.type_of(stmt.expr)
-
-    def visit_function(self, stmt: p.Function) -> None:
-        env: Environment = Environment(self.env)
-        pos_args: list[Function.Argument] = []
-        args: list[Function.Argument] = []
-        kw_args: list[Function.Argument] = []
-
-        def eval_arg_type(arg: p.Function.Argument) -> Type:
-            if arg.type is not None:
-                return arg.type.accept(self)
-            if arg.default is not None:
-                return arg.default.accept(self)
-            return UnknownType()
-
-        for arg in stmt.posonlyargs:
-            pos_args.append(
-                Function.Argument(
-                    name=arg.name,
-                    type=eval_arg_type(arg),
-                    required=arg.default is None,
-                )
-            )
-        for arg in stmt.args:
-            args.append(
-                Function.Argument(
-                    name=arg.name,
-                    type=eval_arg_type(arg),
-                    required=arg.default is None,
-                )
-            )
-        for arg in stmt.kwonlyargs:
-            kw_args.append(
-                Function.Argument(
-                    name=arg.name,
-                    type=eval_arg_type(arg),
-                    required=arg.default is None,
-                )
-            )
-
-        for arg in pos_args + args + kw_args:
-            env.define(arg.name, arg.type)
-
-        returns_hint: Optional[Type] = None
-        if stmt.returns is not None:
-            returns_hint = stmt.returns.accept(self)
-            # Early define to handle simple fully-typed recursion
-            inside_function: Function = Function(
-                name=stmt.name,
-                pos_args=pos_args,
-                args=args,
-                kw_args=kw_args,
-                returns=returns_hint,
-            )
-            self.env.define(stmt.name, inside_function)
-
-        returned: bool = self.process_block(stmt.body, env)
-        inferred_return: Type = UnknownType()
-        if not returned:
-            env.return_types.append(UnitType())
-        return_types: set[Type] = set(env.return_types)
-        if len(return_types) == 1:
-            inferred_return = list(return_types)[0]
-        elif len(return_types) > 1:
-            self.error(
-                stmt.location,
-                f"Mixed return types: {env.return_types}",
-            )
-
-        returns: Type = UnknownType()
-        if returns_hint is not None:
-            assert stmt.returns is not None
-            returns = returns_hint
-            if returns != inferred_return:
-                self.error(
-                    stmt.returns.location,
-                    f"Return type mismatch, annotated {returns} but returns {inferred_return}",
-                )
-        else:
-            returns = inferred_return
-
-        # TODO: handle *args and **kwargs sinks
-        function: Function = Function(
-            name=stmt.name,
-            pos_args=pos_args,
-            args=args,
-            kw_args=kw_args,
-            returns=returns,
-        )
-        self.env.define(stmt.name, function)
-
-    def visit_type_assign(self, stmt: p.TypeAssign) -> None:
-        # TODO check not yet defined locally
-        type: Type = stmt.type.accept(self)
-        self.env.define(stmt.name, type)
-
-    def visit_assign_stmt(self, stmt: p.AssignStmt) -> None:
-        value: Type = self.type_of(stmt.value)
-        for target in stmt.targets:
-            if not isinstance(target, p.VariableExpr):
-                self.logger.warning(f"Unsupported assignment to {target}")
-                self.warning(target.location, f"Unsupported assignment to {target}")
-                continue
-            name: str = target.name
-            var_type: Optional[Type] = self.look_up_variable(name, target)
-
-            if var_type is None:
-                self.env.define(name, value)
-            else:
-                # TODO: implement real comparison method
-                if var_type != value:
-                    self.error(
-                        stmt.location,
-                        f"Cannot assign {value} to {name} of type {var_type}",
-                    )
-
-    def visit_return_stmt(self, stmt: p.ReturnStmt) -> None:
-        type: Type = stmt.value.accept(self) if stmt.value is not None else UnitType()
-        self.env.return_types.append(type)
-        raise ReturnException()
-
-    def visit_if_stmt(self, stmt: p.IfStmt) -> None:
-        # Not evaluated in sub-environment because assignments in the test leak out of the if
-        # For example:
-        # if (m := 1 + 1) < 2:
-        #     ...
-        # print(m)  # <- m is still defined
-        test_type: Type = stmt.test.accept(self)
-
-        # TODO Allow subtypes or any type
-        if test_type != self.ctx.get_type("bool"):
-            self.error(
-                stmt.test.location, f"If test must be a boolean, got {test_type}"
-            )
-
-        env: Environment = Environment(self.env)
-        body_returned: bool = self.process_block(stmt.body, env)
-        else_returned: bool = self.process_block(stmt.orelse, env)
-        self.env.return_types.extend(env.return_types)
-        if body_returned and else_returned:
-            raise ReturnException()
-
-    def visit_binary_expr(self, expr: p.BinaryExpr) -> Type:
-        method: Optional[str] = OPERATOR_METHODS.get(expr.operator.__class__)
-        if method is None:
-            self.logger.warning(f"Unsupported operator {expr.operator}")
-            self.warning(expr.location, f"Unsupported operator {expr.operator}")
-            return UnknownType()
-        left: Type = self.type_of(expr.left)
-        right: Type = self.type_of(expr.right)
-
-        result: Optional[Type] = self.ctx.get_operation_result(left, method, right)
-        if result is None:
-            self.error(
-                expr.location,
-                f"Undefined operation {method} between {left} and {right}",
-            )
-            return UnknownType()
-        return result
-
-    def visit_compare_expr(self, expr: p.CompareExpr) -> Type:
-        method: Optional[str] = COMPARATOR_METHODS.get(expr.operator.__class__)
-        if method is None:
-            self.logger.warning(f"Unsupported operator {expr.operator}")
-            self.warning(expr.location, f"Unsupported operator {expr.operator}")
-            return UnknownType()
-        left: Type = self.type_of(expr.left)
-        right: Type = self.type_of(expr.right)
-
-        result: Optional[Type] = self.ctx.get_operation_result(left, method, right)
-        if result is None:
-            self.error(
-                expr.location,
-                f"Undefined operation {method} between {left} and {right}",
-            )
-            return UnknownType()
-        return result
-
-    def visit_unary_expr(self, expr: p.UnaryExpr) -> Type: ...
-
-    def visit_call_expr(self, expr: p.CallExpr) -> Type:
-        callee: Type = self.type_of(expr.callee)
-        if not isinstance(callee, Function):
-            self.error(expr.callee.location, "Callee is not a function")
-            return UnknownType()
-        function: Function = callee
-        mapped: list[MappedArgument] = self.map_call_arguments(function, expr)
-        for arg in mapped:
-            if arg.type != arg.argument.type:
-                self.error(
-                    arg.expr.location,
-                    f"Wrong type for argument '{arg.argument.name}', expected {arg.argument.type}, got {arg.type}",
-                )
-        return function.returns
-
-    def visit_get_expr(self, expr: p.GetExpr) -> Type: ...
-
-    def visit_literal_expr(self, expr: p.LiteralExpr) -> Type:
-        match expr.value:
-            case bool():  # Must be before int
-                return self.ctx.get_type("bool")
-            case int():
-                return self.ctx.get_type("int")
-            case float():
-                return self.ctx.get_type("float")
-            case str():
-                return self.ctx.get_type("str")
-            case _:
-                self.warning(expr.location, f"Unknown literal {expr}")
-                return UnknownType()
-
-    def visit_variable_expr(self, expr: p.VariableExpr) -> Type:
-        return self.look_up_variable(expr.name, expr) or UnknownType()
-
-    def visit_logical_expr(self, expr: p.LogicalExpr) -> Type:
-        left: Type = expr.left.accept(self)
-        right: Type = expr.right.accept(self)
-        # TODO: union type
-        if left != right:
-            self.error(
-                expr.location,
-                f"Operands must be of the same type, left={left} != right={right}",
-            )
-        return left
-
-    def visit_set_expr(self, expr: p.SetExpr) -> Type: ...
-
-    def visit_cast_expr(self, expr: p.CastExpr) -> Type:
-        return expr.type.accept(self)
-
-    def visit_ternary_expr(self, expr: p.TernaryExpr) -> Type:
-        test_type: Type = expr.test.accept(self)
-
-        # TODO Allow subtypes or any type
-        if test_type != self.ctx.get_type("bool"):
-            self.error(
-                expr.test.location, f"If test must be a boolean, got {test_type}"
-            )
-
-        true_type: Type = expr.if_true.accept(self)
-        false_type: Type = expr.if_false.accept(self)
-        if true_type != false_type:
-            self.error(
-                expr.location,
-                f"Type mismatch in ternary if branches: true={true_type} != false={false_type}",
-            )
-            return UnknownType()
-        return true_type
-
-    def visit_base_type(self, node: p.BaseType) -> Type:
-        return self.ctx.get_type(node.base)
-
-    def visit_constraint_type(self, node: p.ConstraintType) -> Type: ...
-
-    def visit_frame_column(self, node: p.FrameColumn) -> Type: ...
-
-    def visit_frame_type(self, node: p.FrameType) -> Type: ...
-
-    def map_call_arguments(
-        self, function: Function, call: p.CallExpr
-    ) -> list[MappedArgument]:
-        """Map call arguments to function parameters as defined in its signature
-
-        This method maps positional-only, keyword-only and mixed parameter definitions
-        with the arguments passed at the call site
-
-        Any mismatched, missing or unexpected argument is reported as a diagnostic
-
-        Args:
-            function (Function): the function definition
-            call (p.CallExpr): the call expression
-
-        Returns:
-            list[MappedArgument]: the list of mapped arguments
-        """
-        positional: list[tuple[p.Expr, Type]] = [
-            (arg, self.type_of(arg)) for arg in call.arguments
-        ]
-        keywords: dict[str, tuple[p.Expr, Type]] = {
-            name: (arg, self.type_of(arg)) for name, arg in call.keywords.items()
-        }
-        set_args: set[str] = set()
-
-        required_positional: list[str] = [
-            arg.name for arg in function.pos_args + function.args if arg.required
-        ]
-        required_keyword: list[str] = [
-            arg.name for arg in function.kw_args if arg.required
-        ]
-
-        mapped: list[MappedArgument] = []
-
-        pos_params: list[Function.Argument] = list(function.pos_args)
-        mixed_params: list[Function.Argument] = list(function.args)
-        kw_params: dict[str, Function.Argument] = {
-            arg.name: arg for arg in function.kw_args
-        }
-
-        # TODO: handle *args and **kwargs sinks
-        for arg in positional:
-            param: Function.Argument
-            if len(pos_params) != 0:
-                param = pos_params.pop(0)
-            elif len(mixed_params) != 0:
-                param = mixed_params.pop(0)
-            else:
-                self.error(arg[0].location, "Too many positional arguments")
-                break
-            name: str = param.name
-            if name in required_positional:
-                required_positional.remove(name)
-            if name in required_keyword:
-                required_keyword.remove(name)
-            set_args.add(name)
-            mapped.append(
-                MappedArgument(
-                    expr=arg[0],
-                    type=arg[1],
-                    argument=param,
-                )
-            )
-
-        kw_params.update({arg.name: arg for arg in mixed_params})
-        for name, arg in keywords.items():
-            param: Function.Argument
-            if name not in kw_params:
-                if name in set_args:
-                    self.error(
-                        arg[0].location, f"Multiple values for argument '{name}'"
-                    )
-                else:
-                    self.error(arg[0].location, f"Unknown keyword argument '{name}'")
-                continue
-            param = kw_params.pop(name)
-            if name in required_positional:
-                required_positional.remove(name)
-            if name in required_keyword:
-                required_keyword.remove(name)
-            set_args.add(name)
-            mapped.append(
-                MappedArgument(
-                    expr=arg[0],
-                    type=arg[1],
-                    argument=param,
-                )
-            )
-
-        def join_args(args: list[str]) -> str:
-            args = list(map(lambda a: f"'{a}'", args))
-            if len(args) == 0:
-                return ""
-            if len(args) == 1:
-                return args[0]
-            return ", ".join(args[:-1]) + " and " + args[-1]
-
-        if len(required_positional) != 0:
-            plural: str = "" if len(required_positional) == 1 else "s"
-            args: str = join_args(required_positional)
-            self.error(
-                call.location,
-                f"Missing required positional argument{plural}: {args}",
-            )
-
-        if len(required_keyword) != 0:
-            plural: str = "" if len(required_keyword) == 1 else "s"
-            args: str = join_args(required_keyword)
-            self.error(
-                call.location,
-                f"Missing required keyword argument{plural}: {args}",
-            )
-
-        return mapped
+    @property
+    def diagnostics(self) -> list[Diagnostic]:
+        return self.reporter.diagnostics
--- a/midas/checker/diagnostic.py
+++ b/midas/checker/diagnostic.py
@@ -1,6 +1,5 @@
 from dataclasses import dataclass
 from enum import StrEnum
-from pathlib import Path
 from typing import Optional

 from midas.ast.location import Location
@@ -14,12 +13,13 @@ class DiagnosticType(StrEnum):

@dataclass(frozen=True)
 class Diagnostic:
-    file_path: Path
+    file_path: Optional[str]
    location: Location
    type: DiagnosticType
    message: str

-    def __str__(self) -> str:
+    @property
+    def location_str(self) -> str:
        start_loc: str = f"L{self.location.lineno}:{self.location.col_offset+1}"
        end_loc: Optional[str] = ""
        if (
@@ -27,7 +27,16 @@ class Diagnostic:
            and self.location.end_col_offset is not None
        ):
            end_loc = f"L{self.location.end_lineno}:{self.location.end_col_offset+1}"
-        loc: str = (
-            f"at {start_loc}" if end_loc is None else f"from {start_loc} to {end_loc}"
-        )
-        return f"{self.type} in {self.file_path} {loc}: {self.message}"
+
+        loc: str = ""
+        if self.file_path is not None:
+            loc += f" in {self.file_path}"
+        if end_loc is None:
+            loc += f" at {start_loc}"
+        else:
+            loc += f" from {start_loc} to {end_loc}"
+
+        return f"{self.type}{loc}"
+
+    def __str__(self) -> str:
+        return f"{self.location_str}: {self.message}"
--- a/midas/checker/midas.py
+++ b/midas/checker/midas.py
@@ -0,0 +1,171 @@
+import logging
+from typing import Optional
+
+import midas.ast.midas as m
+from midas.checker.builtins import define_builtins
+from midas.checker.registry import TypesRegistry
+from midas.checker.reporter import FileReporter, Reporter
+from midas.checker.types import (
+    AliasType,
+    ComplexType,
+    Function,
+    GenericType,
+    Type,
+    TypeVar,
+    UnknownType,
+)
+from midas.lexer.midas import MidasLexer
+from midas.lexer.token import Token
+from midas.parser.midas import MidasParser
+
+
+class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[None], m.Type.Visitor[Type]):
+    """A resolver which evaluates Midas type definitions and build a registry"""
+
+    def __init__(self, types: TypesRegistry, reporter: Reporter) -> None:
+        self.logger: logging.Logger = logging.getLogger("MidasTyper")
+        self.reporter: FileReporter = reporter.for_file(None)
+
+        self.types: TypesRegistry = types
+        self._local_variables: dict[str, TypeVar] = {}
+
+        define_builtins(self.types)
+
+    def process(self, source: str, path: Optional[str]):
+        self.reporter = self.reporter.for_file(path)
+        lexer: MidasLexer = MidasLexer(source)
+        tokens: list[Token] = lexer.process()
+        parser: MidasParser = MidasParser(tokens)
+        stmts: list[m.Stmt] = parser.parse()
+        self.resolve(stmts)
+
+    def get_type(self, name: str) -> Type:
+        """Get a type from its name
+
+        Args:
+            name (str): the name of the type
+
+        Raises:
+            NameError: if the type is not defined
+
+        Returns:
+            Type: the type
+        """
+        if name in self._local_variables:
+            return self._local_variables[name]
+        return self.types.get_type(name)
+
+    def resolve(self, stmts: list[m.Stmt]):
+        """Process a sequence of statements
+
+        Args:
+            stmts (list[m.Stmt]): the statements
+        """
+        for stmt in stmts:
+            stmt.accept(self)
+
+    def visit_type_stmt(self, stmt: m.TypeStmt) -> None:
+        params: list[TypeVar] = self._resolve_type_params(stmt.params)
+
+        name: str = stmt.name.lexeme
+        type: Type = stmt.type.accept(self)
+        if len(params) != 0:
+            type = GenericType(name=name, params=params, body=type)
+        else:
+            type = AliasType(name=name, type=type)
+        self.types.define_type(name, type)
+        self._local_variables.clear()
+
+    def visit_property_stmt(self, stmt: m.PropertyStmt) -> None: ...
+
+    def visit_extend_stmt(self, stmt: m.ExtendStmt) -> None:
+        self._resolve_type_params(stmt.params)
+        base: Type = stmt.type.accept(self)
+        for op in stmt.operations:
+            right: Type = op.operand.accept(self)
+            result: Type = op.result.accept(self)
+            self.types.define_operation(
+                left=base,
+                operator=op.name.lexeme,
+                right=right,
+                result=result,
+            )
+
+    def visit_op_stmt(self, stmt: m.OpStmt) -> None: ...
+
+    def visit_predicate_stmt(self, stmt: m.PredicateStmt) -> None: ...
+
+    def visit_logical_expr(self, expr: m.LogicalExpr) -> None: ...
+
+    def visit_binary_expr(self, expr: m.BinaryExpr) -> None: ...
+
+    def visit_unary_expr(self, expr: m.UnaryExpr) -> None: ...
+
+    def visit_get_expr(self, expr: m.GetExpr) -> None: ...
+
+    def visit_variable_expr(self, expr: m.VariableExpr) -> None: ...
+
+    def visit_grouping_expr(self, expr: m.GroupingExpr) -> None:
+        return expr.expr.accept(self)
+
+    def visit_literal_expr(self, expr: m.LiteralExpr) -> None: ...
+
+    def visit_wildcard_expr(self, expr: m.WildcardExpr) -> None: ...
+
+    def visit_named_type(self, type: m.NamedType) -> Type:
+        return self.get_type(type.name.lexeme)
+
+    def visit_generic_type(self, type: m.GenericType) -> Type:
+        type_: Type = type.type.accept(self)
+        args: list[Type] = [arg.accept(self) for arg in type.args]
+        return self.types.apply_generic(type_, args)
+
+    def visit_constraint_type(self, type: m.ConstraintType) -> Type:
+        type_: Type = type.type.accept(self)
+        type.constraint.accept(self)
+        # TODO
+        return UnknownType()
+
+    def visit_complex_type(self, type: m.ComplexType) -> Type:
+        return ComplexType(
+            properties={
+                prop.name.lexeme: prop.type.accept(self) for prop in type.properties
+            }
+        )
+
+    def visit_function_type(self, type: m.FunctionType) -> Type:
+        return Function(
+            name="<anonymous>",
+            pos_args=[
+                Function.Argument(
+                    pos=i,
+                    name=arg.name.lexeme if arg.name is not None else str(i),
+                    type=arg.type.accept(self),
+                    required=arg.required,
+                )
+                for i, arg in enumerate(type.pos_args)
+            ],
+            args=[],
+            kw_args=[
+                Function.Argument(
+                    pos=i,
+                    name=arg.name.lexeme if arg.name is not None else str(i),
+                    type=arg.type.accept(self),
+                    required=arg.required,
+                )
+                for i, arg in enumerate(type.kw_args, start=len(type.pos_args))
+            ],
+            returns=type.returns.accept(self),
+        )
+
+    def _resolve_type_params(self, params: list[m.TypeParam]):
+        vars: list[TypeVar] = []
+        for param in params:
+            name: str = param.name.lexeme
+            bound: Optional[Type] = None
+            if param.bound is not None:
+                bound = param.bound.accept(self)
+            var = TypeVar(name=name, bound=bound)
+            self._local_variables[name] = var
+            vars.append(var)
+        return vars
--- a/midas/checker/python.py
+++ b/midas/checker/python.py
@@ -0,0 +1,646 @@
+import ast
+import logging
+from dataclasses import dataclass
+from typing import Optional
+
+import midas.ast.python as p
+from midas.ast.location import Location
+from midas.checker.environment import Environment
+from midas.checker.operators import COMPARATOR_METHODS, OPERATOR_METHODS
+from midas.checker.registry import TypesRegistry
+from midas.checker.reporter import FileReporter, Reporter
+from midas.checker.resolver import Resolver
+from midas.checker.types import (
+    ComplexType,
+    Function,
+    Operation,
+    Type,
+    UnitType,
+    UnknownType,
+    unfold_type,
+)
+from midas.parser.python import PythonParser
+
+
+class ReturnException(Exception):
+    pass
+
+
+@dataclass(frozen=True, kw_only=True)
+class MappedArgument:
+    expr: p.Expr
+    type: Type
+    argument: Function.Argument
+
+
+class PythonTyper(
+    p.Stmt.Visitor[None],
+    p.Expr.Visitor[Type],
+    p.MidasType.Visitor[Type],
+):
+    """A type checker which can use custom type definitions"""
+
+    def __init__(
+        self,
+        types: TypesRegistry,
+        reporter: Reporter,
+    ):
+        self.logger: logging.Logger = logging.getLogger("PythonTyper")
+        self.reporter: FileReporter = reporter.for_file(None)
+        self.types: TypesRegistry = types
+        self.global_env: Environment = Environment()
+        self.env: Environment = self.global_env
+        self.locals: dict[p.Expr, int] = {}
+        self.judgements: list[tuple[p.Expr, Type]] = []
+
+    def process(self, source: str, path: Optional[str]):
+        self.reporter = self.reporter.for_file(path)
+
+        tree: ast.Module = ast.parse(source, filename=path or "<unknown>")
+        parser = PythonParser()
+        stmts: list[p.Stmt] = parser.parse_module(tree)
+        resolver = Resolver()
+        resolver.resolve(*stmts)
+
+        self.env = self.global_env
+        self.locals = resolver.locals
+        self.judgements = []
+
+        self.check(stmts)
+
+    def type_of(self, expr: p.Expr) -> Type:
+        """Evaluate the type of an expression
+
+        Args:
+            expr (p.Expr): the expression to evaluate
+
+        Returns:
+            Type: the type of the given expression
+        """
+        type: Type = expr.accept(self)
+        self.judgements.append((expr, type))
+        return type
+
+    def process_block(self, block: list[p.Stmt], env: Environment) -> bool:
+        """Evaluate a sequence of statements
+
+        Args:
+            block (list[p.Stmt]): the statements to evaluate
+            env (Environment): the environment in which to evaluate
+
+        Returns:
+            bool: whether a return statement is present in the block
+        """
+        previous_env: Environment = self.env
+        self.env = env
+        returned: bool = False
+        for i, stmt in enumerate(block):
+            try:
+                stmt.accept(self)
+            except ReturnException:
+                returned = True
+                if i < len(block) - 1:
+                    self.reporter.warning(
+                        block[i + 1].location, "Unreachable statement"
+                    )
+                break
+        self.env = previous_env
+        return returned
+
+    def check(self, statements: list[p.Stmt]) -> None:
+        """Type check a sequence of statements and returns diagnostics
+
+        Args:
+            statements (list[p.Stmt]): the statements to evaluate and check
+        """
+        for stmt in statements:
+            stmt.accept(self)
+
+        self.logger.debug(f"Final environment: {self.env.flat_dict()}")
+
+    def look_up_variable(self, name: str, expr: p.Expr) -> Optional[Type]:
+        """Look up a variable in the environment it was declared
+
+        Args:
+            name (str): the name of the variable
+            expr (p.Expr): the variable expression, used to lookup the scope distance
+
+        Returns:
+            Optional[Type]: the type of the variable, or None if it was not found
+        """
+        distance: Optional[int] = self.locals.get(expr)
+        if distance is not None:
+            return self.env.get_at(distance, name)
+        return self.global_env.get(name)
+
+    def is_subtype(self, type1: Type, type2: Type) -> bool:
+        return self.types.is_subtype(type1, type2)
+
+    def visit_expression_stmt(self, stmt: p.ExpressionStmt) -> None:
+        self.type_of(stmt.expr)
+
+    def visit_function(self, stmt: p.Function) -> None:
+        env: Environment = Environment(self.env)
+        pos_args: list[Function.Argument] = []
+        args: list[Function.Argument] = []
+        kw_args: list[Function.Argument] = []
+
+        def eval_arg_type(arg: p.Function.Argument) -> Type:
+            if arg.type is not None:
+                return arg.type.accept(self)
+            if arg.default is not None:
+                return arg.default.accept(self)
+            return UnknownType()
+
+        pos: int = 0
+        for arg in stmt.posonlyargs:
+            pos_args.append(
+                Function.Argument(
+                    pos=pos,
+                    name=arg.name,
+                    type=eval_arg_type(arg),
+                    required=arg.default is None,
+                )
+            )
+            pos += 1
+        for arg in stmt.args:
+            args.append(
+                Function.Argument(
+                    pos=pos,
+                    name=arg.name,
+                    type=eval_arg_type(arg),
+                    required=arg.default is None,
+                )
+            )
+            pos += 1
+        for arg in stmt.kwonlyargs:
+            kw_args.append(
+                Function.Argument(
+                    pos=pos,  # not relevant
+                    name=arg.name,
+                    type=eval_arg_type(arg),
+                    required=arg.default is None,
+                )
+            )
+            pos += 1
+
+        for arg in pos_args + args + kw_args:
+            env.define(arg.name, arg.type)
+
+        returns_hint: Optional[Type] = None
+        if stmt.returns is not None:
+            returns_hint = stmt.returns.accept(self)
+            # Early define to handle simple fully-typed recursion
+            inside_function: Function = Function(
+                name=stmt.name,
+                pos_args=pos_args,
+                args=args,
+                kw_args=kw_args,
+                returns=returns_hint,
+            )
+            self.env.define(stmt.name, inside_function)
+
+        returned: bool = self.process_block(stmt.body, env)
+        inferred_return: Type = UnknownType()
+        if not returned:
+            env.return_types.append(UnitType())
+        return_types: list[Type] = self.types.reduce_types(env.return_types)
+        if len(return_types) == 1:
+            inferred_return = return_types[0]
+        elif len(return_types) > 1:
+            self.reporter.error(
+                stmt.location,
+                f"Mixed return types: {return_types}",
+            )
+
+        returns: Type = UnknownType()
+        if returns_hint is not None:
+            assert stmt.returns is not None
+            returns = returns_hint
+            if returns != inferred_return:
+                self.reporter.error(
+                    stmt.returns.location,
+                    f"Return type mismatch, annotated {returns} but returns {inferred_return}",
+                )
+        else:
+            returns = inferred_return
+
+        # TODO: handle *args and **kwargs sinks
+        function: Function = Function(
+            name=stmt.name,
+            pos_args=pos_args,
+            args=args,
+            kw_args=kw_args,
+            returns=returns,
+        )
+        self.env.define(stmt.name, function)
+
+    def visit_type_assign(self, stmt: p.TypeAssign) -> None:
+        # TODO check not yet defined locally
+        type: Type = stmt.type.accept(self)
+        self.env.define(stmt.name, type)
+
+    def visit_assign_stmt(self, stmt: p.AssignStmt) -> None:
+        value_type: Type = self.type_of(stmt.value)
+        for target in stmt.targets:
+            self._assign(stmt.location, target, value_type)
+
+    def _assign(self, location: Location, target: p.Expr, value_type: Type):
+        match target:
+            case p.VariableExpr():
+                self._assign_var(location, target, value_type)
+
+            case p.GetExpr():
+                self._assign_attr(location, target, value_type)
+
+            case _:
+                if not isinstance(target, p.VariableExpr):
+                    self.logger.warning(f"Unsupported assignment to {target}")
+                    self.reporter.warning(
+                        target.location, f"Unsupported assignment to {target}"
+                    )
+
+    def _assign_var(self, location: Location, target: p.VariableExpr, value_type: Type):
+        name: str = target.name
+        var_type: Optional[Type] = self.look_up_variable(name, target)
+
+        if var_type is None:
+            self.env.define(name, value_type)
+        else:
+            # S <: T
+            # Γ, x: T   v: S
+            # x = v
+            if not self.is_subtype(value_type, var_type):
+                self.reporter.error(
+                    location,
+                    f"Cannot assign {value_type} to variable '{name}' of type {var_type}",
+                )
+
+    def _assign_attr(self, location: Location, target: p.GetExpr, value_type: Type):
+        object: Type = self.type_of(target.object)
+        base_object: Type = unfold_type(object)
+        match base_object:
+            case ComplexType(properties=properties):
+                if target.name not in properties:
+                    self.reporter.error(
+                        target.location, f"Unknown property '{object}.{target.name}'"
+                    )
+                    return
+
+                prop_type: Type = properties[target.name]
+                if not self.is_subtype(value_type, prop_type):
+                    self.reporter.error(
+                        location,
+                        f"Cannot assign {value_type} to property '{object}.{target.name}' of type {prop_type}",
+                    )
+                    return
+
+            case UnknownType():
+                pass
+
+            case _:
+                self.reporter.error(
+                    target.location,
+                    f"Cannot assign {value_type} to unknown property '{object}.{target.name}'",
+                )
+
+    def visit_return_stmt(self, stmt: p.ReturnStmt) -> None:
+        type: Type = stmt.value.accept(self) if stmt.value is not None else UnitType()
+        self.env.return_types.append(type)
+        raise ReturnException()
+
+    def visit_if_stmt(self, stmt: p.IfStmt) -> None:
+        # Not evaluated in sub-environment because assignments in the test leak out of the if
+        # For example:
+        # if (m := 1 + 1) < 2:
+        #     ...
+        # print(m)  # <- m is still defined
+        test_type: Type = stmt.test.accept(self)
+
+        # TODO Allow subtypes or any type
+        if test_type != self.types.get_type("bool"):
+            self.reporter.error(
+                stmt.test.location, f"If test must be a boolean, got {test_type}"
+            )
+
+        env: Environment = Environment(self.env)
+        body_returned: bool = self.process_block(stmt.body, env)
+        else_returned: bool = self.process_block(stmt.orelse, env)
+        self.env.return_types.extend(env.return_types)
+        if body_returned and else_returned:
+            raise ReturnException()
+
+    def visit_binary_expr(self, expr: p.BinaryExpr) -> Type:
+        method: Optional[str] = OPERATOR_METHODS.get(expr.operator.__class__)
+        if method is None:
+            self.logger.warning(f"Unsupported operator {expr.operator}")
+            self.reporter.warning(
+                expr.location, f"Unsupported operator {expr.operator}"
+            )
+            return UnknownType()
+        left: Type = self.type_of(expr.left)
+        right: Type = self.type_of(expr.right)
+
+        operations: list[Operation] = self.types.get_operations_by_name(method)
+        valid_operations: list[Operation] = []
+        for op in operations:
+            sig: Operation.CallSignature = op.signature
+            if self.is_subtype(left, sig.left) and self.is_subtype(right, sig.right):
+                valid_operations.append(op)
+
+        if len(valid_operations) == 0:
+            self.reporter.error(
+                expr.location,
+                f"Undefined operation {method} between {left} and {right}",
+            )
+            return UnknownType()
+        elif len(valid_operations) == 1:
+            self.logger.debug(f"Unique operation {method} between {left} and {right}")
+            return valid_operations[0].result
+
+        for i, op1 in enumerate(valid_operations):
+            sig1: Operation.CallSignature = op1.signature
+            best_match: bool = True
+            for j, op2 in enumerate(valid_operations):
+                if i == j:
+                    continue
+                sig2: Operation.CallSignature = op2.signature
+
+                # If op1 is not a full overload of op2 (i.e. operands of op1 are subtypes of op2's)
+                # ambiguity -> not best match
+                if not self.is_subtype(sig1.left, sig2.left) or not self.is_subtype(
+                    sig1.right, sig2.right
+                ):
+                    best_match = False
+                    break
+                self.logger.debug(f"{op1} is a full overload of {op2}")
+            if best_match:
+                return op1.result
+
+        self.reporter.error(
+            expr.location,
+            f"Ambiguous operation {method} between {left} and {right}, multiple matching overloads: {', '.join(map(str, valid_operations))}",
+        )
+        return UnknownType()
+
+    def visit_compare_expr(self, expr: p.CompareExpr) -> Type:
+        method: Optional[str] = COMPARATOR_METHODS.get(expr.operator.__class__)
+        if method is None:
+            self.logger.warning(f"Unsupported operator {expr.operator}")
+            self.reporter.warning(
+                expr.location, f"Unsupported operator {expr.operator}"
+            )
+            return UnknownType()
+        left: Type = self.type_of(expr.left)
+        right: Type = self.type_of(expr.right)
+
+        result: Optional[Type] = self.types.get_operation_result(left, method, right)
+        if result is None:
+            self.reporter.error(
+                expr.location,
+                f"Undefined operation {method} between {left} and {right}",
+            )
+            return UnknownType()
+        return result
+
+    def visit_unary_expr(self, expr: p.UnaryExpr) -> Type: ...
+
+    def visit_call_expr(self, expr: p.CallExpr) -> Type:
+        callee: Type = self.type_of(expr.callee)
+        if not isinstance(callee, Function):
+            self.reporter.error(expr.callee.location, "Callee is not a function")
+            return UnknownType()
+        function: Function = callee
+        mapped: list[MappedArgument] = self.map_call_arguments(function, expr)
+        for arg in mapped:
+            if not self.is_subtype(arg.type, arg.argument.type):
+                self.reporter.error(
+                    arg.expr.location,
+                    f"Wrong type for argument '{arg.argument.name}', expected {arg.argument.type}, got {arg.type}",
+                )
+        return function.returns
+
+    def visit_get_expr(self, expr: p.GetExpr) -> Type:
+        object: Type = self.type_of(expr.object)
+        base_object: Type = unfold_type(object)
+        match base_object:
+            case ComplexType(properties=properties):
+                if expr.name not in properties:
+                    self.reporter.error(
+                        expr.location, f"Unknown property '{expr.name} on {object}"
+                    )
+                    return UnknownType()
+                return properties[expr.name]
+
+            case UnknownType():
+                return UnknownType()
+
+            case _:
+                self.reporter.error(
+                    expr.location, f"Cannot get property '{expr.name}' on {object}"
+                )
+                return UnknownType()
+
+    def visit_literal_expr(self, expr: p.LiteralExpr) -> Type:
+        match expr.value:
+            case bool():  # Must be before int
+                return self.types.get_type("bool")
+            case int():
+                return self.types.get_type("int")
+            case float():
+                return self.types.get_type("float")
+            case str():
+                return self.types.get_type("str")
+            case _:
+                self.reporter.warning(expr.location, f"Unknown literal {expr}")
+                return UnknownType()
+
+    def visit_variable_expr(self, expr: p.VariableExpr) -> Type:
+        return self.look_up_variable(expr.name, expr) or UnknownType()
+
+    def visit_logical_expr(self, expr: p.LogicalExpr) -> Type:
+        left: Type = expr.left.accept(self)
+        right: Type = expr.right.accept(self)
+
+        if self.is_subtype(left, right):
+            return right
+        if self.is_subtype(right, left):
+            return left
+
+        self.reporter.error(
+            expr.location,
+            f"Incompatible operand types, {left=} and {right=}",
+        )
+        return UnknownType()
+
+    def visit_cast_expr(self, expr: p.CastExpr) -> Type:
+        return expr.type.accept(self)
+
+    def visit_ternary_expr(self, expr: p.TernaryExpr) -> Type:
+        test_type: Type = expr.test.accept(self)
+
+        # TODO Allow subtypes or any type
+        if test_type != self.types.get_type("bool"):
+            self.reporter.error(
+                expr.test.location, f"If test must be a boolean, got {test_type}"
+            )
+
+        true_type: Type = expr.if_true.accept(self)
+        false_type: Type = expr.if_false.accept(self)
+        if self.is_subtype(true_type, false_type):
+            return false_type
+        if self.is_subtype(false_type, true_type):
+            return true_type
+
+        self.reporter.error(
+            expr.location,
+            f"Incompatible types in ternary if branches: true={true_type} and false={false_type}",
+        )
+        return UnknownType()
+
+    def visit_list_expr(self, expr: p.ListExpr) -> Type:
+        list_type: Type = self.types.get_type("list")
+        item_types: list[Type] = [self.type_of(item) for item in expr.items]
+        item_types = self.types.reduce_types(item_types)
+
+        if len(item_types) == 0:
+            return list_type
+
+        if len(item_types) == 1:
+            item_type: Type = item_types[0]
+            return self.types.apply_generic(list_type, [item_type])
+        self.reporter.error(
+            expr.location,
+            f"Heterogeneous list items: {item_types}",
+        )
+        return self.types.apply_generic(list_type, [UnknownType()])
+
+    def visit_base_type(self, node: p.BaseType) -> Type:
+        base: Type = self.types.get_type(node.base)
+        if node.param is not None:
+            param: Type = node.param.accept(self)
+            return self.types.apply_generic(base, [param])
+        return base
+
+    def visit_constraint_type(self, node: p.ConstraintType) -> Type: ...
+
+    def visit_frame_column(self, node: p.FrameColumn) -> Type: ...
+
+    def visit_frame_type(self, node: p.FrameType) -> Type: ...
+
+    def map_call_arguments(
+        self, function: Function, call: p.CallExpr
+    ) -> list[MappedArgument]:
+        """Map call arguments to function parameters as defined in its signature
+
+        This method maps positional-only, keyword-only and mixed parameter definitions
+        with the arguments passed at the call site
+
+        Any mismatched, missing or unexpected argument is reported as a diagnostic
+
+        Args:
+            function (Function): the function definition
+            call (p.CallExpr): the call expression
+
+        Returns:
+            list[MappedArgument]: the list of mapped arguments
+        """
+        positional: list[tuple[p.Expr, Type]] = [
+            (arg, self.type_of(arg)) for arg in call.arguments
+        ]
+        keywords: dict[str, tuple[p.Expr, Type]] = {
+            name: (arg, self.type_of(arg)) for name, arg in call.keywords.items()
+        }
+        set_args: set[str] = set()
+
+        required_positional: list[str] = [
+            arg.name for arg in function.pos_args + function.args if arg.required
+        ]
+        required_keyword: list[str] = [
+            arg.name for arg in function.kw_args if arg.required
+        ]
+
+        mapped: list[MappedArgument] = []
+
+        pos_params: list[Function.Argument] = list(function.pos_args)
+        mixed_params: list[Function.Argument] = list(function.args)
+        kw_params: dict[str, Function.Argument] = {
+            arg.name: arg for arg in function.kw_args
+        }
+
+        # TODO: handle *args and **kwargs sinks
+        for arg in positional:
+            param: Function.Argument
+            if len(pos_params) != 0:
+                param = pos_params.pop(0)
+            elif len(mixed_params) != 0:
+                param = mixed_params.pop(0)
+            else:
+                self.reporter.error(arg[0].location, "Too many positional arguments")
+                break
+            name: str = param.name
+            if name in required_positional:
+                required_positional.remove(name)
+            if name in required_keyword:
+                required_keyword.remove(name)
+            set_args.add(name)
+            mapped.append(
+                MappedArgument(
+                    expr=arg[0],
+                    type=arg[1],
+                    argument=param,
+                )
+            )
+
+        kw_params.update({arg.name: arg for arg in mixed_params})
+        for name, arg in keywords.items():
+            param: Function.Argument
+            if name not in kw_params:
+                if name in set_args:
+                    self.reporter.error(
+                        arg[0].location, f"Multiple values for argument '{name}'"
+                    )
+                else:
+                    self.reporter.error(
+                        arg[0].location, f"Unknown keyword argument '{name}'"
+                    )
+                continue
+            param = kw_params.pop(name)
+            if name in required_positional:
+                required_positional.remove(name)
+            if name in required_keyword:
+                required_keyword.remove(name)
+            set_args.add(name)
+            mapped.append(
+                MappedArgument(
+                    expr=arg[0],
+                    type=arg[1],
+                    argument=param,
+                )
+            )
+
+        def join_args(args: list[str]) -> str:
+            args = list(map(lambda a: f"'{a}'", args))
+            if len(args) == 0:
+                return ""
+            if len(args) == 1:
+                return args[0]
+            return ", ".join(args[:-1]) + " and " + args[-1]
+
+        if len(required_positional) != 0:
+            plural: str = "" if len(required_positional) == 1 else "s"
+            args: str = join_args(required_positional)
+            self.reporter.error(
+                call.location,
+                f"Missing required positional argument{plural}: {args}",
+            )
+
+        if len(required_keyword) != 0:
+            plural: str = "" if len(required_keyword) == 1 else "s"
+            args: str = join_args(required_keyword)
+            self.reporter.error(
+                call.location,
+                f"Missing required keyword argument{plural}: {args}",
+            )
+
+        return mapped
--- a/midas/checker/registry.py
+++ b/midas/checker/registry.py
@@ -0,0 +1,313 @@
+from typing import Optional
+
+from midas.checker.builtins import BUILTIN_SUBTYPES
+from midas.checker.types import (
+    AliasType,
+    AppliedType,
+    BaseType,
+    ComplexType,
+    Function,
+    GenericType,
+    Operation,
+    Type,
+    substitute_typevars,
+)
+
+
+class TypesRegistry:
+    def __init__(self) -> None:
+        self._types: dict[str, Type] = {}
+        self._operations: dict[Operation.CallSignature, Type] = {}
+
+    def get_type(self, name: str) -> Type:
+        """Get a type from its name
+
+        Args:
+            name (str): the name of the type
+
+        Raises:
+            NameError: if the type is not defined
+
+        Returns:
+            Type: the type
+        """
+        if name in self._types:
+            return self._types[name]
+        raise NameError(f"Undefined type {name}")
+
+    def get_operation_result(
+        self, left: Type, operator: str, right: Type
+    ) -> Optional[Type]:
+        """Get the resulting type of an operation
+
+        Args:
+            left (Type): the type of the left operand
+            operator (str): the operation name
+            right (Type): the type of the right operand
+
+        Returns:
+            Optional[Type]: the result type, or None if no matching operation was found
+        """
+        signature: Operation.CallSignature = Operation.CallSignature(
+            left=left,
+            method=operator,
+            right=right,
+        )
+        result: Optional[Type] = self._operations.get(signature)
+        return result
+
+    def get_operations_by_name(self, name: str) -> list[Operation]:
+        operations: list[Operation] = []
+        for signature, result in self._operations.items():
+            if signature.method == name:
+                operations.append(
+                    Operation(
+                        signature=signature,
+                        result=result,
+                    )
+                )
+        return operations
+
+    def define_type(self, name: str, type: Type) -> Type:
+        """Define a type in the registry
+
+        Args:
+            name (str): the name of the type
+            type (Type): the type to define
+
+        Raises:
+            ValueError: if a type is already defined with that name
+
+        Returns:
+            Type: the defined type
+        """
+        if name in self._types:
+            raise ValueError(f"Type {name} already defined")
+        self._types[name] = type
+        return type
+
+    def define_operation(self, left: Type, operator: str, right: Type, result: Type):
+        """Define an operation in the registry
+
+        Args:
+            left (Type): the type of the left operand
+            operator (str): the operation name
+            right (Type): the type of the right operand
+            result (Type): the result type
+
+        Raises:
+            ValueError: if an operation is already defined with these operands and name
+        """
+        signature: Operation.CallSignature = Operation.CallSignature(
+            left=left,
+            method=operator,
+            right=right,
+        )
+        if signature in self._operations:
+            raise ValueError(
+                f"Operation {operator} already defined between {left} and {right}"
+            )
+        self._operations[signature] = result
+
+    def is_subtype(self, type1: Type, type2: Type) -> bool:
+        """Check whether `type1` is a subtype of `type2`
+
+        For more details on the rules checked here, see TAPL Chap. 15-16-17
+
+        Args:
+            type1 (Type): the potential subtype
+            type2 (Type): the potential supertype
+
+        Returns:
+            bool: whether `type1` is a subtype of `type2`
+        """
+
+        if type1 == type2:
+            return True
+
+        match (type1, type2):
+            case (AliasType(type=base1), _):
+                return self.is_subtype(base1, type2)
+
+            case (BaseType(name=name1), BaseType(name=name2)):
+                return name1 in BUILTIN_SUBTYPES.get(name2, set())
+
+            case (ComplexType(properties=props1), ComplexType(properties=props2)):
+                for k, t in props2.items():
+                    if k not in props1:
+                        return False
+                    if not self.is_subtype(props1[k], t):
+                        return False
+                return True
+
+            case (Function(), Function()):
+                return self.is_func_subtype(type1, type2)
+
+        return False
+
+    # TODO: verify the logic in here
+    def is_func_subtype(self, func1: Function, func2: Function) -> bool:
+        """Check whether a function is a subtype of another
+
+        Args:
+            func1 (Function): the potential function subtype
+            func2 (Function): the potential function supertype
+
+        Returns:
+            bool: whether `func1` is a subtype of `func2`
+        """
+        if not self.is_subtype(func1.returns, func2.returns):
+            return False
+
+        pos1: list[Function.Argument] = func1.pos_args
+        mixed1: list[Function.Argument] = func1.args
+        kw1: dict[str, Function.Argument] = {a.name: a for a in func1.kw_args}
+        pos2: list[Function.Argument] = func2.pos_args
+        mixed2: list[Function.Argument] = func2.args
+        kw2: dict[str, Function.Argument] = {a.name: a for a in func2.kw_args}
+
+        mixed_by_pos: dict[int, Function.Argument] = {arg.pos: arg for arg in mixed2}
+        mixed_by_name: dict[str, Function.Argument] = {arg.name: arg for arg in mixed2}
+
+        def is_arg_subtype(sub: Function.Argument, sup: Function.Argument) -> bool:
+            if not self.is_subtype(sub.type, sup.type):
+                return False
+            if not sup.required and sub.required:
+                return False
+            return True
+
+        for arg1 in pos1:
+            arg2: Function.Argument
+            if arg1.pos < len(pos2):
+                arg2 = pos2[arg1.pos]
+            elif arg1.pos in mixed_by_pos:
+                arg2 = mixed_by_pos[arg1.pos]
+            elif not arg1.required:
+                continue
+            else:
+                return False
+            if not is_arg_subtype(arg2, arg1):
+                return False
+
+        for name, arg1 in kw1.items():
+            arg2: Function.Argument
+            if name in kw2:
+                arg2 = kw2[name]
+            elif name in mixed_by_name:
+                arg2 = mixed_by_name[name]
+            elif not arg1.required:
+                continue
+            else:
+                return False
+            if not is_arg_subtype(arg2, arg1):
+                return False
+
+        for arg1 in mixed1:
+            pos_arg2: Optional[Function.Argument] = None
+            kw_arg2: Optional[Function.Argument] = None
+            if arg1.name in kw2:
+                kw_arg2 = kw2[arg1.name]
+            elif arg1.name in mixed_by_name:
+                kw_arg2 = mixed_by_name[arg1.name]
+            if arg1.pos < len(pos2):
+                pos_arg2 = pos2[arg1.pos]
+            elif arg1.pos in mixed_by_pos:
+                pos_arg2 = mixed_by_pos[arg1.pos]
+
+            # No match in func2 and arg is required
+            if pos_arg2 is None and kw_arg2 is None and arg1.required:
+                return False
+
+            # Matching keyword argument
+            if kw_arg2 is not None and not is_arg_subtype(kw_arg2, arg1):
+                return False
+
+            # Matching positional argument
+            if pos_arg2 is not None and not is_arg_subtype(pos_arg2, arg1):
+                return False
+
+        mixed_positions: set[int] = {a.pos for a in mixed1}
+        mixed_names: set[str] = {a.name for a in mixed1}
+        for arg2 in pos2:
+            if not arg2.required:
+                continue
+            if arg2.pos >= len(pos1) and arg2.pos not in mixed_positions:
+                return False
+
+        for name, arg2 in kw2.items():
+            if not arg2.required:
+                continue
+            if name not in kw1 and name not in mixed_names:
+                return False
+
+        for arg2 in mixed2:
+            if arg2.required:
+                continue
+            pos_match: bool = arg2.pos < len(pos1) or arg2.pos in mixed_positions
+            kw_match: bool = arg2.name in kw1 or arg2.name in mixed_names
+            if not pos_match or not kw_match:
+                return False
+
+        return True
+
+    def apply_generic(self, type: Type, args: list[Type]) -> Type:
+        match type:
+            case AliasType(name=name, type=base):
+                return AliasType(name=name, type=self.apply_generic(base, args))
+
+            case GenericType(name=name, params=type_vars, body=body):
+                n_args: int = len(args)
+                n_type_vars: int = len(type_vars)
+                if n_args < n_type_vars:
+                    raise ValueError(
+                        f"Missing type arguments, expected {n_type_vars} but only {n_args} provided"
+                    )
+                if n_args > n_type_vars:
+                    raise ValueError(
+                        f"Too many type arguments, expected {n_type_vars} but {n_args} provided"
+                    )
+                substitutions: dict[str, Type] = {}
+                for arg, type_var in zip(args, type_vars):
+                    if type_var.bound is not None and not self.is_subtype(
+                        arg, type_var.bound
+                    ):
+                        raise ValueError(
+                            f"Type argument {arg} is not a subtype of {type_var.bound}"
+                        )
+                    substitutions[type_var.name] = arg
+                return AppliedType(
+                    name=name,
+                    args=args,
+                    body=substitute_typevars(body, substitutions),
+                )
+
+            case _:
+                raise ValueError(f"{type} is not a generic type")
+
+    def reduce_types(self, types: list[Type]) -> list[Type]:
+        """Reduce a list of types to remove subtypes and only keep the highest types
+
+        Args:
+            types (list[Type]): the types to reduce
+
+        Returns:
+            list[Type]: the reduced list of types
+        """
+
+        reduced: bool = True
+        keep: list[int] = list(range(len(types)))
+        while reduced:
+            reduced = False
+            for i, i1 in enumerate(keep):
+                type1: Type = types[i1]
+                for i2 in keep[i + 1 :]:
+                    type2 = types[i2]
+                    if self.is_subtype(type1, type2):
+                        keep.remove(i1)
+                    elif self.is_subtype(type2, type1):
+                        keep.remove(i2)
+                    else:
+                        continue
+                    reduced = True
+                    break
+        return [types[i] for i in keep]
--- a/midas/checker/reporter.py
+++ b/midas/checker/reporter.py
@@ -0,0 +1,63 @@
+from __future__ import annotations
+
+from typing import Optional
+
+from midas.ast.location import Location
+from midas.checker.diagnostic import Diagnostic, DiagnosticType
+
+
+class Reporter:
+    def __init__(self):
+        self.diagnostics: list[Diagnostic] = []
+
+    def report(
+        self,
+        path: Optional[str],
+        type: DiagnosticType,
+        location: Location,
+        message: str,
+    ):
+        self.diagnostics.append(
+            Diagnostic(
+                file_path=path,
+                location=location,
+                type=type,
+                message=message,
+            )
+        )
+
+    def for_file(self, path: Optional[str]) -> FileReporter:
+        return FileReporter(self, path)
+
+
+class FileReporter:
+    def __init__(self, base_reporter: Reporter, path: Optional[str]) -> None:
+        self.base_reporter: Reporter = base_reporter
+        self.path: Optional[str] = path
+
+    def for_file(self, path: Optional[str]) -> FileReporter:
+        return FileReporter(self.base_reporter, path)
+
+    def report(self, type: DiagnosticType, location: Location, message: str):
+        self.base_reporter.report(self.path, type, location, message)
+
+    def error(self, location: Location, message: str):
+        self.report(
+            type=DiagnosticType.ERROR,
+            location=location,
+            message=message,
+        )
+
+    def warning(self, location: Location, message: str):
+        self.report(
+            type=DiagnosticType.WARNING,
+            location=location,
+            message=message,
+        )
+
+    def info(self, location: Location, message: str):
+        self.report(
+            type=DiagnosticType.INFO,
+            location=location,
+            message=message,
+        )
--- a/midas/resolver/resolver.py
+++ b/midas/resolver/resolver.py
@@ -13,7 +13,7 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):

    def __init__(self):
        self.locals: dict[p.Expr, int] = {}
-        self.scopes: list[dict[str, bool]] = []
+        self.scopes: list[dict[str, bool]] = [{}]

    def resolve(self, *objects: p.Stmt | p.Expr) -> None:
        """Resolve the given statements or expressions"""
@@ -77,6 +77,12 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
                self.locals[expr] = i
                return

+    def is_defined(self, name: str) -> bool:
+        for scope in self.scopes:
+            if name in scope:
+                return True
+        return False
+
    def resolve_function(self, function: p.Function) -> None:
        """Resolve a function definition

@@ -112,8 +118,13 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
        for target in stmt.targets:
            match target:
                case p.VariableExpr(name=name):
-                    self.resolve_local(target, name)
-                    # TODO: declare if not found
+                    if not self.is_defined(name):
+                        self.declare(name)
+                        self.define(name)
+                    target.accept(self)
+
+                case p.GetExpr():
+                    target.accept(self)
                case _:
                    raise Exception(f"Unsupported assignment to {target}")

@@ -174,10 +185,6 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
        self.resolve(expr.left)
        self.resolve(expr.right)

-    def visit_set_expr(self, expr: p.SetExpr) -> None:
-        self.resolve(expr.value)
-        self.resolve(expr.object)
-
    def visit_cast_expr(self, expr: p.CastExpr) -> None:
        self.resolve(expr.expr)

@@ -185,3 +192,7 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
        self.resolve(expr.test)
        self.resolve(expr.if_true)
        self.resolve(expr.if_false)
+
+    def visit_list_expr(self, expr: p.ListExpr) -> None:
+        for item in expr.items:
+            self.resolve(item)
--- a/midas/checker/types.py
+++ b/midas/checker/types.py
@@ -1,27 +1,36 @@
 from __future__ import annotations

 from dataclasses import dataclass
+from typing import Optional


@dataclass(frozen=True, kw_only=True)
 class BaseType:
    name: str

+    def __str__(self) -> str:
+        return self.name
+

@dataclass(frozen=True, kw_only=True)
 class AliasType:
    name: str
    type: Type

+    def __str__(self) -> str:
+        return self.name
+

@dataclass(frozen=True, kw_only=True)
 class UnknownType:
-    pass
+    def __str__(self) -> str:
+        return "<Unknown>"


@dataclass(frozen=True, kw_only=True)
 class UnitType:
-    pass
+    def __str__(self) -> str:
+        return "None"


@dataclass(frozen=True, kw_only=True)
@@ -32,16 +41,159 @@ class Function:
    kw_args: list[Argument]
    returns: Type

+    def __str__(self) -> str:
+        args: list[str] = []
+        if len(self.pos_args) != 0:
+            args += list(map(str, self.pos_args))
+            if len(self.args) + len(self.kw_args) != 0:
+                args.append("/")
+
+        if len(self.args) != 0:
+            args += list(map(str, self.args))
+
+        if len(self.kw_args) != 0:
+            if len(args) != 0:
+                args.append("*")
+            args += list(map(str, self.kw_args))
+
+        return f"{self.name}({', '.join(args)}) -> {self.returns}"
+
    @dataclass(frozen=True, kw_only=True)
    class Argument:
+        pos: int
        name: str
        type: Type
        required: bool

+        def __str__(self) -> str:
+            opt: str = "" if self.required else "?"
+            return f"{self.name}: {self.type}{opt}"
+

@dataclass(frozen=True, kw_only=True)
 class ComplexType:
    properties: dict[str, Type]

+    def __str__(self) -> str:
+        props: list[str] = [f"{name}: {type}" for name, type in self.properties.items()]
+        return f"{{{', '.join(props)}}}"

-Type = BaseType | AliasType | UnknownType | UnitType | Function | ComplexType
+
+@dataclass(frozen=True, kw_only=True)
+class Operation:
+    signature: CallSignature
+    result: Type
+
+    def __str__(self) -> str:
+        return f"{self.signature} -> {self.result}"
+
+    @dataclass(frozen=True, kw_only=True)
+    class CallSignature:
+        left: Type
+        method: str
+        right: Type
+
+        def __str__(self) -> str:
+            return f"{self.method}({self.left}, {self.right})"
+
+
+@dataclass(frozen=True, kw_only=True)
+class TypeVar:
+    name: str
+    bound: Optional[Type]
+
+    def __str__(self) -> str:
+        if self.bound is not None:
+            return f"{self.name} <: {self.bound}"
+        return self.name
+
+
+@dataclass(frozen=True, kw_only=True)
+class GenericType:
+    name: str
+    params: list[TypeVar]
+    body: Type
+
+    def __str__(self) -> str:
+        return f"{self.name}[{', '.join(map(str, self.params))}]"
+
+
+@dataclass(frozen=True, kw_only=True)
+class AppliedType:
+    name: str
+    args: list[Type]
+    body: Type
+
+    def __str__(self) -> str:
+        return f"{self.name}[{', '.join(map(str, self.args))}]"
+
+
+def substitute_typevars(type: Type, substitutions: dict[str, Type]) -> Type:
+    def sub_argument(arg: Function.Argument):
+        return Function.Argument(
+            pos=arg.pos,
+            name=arg.name,
+            type=substitute_typevars(arg.type, substitutions),
+            required=arg.required,
+        )
+
+    match type:
+        case BaseType(name=name) if name in substitutions:
+            return substitutions[name]
+
+        case AliasType(name=name, type=type2):
+            return AliasType(name=name, type=substitute_typevars(type2, substitutions))
+
+        case Function(
+            name=name,
+            pos_args=pos_args,
+            args=args,
+            kw_args=kw_args,
+            returns=returns,
+        ):
+            return Function(
+                name=name,
+                pos_args=list(map(sub_argument, pos_args)),
+                args=list(map(sub_argument, args)),
+                kw_args=list(map(sub_argument, kw_args)),
+                returns=substitute_typevars(returns, substitutions),
+            )
+
+        case ComplexType(properties=properties):
+            properties2: dict[str, Type] = {
+                name: substitute_typevars(prop, substitutions)
+                for name, prop in properties.items()
+            }
+            return ComplexType(properties=properties2)
+
+        case TypeVar(name=name):
+            if name in substitutions:
+                return substitutions[name]
+            raise ValueError(f"Missing TypeVar substitution for {name}")
+
+        case UnknownType() | UnitType():
+            return type
+
+        case _:
+            raise NotImplementedError(f"Unsupported type {type}")
+
+
+def unfold_type(type: Type) -> Type:
+    match type:
+        case AliasType(type=ref_type):
+            return unfold_type(ref_type)
+        case _:
+            return type
+
+
+Type = (
+    BaseType
+    | AliasType
+    | UnknownType
+    | UnitType
+    | Function
+    | ComplexType
+    | TypeVar
+    | GenericType
+    | AppliedType
+)
--- a/midas/cli/ansi.py
+++ b/midas/cli/ansi.py
@@ -0,0 +1,41 @@
+class Ansi:
+    CTRL = "\x1b["
+    RESET = CTRL + "0m"
+    BOLD = CTRL + "1m"
+    DIM = CTRL + "2m"
+    ITALIC = CTRL + "3m"
+    UNDERLINE = CTRL + "4m"
+
+    BLACK = 0
+    RED = 1
+    GREEN = 2
+    YELLOW = 3
+    BLUE = 4
+    MAGENTA = 5
+    CYAN = 6
+    WHITE = 7
+
+    BRIGHT_BLACK = 60
+    BRIGHT_RED = 61
+    BRIGHT_GREEN = 62
+    BRIGHT_YELLOW = 63
+    BRIGHT_BLUE = 64
+    BRIGHT_MAGENTA = 65
+    BRIGHT_CYAN = 66
+    BRIGHT_WHITE = 67
+
+    @classmethod
+    def FG(cls, col: int) -> str:
+        return f"{cls.CTRL}{30 + col}m"
+
+    @classmethod
+    def BG(cls, col: int) -> str:
+        return f"{cls.CTRL}{40 + col}m"
+
+    @classmethod
+    def FG_RGB(cls, r: int, g: int, b: int) -> str:
+        return f"{cls.CTRL}38;2;{r};{g};{b}m"
+
+    @classmethod
+    def BG_RGB(cls, r: int, g: int, b: int) -> str:
+        return f"{cls.CTRL}48;2;{r};{g};{b}m"
--- a/midas/cli/highlighter.py
+++ b/midas/cli/highlighter.py
@@ -210,12 +210,14 @@ class PythonHighlighter(

    def visit_logical_expr(self, expr: p.LogicalExpr) -> None: ...

-    def visit_set_expr(self, expr: p.SetExpr) -> None: ...
-
    def visit_cast_expr(self, expr: p.CastExpr) -> None: ...

    def visit_ternary_expr(self, expr: p.TernaryExpr) -> None: ...

+    def visit_list_expr(self, expr: p.ListExpr) -> None:
+        for item in expr.items:
+            item.accept(self)
+

 class MidasHighlighter(
    Highlighter, m.Stmt.Visitor[None], m.Expr.Visitor[None], m.Type.Visitor[None]
@@ -286,8 +288,8 @@ class MidasHighlighter(
    def visit_generic_type(self, type: m.GenericType) -> None:
        self.wrap(type, "generic-type")
        type.type.accept(self)
-        for param in type.params:
-            param.accept(self)
+        for arg in type.args:
+            arg.accept(self)

    def visit_constraint_type(self, type: m.ConstraintType) -> None:
        self.wrap(type, "constraint-type")
@@ -299,6 +301,12 @@ class MidasHighlighter(
        for prop in type.properties:
            prop.accept(self)

+    def visit_function_type(self, type: m.FunctionType) -> None:
+        self.wrap(type, "function")
+        for arg in type.pos_args + type.kw_args:
+            arg.type.accept(self)
+        type.returns.accept(self)
+

 class DiagnosticsHighlighter(Highlighter):
    EXTRA_CSS_PATH: Optional[Path] = Path(__file__).parent / "hl_diagnostic.css"
--- a/midas/cli/main.py
+++ b/midas/cli/main.py
@@ -8,10 +8,12 @@ import click

 import midas.ast.midas as m
 import midas.ast.python as p
+from midas.ast.location import Location
 from midas.ast.printer import MidasAstPrinter, MidasPrinter, PythonAstPrinter
-from midas.checker.checker import Checker
-from midas.checker.diagnostic import Diagnostic
+from midas.checker.checker import TypeChecker
+from midas.checker.diagnostic import Diagnostic, DiagnosticType
 from midas.checker.types import Type
+from midas.cli.ansi import Ansi
 from midas.cli.highlighter import (
    DiagnosticsHighlighter,
    Highlighter,
@@ -23,7 +25,6 @@ from midas.lexer.midas import MidasLexer
 from midas.lexer.token import Token, TokenType
 from midas.parser.midas import MidasParser
 from midas.parser.python import PythonParser
-from midas.resolver.resolver import Resolver
 from midas.utils import UniversalJSONDumper


@@ -32,38 +33,92 @@ def midas():
    pass


+def print_diagnostic(lines: list[str], diagnostic: Diagnostic, indent: int = 4):
+    """Pretty-print a diagnostic, showing some context if possible
+
+    If the diagnostic concerns a specific part of one line, the line is shown
+    with the affected part highlighted. The message is clearly printed under the
+    line with an underline further indicating the target expression.
+
+    If multiple lines are concerned, no context is shown, only the
+    diagnostic type, location and message
+
+    Args:
+        lines (list[str]): source code lines
+        diagnostic (Diagnostic): the diagnostic to print
+        indent (int, optional): the number of spaces added before the target line to indent if from the location header. Defaults to 4.
+    """
+
+    loc: Location = diagnostic.location
+    if loc.lineno != loc.end_lineno:
+        print(diagnostic)
+        return
+
+    start_offset: int = loc.col_offset
+    end_offset: int = loc.end_col_offset or (start_offset + 1)
+
+    line: str = lines[loc.lineno - 1]
+    before: str = line[:start_offset]
+    after: str = line[end_offset:]
+
+    color: int = {
+        DiagnosticType.ERROR: Ansi.RED,
+        DiagnosticType.WARNING: Ansi.YELLOW,
+        DiagnosticType.INFO: Ansi.CYAN,
+    }.get(diagnostic.type, Ansi.WHITE)
+
+    subject: str = Ansi.FG(color) + line[start_offset:end_offset] + Ansi.RESET
+    cursor: str = (
+        " " * start_offset
+        + Ansi.FG(color)
+        + "~" * (end_offset - start_offset)
+        + "> "
+        + diagnostic.message
+        + Ansi.RESET
+    )
+
+    indent_str: str = " " * indent
+    print(diagnostic.location_str + ":")
+    print(indent_str + before + subject + after)
+    print(indent_str + cursor)
+    print()
+
+
@midas.command()
@click.option("-l", "--highlight", type=click.File("w"))
@click.option("-t", "--types", type=click.File("r"), multiple=True)
+@click.option("-v", "--verbose", is_flag=True)
@click.argument("file", type=click.File("r"))
-def compile(highlight: Optional[TextIO], file: TextIO, types: tuple[TextIO]):
-    logging.basicConfig(level=logging.DEBUG)
+def compile(
+    highlight: Optional[TextIO],
+    types: tuple[TextIO],
+    verbose: bool,
+    file: TextIO,
+):
+    logging.basicConfig(level=logging.DEBUG if verbose else logging.WARN)
    source: str = file.read()
-    tree: ast.Module = ast.parse(source, filename=file.name)
-    parser = PythonParser()
-    stmts: list[p.Stmt] = parser.parse_module(tree)
-    resolver = Resolver()
-    resolver.resolve(*stmts)
-    types_paths: list[Path] = [Path(t.name).resolve() for t in types]
-    checker = Checker(
-        resolver.locals,
-        source_path=Path(file.name).resolve(),
-        types_paths=types_paths,
-    )
-    diagnostics: list[Diagnostic] = checker.check(stmts)
-    for diagnostic in diagnostics:
-        print(diagnostic)

-    print(
-        json.dumps(
-            UniversalJSONDumper.dump(
-                checker.global_env,
-                [("Environment", "_children")],
-                lambda obj: isinstance(obj, get_args(Type)),
-            ),
-            indent=4,
+    checker = TypeChecker()
+    for path in types:
+        checker.import_midas(Path(path.name).resolve())
+
+    checker.type_check_source(source, str(Path(file.name).resolve()))
+    diagnostics: list[Diagnostic] = checker.diagnostics
+    lines: list[str] = source.split("\n")
+    for diagnostic in diagnostics:
+        print_diagnostic(lines, diagnostic)
+
+    if verbose:
+        print(
+            json.dumps(
+                UniversalJSONDumper.dump(
+                    checker.python_typer.global_env,
+                    [("Environment", "_children")],
+                    lambda obj: isinstance(obj, get_args(Type)),
+                ),
+                indent=4,
+            )
        )
-    )
    if highlight is not None:
        highlighter = DiagnosticsHighlighter(source)
        highlighter.highlight(diagnostics)
--- a/midas/lexer/midas.py
+++ b/midas/lexer/midas.py
@@ -50,12 +50,14 @@ class MidasLexer(Lexer):
            #     self.add_token(TokenType.PLUS)
            case "-":
                self.add_token(TokenType.MINUS)
-            # case "*":
-            #     self.add_token(TokenType.STAR)
+            case "*":
+                self.add_token(TokenType.STAR)
            case "/" if self.match("/"):
                self.scan_comment()
            case "/" if self.match("*"):
                self.scan_comment_multiline()
+            case "/":
+                self.add_token(TokenType.SLASH)
            case "\n":
                self.add_token(TokenType.NEWLINE)
            case " " | "\r" | "\t":
--- a/midas/lexer/token.py
+++ b/midas/lexer/token.py
@@ -27,8 +27,8 @@ class TokenType(Enum):
    # Operators
    # PLUS = auto()
    MINUS = auto()
-    # STAR = auto()
-    # SLASH = auto()
+    STAR = auto()
+    SLASH = auto()
    GREATER = auto()
    GREATER_EQUAL = auto()
    LESS = auto()
--- a/midas/parser/midas.py
+++ b/midas/parser/midas.py
@@ -7,6 +7,7 @@ from midas.ast.midas import (
    ConstraintType,
    Expr,
    ExtendStmt,
+    FunctionType,
    GenericType,
    GetExpr,
    GroupingExpr,
@@ -18,12 +19,13 @@ from midas.ast.midas import (
    PropertyStmt,
    Stmt,
    Type,
+    TypeParam,
    TypeStmt,
    UnaryExpr,
    VariableExpr,
    WildcardExpr,
 )
-from midas.lexer.token import Token, TokenType
+from midas.lexer.token import KEYWORDS, Token, TokenType
 from midas.parser.base import Parser
 from midas.parser.errors import ParsingError

@@ -107,10 +109,8 @@ class MidasParser(Parser):
            TypeStmt: the parsed type declaration statement
        """
        keyword: Token = self.previous()
-        name: Token = self.consume(TokenType.IDENTIFIER, "Expected type name")
-        params: list[TypeStmt.Param] = []
-        if self.check(TokenType.LEFT_BRACKET):
-            params = self.type_stmt_params()
+        name: Token = self.consume_identifier("Expected type name")
+        params: list[TypeParam] = self.type_params()

        self.consume(TokenType.EQUAL, "Expected '=' before type definition")

@@ -123,24 +123,27 @@ class MidasParser(Parser):
            type=type,
        )

-    def type_stmt_params(self) -> list[TypeStmt.Param]:
-        """Parse a generic template expression
+    def type_params(self) -> list[TypeParam]:
+        """Parse a list of type parameters

-        A template is written `[TypeExpr]`
+        Type parameters are a comma-separated list of type variables wrapped in brackets.
+        Each type variable is either a simple variable, or a bounded variable written `S <: T`

        Returns:
-            TemplateExpr: the parsed template expression
+            list[TypeParam]: the list of type parameters, if any, or an empty list
        """
-        self.consume(TokenType.LEFT_BRACKET, "Missing '[' before template expression")
-        params: list[TypeStmt.Param] = []
+        if not self.match(TokenType.LEFT_BRACKET):
+            return []
+
+        params: list[TypeParam] = []
        while not self.is_at_end() and not self.check(TokenType.RIGHT_BRACKET):
-            name: Token = self.consume(TokenType.IDENTIFIER, "Expected type variable")
+            name: Token = self.consume_identifier("Expected type variable")
            bound: Optional[Type] = None
            if self.match(TokenType.LESS):
                self.consume(TokenType.COLON, "Expected ':' after '<'")
                bound = self.type_expr()
            params.append(
-                TypeStmt.Param(
+                TypeParam(
                    location=name.location_to(self.previous()),
                    name=name,
                    bound=bound,
@@ -148,7 +151,7 @@ class MidasParser(Parser):
            )
            if not self.match(TokenType.COMMA):
                break
-        self.consume(TokenType.RIGHT_BRACKET, "Missing ']' after template expression")
+        self.consume(TokenType.RIGHT_BRACKET, "Missing ']' after type parameters")
        return params

    def type_expr(self) -> Type:
@@ -187,26 +190,26 @@ class MidasParser(Parser):
    def generic_type(self) -> Type:
        type: Type = self.named_type()
        if self.check(TokenType.LEFT_BRACKET):
-            params: list[Type] = self.type_params()
+            args: list[Type] = self.type_args()
            return GenericType(
                location=Location.span(type.location, self.previous().get_location()),
                type=type,
-                params=params,
+                args=args,
            )
        return type

-    def type_params(self) -> list[Type]:
-        params: list[Type] = []
-        self.consume(TokenType.LEFT_BRACKET, "Missing '[' before generic parameters")
+    def type_args(self) -> list[Type]:
+        args: list[Type] = []
+        self.consume(TokenType.LEFT_BRACKET, "Missing '[' before generic arguments")
        while not self.is_at_end() and not self.check(TokenType.RIGHT_BRACKET):
-            params.append(self.type_expr())
+            args.append(self.type_expr())
            if not self.match(TokenType.COMMA):
                break
-        self.consume(TokenType.RIGHT_BRACKET, "Missing ']' after generic parameters")
-        return params
+        self.consume(TokenType.RIGHT_BRACKET, "Missing ']' after generic arguments")
+        return args

    def named_type(self) -> Type:
-        name: Token = self.consume(TokenType.IDENTIFIER, "Expected type name")
+        name: Token = self.consume_identifier("Expected type name")
        return NamedType(
            location=name.get_location(),
            name=name,
@@ -322,9 +325,7 @@ class MidasParser(Parser):
        """
        expr: Expr = self.primary()
        while self.match(TokenType.DOT):
-            name: Token = self.consume(
-                TokenType.IDENTIFIER, "Expected property name after '.'"
-            )
+            name: Token = self.consume_identifier("Expected property name after '.'")
            location: Location = Location.span(expr.location, name.get_location())
            expr = GetExpr(location=location, expr=expr, name=name)
        return expr
@@ -348,7 +349,7 @@ class MidasParser(Parser):
        if self.match(TokenType.NUMBER):
            return LiteralExpr(location=token.get_location(), value=token.value)

-        if self.match(TokenType.IDENTIFIER):
+        if self.match_identifier():
            return VariableExpr(location=token.get_location(), name=token)

        if self.match(TokenType.UNDERSCORE):
@@ -361,6 +362,20 @@ class MidasParser(Parser):

        raise self.error(self.peek(), "Expected expression")

+    def consume_identifier(self, message: str = "Expected identifier") -> Token:
+        if not self.match_identifier():
+            raise self.error(self.peek(), message)
+        return self.previous()
+
+    def match_identifier(self) -> bool:
+        return self.match(TokenType.IDENTIFIER, *KEYWORDS.values())
+
+    def check_identifier(self) -> bool:
+        for tt in [TokenType.IDENTIFIER, *KEYWORDS.values()]:
+            if self.check(tt):
+                return True
+        return False
+
    def property_stmt(self) -> PropertyStmt:
        """Parse a property statement

@@ -369,7 +384,7 @@ class MidasParser(Parser):
        Returns:
            PropertyStmt: the parsed property statement
        """
-        name: Token = self.consume(TokenType.IDENTIFIER, "Expected property name")
+        name: Token = self.consume_identifier("Expected property name")
        self.consume(TokenType.COLON, "Expected ':' after property name")
        type: Type = self.type_expr()
        return PropertyStmt(
@@ -381,12 +396,14 @@ class MidasParser(Parser):
    def extend_declaration(self) -> ExtendStmt:
        """Parse an extension definition

-        An extension is written `extend Type { operations }`
+        An extension is written `extend Type { operations }` or `extend[S <: T, U] Type { operations }`

        Returns:
            ExtendStmt: the parsed extension statement
        """
        keyword: Token = self.previous()
+        params: list[TypeParam] = self.type_params()
+
        type: Type = self.type_expr()
        self.consume(TokenType.LEFT_BRACE, "Expected '{' to start extend body")
        operations: list[OpStmt] = []
@@ -394,7 +411,12 @@ class MidasParser(Parser):
            operations.append(self.op_declaration())
        self.consume(TokenType.RIGHT_BRACE, "Unclosed extend body")
        location: Location = keyword.location_to(self.previous())
-        return ExtendStmt(location=location, type=type, operations=operations)
+        return ExtendStmt(
+            location=location,
+            params=params,
+            type=type,
+            operations=operations,
+        )

    def op_declaration(self) -> OpStmt:
        """Parse an operation definition
@@ -430,9 +452,9 @@ class MidasParser(Parser):
            PredicateStmt: the parsed predicate declaration statement
        """
        keyword: Token = self.previous()
-        name: Token = self.consume(TokenType.IDENTIFIER, "Expected predicate name")
+        name: Token = self.consume_identifier("Expected predicate name")
        self.consume(TokenType.LEFT_PAREN, "Expected '(' before predicate subject")
-        subject: Token = self.consume(TokenType.IDENTIFIER, "Expected subject name")
+        subject: Token = self.consume_identifier("Expected subject name")
        self.consume(TokenType.COLON, "Expected ':' after subject name")
        type: Type = self.type_expr()
        self.consume(TokenType.RIGHT_PAREN, "Expected ')' after predicate subject")
@@ -445,3 +467,48 @@ class MidasParser(Parser):
            type=type,
            condition=condition,
        )
+
+    def function(self) -> FunctionType:
+        l_paren: Token = self.consume(
+            TokenType.LEFT_PAREN, "Expected '(' before function parameters"
+        )
+        pos_args: list[FunctionType.Argument] = []
+        kw_args: list[FunctionType.Argument] = []
+
+        positional: bool = True
+        while not self.is_at_end() and not self.check(TokenType.RIGHT_PAREN):
+            if positional and (
+                self.match(TokenType.STAR) or self.match(TokenType.SLASH)
+            ):
+                positional = False
+            else:
+                name: Optional[Token] = None
+                if self.check_identifier() and self.check_next(TokenType.COLON):
+                    name = self.advance()
+                    self.advance()
+                type: Type = self.type_expr()
+                required: bool = self.match(TokenType.QMARK)
+                arg = FunctionType.Argument(
+                    location=None,
+                    name=name,
+                    type=type,
+                    required=required,
+                )
+                if positional:
+                    pos_args.append(arg)
+                else:
+                    kw_args.append(arg)
+
+            if not self.match(TokenType.COMMA):
+                break
+        self.consume(TokenType.RIGHT_PAREN, "Expected ')' after function parameters")
+
+        self.consume(TokenType.ARROW, "Expected '->' before result type")
+        result: Type = self.type_expr()
+
+        return FunctionType(
+            location=l_paren.location_to(self.previous()),
+            pos_args=pos_args,
+            kw_args=kw_args,
+            returns=result,
+        )
--- a/midas/parser/python.py
+++ b/midas/parser/python.py
@@ -17,6 +17,7 @@ from midas.ast.python import (
    Function,
    GetExpr,
    IfStmt,
+    ListExpr,
    LiteralExpr,
    LogicalExpr,
    MidasType,
@@ -416,6 +417,12 @@ class PythonParser:
            case ast.Name(id=name):
                return VariableExpr(location=location, name=name)

+            case ast.List(elts=items):
+                return ListExpr(
+                    location=location,
+                    items=[self.parse_expr(item) for item in items],
+                )
+
            case _:
                raise UnsupportedSyntaxError(node)

--- a/midas/resolver/builtin.py
+++ b/midas/resolver/builtin.py
@@ -1,72 +0,0 @@
-from __future__ import annotations
-
-from typing import TYPE_CHECKING
-
-from midas.checker.types import BaseType, Type, UnitType
-
-if TYPE_CHECKING:
-    from midas.resolver.midas import MidasResolver
-
-
-def op(ctx: MidasResolver, t1: Type, operator: str, t2: Type, t3: Type):
-    ctx.define_operation(
-        left=t1,
-        operator=operator,
-        right=t2,
-        result=t3,
-    )
-
-
-def basic_op(ctx: MidasResolver, type: Type, op: str):
-    ctx.define_operation(
-        left=type,
-        operator=op,
-        right=type,
-        result=type,
-    )
-
-
-def define_builtins(ctx: MidasResolver):
-    """Define builtin types and operations"""
-    unit = ctx.define_type("None", UnitType())
-    bool = ctx.define_type("bool", BaseType(name="bool"))
-    int = ctx.define_type("int", BaseType(name="int"))
-    float = ctx.define_type("float", BaseType(name="float"))
-    str = ctx.define_type("str", BaseType(name="str"))
-
-    basic_op(ctx, int, "__add__")  # int + int = int
-    basic_op(ctx, int, "__sub__")  # int - int = int
-    basic_op(ctx, int, "__mul__")  # int * int = int
-    basic_op(ctx, int, "__pow__")  # int ** int = int
-    basic_op(ctx, int, "__mod__")  # int % int = int
-    basic_op(ctx, int, "__and__")  # int & int = int
-    basic_op(ctx, int, "__or__")  # int | int = int
-    basic_op(ctx, int, "__xor__")  # int ^ int = int
-    op(ctx, int, "__lt__", int, bool)  # int < int = bool
-    op(ctx, int, "__gt__", int, bool)  # int > int = bool
-    op(ctx, int, "__le__", int, bool)  # int <= int = bool
-    op(ctx, int, "__ge__", int, bool)  # int >= int = bool
-    op(ctx, int, "__eq__", int, bool)  # int == int = bool
-    basic_op(ctx, float, "__add__")  # float + float = float
-    basic_op(ctx, float, "__sub__")  # float - float = float
-    basic_op(ctx, float, "__mul__")  # float * float = float
-    basic_op(ctx, float, "__truediv__")  # float / float = float
-    op(ctx, float, "__lt__", float, bool)  # float < float = bool
-    op(ctx, float, "__gt__", float, bool)  # float > float = bool
-    op(ctx, float, "__le__", float, bool)  # float <= float = bool
-    op(ctx, float, "__ge__", float, bool)  # float >= float = bool
-    op(ctx, float, "__eq__", float, bool)  # float == float = bool
-    basic_op(ctx, str, "__add__")  # str + str = str
-    op(ctx, str, "__eq__", str, bool)  # str == str = bool
-
-    op(ctx, int, "__lt__", float, bool)  # int < float = bool
-    op(ctx, int, "__gt__", float, bool)  # int > float = bool
-    op(ctx, int, "__le__", float, bool)  # int <= float = bool
-    op(ctx, int, "__ge__", float, bool)  # int >= float = bool
-    op(ctx, int, "__eq__", float, bool)  # int == float = bool
-
-    op(ctx, float, "__lt__", int, bool)  # float < int = bool
-    op(ctx, float, "__gt__", int, bool)  # float > int = bool
-    op(ctx, float, "__le__", int, bool)  # float <= int = bool
-    op(ctx, float, "__ge__", int, bool)  # float >= int = bool
-    op(ctx, float, "__eq__", int, bool)  # float == int = bool
--- a/midas/resolver/midas.py
+++ b/midas/resolver/midas.py
@@ -1,163 +0,0 @@
-from typing import Optional
-
-import midas.ast.midas as m
-from midas.checker.types import (
-    AliasType,
-    Type,
-    UnknownType,
-)
-from midas.resolver.builtin import define_builtins
-
-
-class MidasResolver(m.Stmt.Visitor[None], m.Expr.Visitor[None], m.Type.Visitor[Type]):
-    """A resolver which evaluates Midas type definitions and build a registry"""
-
-    def __init__(self) -> None:
-        self._types: dict[str, Type] = {}
-        self._operations: dict[tuple[Type, str, Type], Type] = {}
-
-        define_builtins(self)
-
-    def get_type(self, name: str) -> Type:
-        """Get a type from its name
-
-        Args:
-            name (str): the name of the type
-
-        Raises:
-            NameError: if the type is not defined
-
-        Returns:
-            Type: the type
-        """
-        type: Optional[Type] = self._types.get(name)
-        if type is None:
-            raise NameError(f"Undefined type {name}")
-        return type
-
-    def get_operation_result(
-        self, left: Type, operator: str, right: Type
-    ) -> Optional[Type]:
-        """Get the resulting type of an operation
-
-        Args:
-            left (Type): the type of the left operand
-            operator (str): the operation name
-            right (Type): the type of the right operand
-
-        Returns:
-            Optional[Type]: the result type, or None if no matching operation was found
-        """
-        operation: tuple[Type, str, Type] = (left, operator, right)
-        result: Optional[Type] = self._operations.get(operation)
-        return result
-
-    def define_type(self, name: str, type: Type) -> Type:
-        """Define a type in the registry
-
-        Args:
-            name (str): the name of the type
-            type (Type): the type to define
-
-        Raises:
-            ValueError: if a type is already defined with that name
-
-        Returns:
-            Type: the defined type
-        """
-        if name in self._types:
-            raise ValueError(f"Type {name} already defined")
-        self._types[name] = type
-        return type
-
-    def define_operation(self, left: Type, operator: str, right: Type, result: Type):
-        """Define an operation in the registry
-
-        Args:
-            left (Type): the type of the left operand
-            operator (str): the operation name
-            right (Type): the type of the right operand
-            result (Type): the result type
-
-        Raises:
-            ValueError: if an operation is already defined with these operands and name
-        """
-        operation: tuple[Type, str, Type] = (left, operator, right)
-        if operation in self._operations:
-            raise ValueError(
-                f"Operation {operator} already defined between {left} and {right}"
-            )
-        self._operations[operation] = result
-
-    def resolve(self, stmts: list[m.Stmt]):
-        """Process a sequence of statements
-
-        Args:
-            stmts (list[m.Stmt]): the statements
-        """
-        for stmt in stmts:
-            stmt.accept(self)
-
-    def visit_type_stmt(self, stmt: m.TypeStmt) -> None:
-        type: Type = stmt.type.accept(self)
-        for param in stmt.params:
-            if param.bound is not None:
-                param.bound.accept(self)
-        name: str = stmt.name.lexeme
-        self.define_type(name, AliasType(name=name, type=type))
-
-    def visit_property_stmt(self, stmt: m.PropertyStmt) -> None: ...
-
-    def visit_extend_stmt(self, stmt: m.ExtendStmt) -> None:
-        base: Type = stmt.type.accept(self)
-        for op in stmt.operations:
-            right: Type = op.operand.accept(self)
-            result: Type = op.result.accept(self)
-            self.define_operation(
-                left=base,
-                operator=op.name.lexeme,
-                right=right,
-                result=result,
-            )
-
-    def visit_op_stmt(self, stmt: m.OpStmt) -> None: ...
-
-    def visit_predicate_stmt(self, stmt: m.PredicateStmt) -> None: ...
-
-    def visit_logical_expr(self, expr: m.LogicalExpr) -> None: ...
-
-    def visit_binary_expr(self, expr: m.BinaryExpr) -> None: ...
-
-    def visit_unary_expr(self, expr: m.UnaryExpr) -> None: ...
-
-    def visit_get_expr(self, expr: m.GetExpr) -> None: ...
-
-    def visit_variable_expr(self, expr: m.VariableExpr) -> None: ...
-
-    def visit_grouping_expr(self, expr: m.GroupingExpr) -> None:
-        return expr.expr.accept(self)
-
-    def visit_literal_expr(self, expr: m.LiteralExpr) -> None: ...
-
-    def visit_wildcard_expr(self, expr: m.WildcardExpr) -> None: ...
-
-    def visit_named_type(self, type: m.NamedType) -> Type:
-        return self.get_type(type.name.lexeme)
-
-    def visit_generic_type(self, type: m.GenericType) -> Type:
-        type_: Type = type.type.accept(self)
-        params: list[Type] = [param.accept(self) for param in type.params]
-        # TODO
-        return UnknownType()
-
-    def visit_constraint_type(self, type: m.ConstraintType) -> Type:
-        type_: Type = type.type.accept(self)
-        type.constraint.accept(self)
-        # TODO
-        return UnknownType()
-
-    def visit_complex_type(self, type: m.ComplexType) -> Type:
-        for prop in type.properties:
-            prop.accept(self)
-        # TODO
-        return UnknownType()
--- a/tests/base.py
+++ b/tests/base.py
@@ -29,7 +29,7 @@ class Tester(ABC):
    def _list_tests(self) -> list[Path]: ...

    def run_all_tests(self) -> bool:
-        paths: list[Path] = self._list_tests()
+        paths: list[Path] = sorted(self._list_tests())
        return self.run_tests(paths)

    def run_tests(self, tests: list[Path]) -> bool:
@@ -40,7 +40,7 @@ class Tester(ABC):

        print(rule)
        for i, test in enumerate(tests):
-            print(f"Case {i+1}/{n}: {test.relative_to(self.CASES_DIR)}")
+            print(f"Case {i+1}/{n}: {test.resolve().relative_to(self.CASES_DIR)}")
            success: bool = self._run_test(test)
            if success:
                successes += 1
@@ -78,7 +78,7 @@ class Tester(ABC):
    def _exec_case(self, path: Path) -> CaseResult: ...

    def update_all_tests(self):
-        paths: list[Path] = self._list_tests()
+        paths: list[Path] = sorted(self._list_tests())
        return self.update_tests(paths)

    def update_tests(self, tests: list[Path]):
--- a/tests/cases/checker/01_simple_types.py.ref.json
+++ b/tests/cases/checker/01_simple_types.py.ref.json
@@ -1,3 +1,4 @@
 {
-  "diagnostics": []
+  "diagnostics": [],
+  "judgments": []
 }
--- a/tests/cases/checker/02_simple_operations.py.ref.json
+++ b/tests/cases/checker/02_simple_operations.py.ref.json
@@ -12,35 +12,168 @@
          13
        ]
      },
-      "message": "Cannot assign BaseType(name='str') to c of type BaseType(name='int')"
+      "message": "Cannot assign str to variable 'c' of type int"
+    }
+  ],
+  "judgments": [
+    {
+      "location": {
+        "from": "L1:9",
+        "to": "L1:10"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": 3
+      },
+      "type": {
+        "name": "int"
+      }
    },
    {
-      "type": "Error",
      "location": {
-        "start": [
-          9,
-          4
-        ],
-        "end": [
-          9,
-          9
-        ]
+        "from": "L2:9",
+        "to": "L2:10"
      },
-      "message": "Undefined operation __add__ between BaseType(name='bool') and BaseType(name='bool')"
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": 4
+      },
+      "type": {
+        "name": "int"
+      }
    },
    {
-      "type": "Error",
      "location": {
-        "start": [
-          11,
-          0
-        ],
-        "end": [
-          11,
-          12
-        ]
+        "from": "L4:4",
+        "to": "L4:5"
      },
-      "message": "Cannot assign BaseType(name='int') to f of type BaseType(name='float')"
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L4:8",
+        "to": "L4:9"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L4:4",
+        "to": "L4:9"
+      },
+      "expr": {
+        "_type": "BinaryExpr",
+        "left": {
+          "_type": "VariableExpr",
+          "name": "a"
+        },
+        "operator": "+",
+        "right": {
+          "_type": "VariableExpr",
+          "name": "b"
+        }
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L6:4",
+        "to": "L6:13"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": "invalid"
+      },
+      "type": {
+        "name": "str"
+      }
+    },
+    {
+      "location": {
+        "from": "L8:4",
+        "to": "L8:8"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": true
+      },
+      "type": {
+        "name": "bool"
+      }
+    },
+    {
+      "location": {
+        "from": "L9:4",
+        "to": "L9:5"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "d"
+      },
+      "type": {
+        "name": "bool"
+      }
+    },
+    {
+      "location": {
+        "from": "L9:8",
+        "to": "L9:9"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "d"
+      },
+      "type": {
+        "name": "bool"
+      }
+    },
+    {
+      "location": {
+        "from": "L9:4",
+        "to": "L9:9"
+      },
+      "expr": {
+        "_type": "BinaryExpr",
+        "left": {
+          "_type": "VariableExpr",
+          "name": "d"
+        },
+        "operator": "+",
+        "right": {
+          "_type": "VariableExpr",
+          "name": "d"
+        }
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L11:11",
+        "to": "L11:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
    }
  ]
 }
--- a/tests/cases/checker/03_functions.py.ref.json
+++ b/tests/cases/checker/03_functions.py.ref.json
--- a/tests/cases/checker/04_custom_types.py.ref.json
+++ b/tests/cases/checker/04_custom_types.py.ref.json
@@ -1,3 +1,109 @@
 {
-  "diagnostics": []
+  "diagnostics": [],
+  "judgments": [
+    {
+      "location": {
+        "from": "L4:18",
+        "to": "L4:37"
+      },
+      "expr": {
+        "_type": "CastExpr",
+        "type": {
+          "_type": "BaseType",
+          "base": "Meter",
+          "param": null
+        },
+        "expr": {
+          "_type": "LiteralExpr",
+          "value": 123.45
+        }
+      },
+      "type": {
+        "name": "Meter",
+        "type": {
+          "name": "float"
+        }
+      }
+    },
+    {
+      "location": {
+        "from": "L5:15",
+        "to": "L5:32"
+      },
+      "expr": {
+        "_type": "CastExpr",
+        "type": {
+          "_type": "BaseType",
+          "base": "Second",
+          "param": null
+        },
+        "expr": {
+          "_type": "LiteralExpr",
+          "value": 6.7
+        }
+      },
+      "type": {
+        "name": "Second",
+        "type": {
+          "name": "float"
+        }
+      }
+    },
+    {
+      "location": {
+        "from": "L6:8",
+        "to": "L6:16"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "distance"
+      },
+      "type": {
+        "name": "Meter",
+        "type": {
+          "name": "float"
+        }
+      }
+    },
+    {
+      "location": {
+        "from": "L6:19",
+        "to": "L6:23"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "time"
+      },
+      "type": {
+        "name": "Second",
+        "type": {
+          "name": "float"
+        }
+      }
+    },
+    {
+      "location": {
+        "from": "L6:8",
+        "to": "L6:23"
+      },
+      "expr": {
+        "_type": "BinaryExpr",
+        "left": {
+          "_type": "VariableExpr",
+          "name": "distance"
+        },
+        "operator": "/",
+        "right": {
+          "_type": "VariableExpr",
+          "name": "time"
+        }
+      },
+      "type": {
+        "name": "MeterPerSecond",
+        "type": {
+          "name": "float"
+        }
+      }
+    }
+  ]
 }
--- a/tests/cases/checker/05_control_flow.py.ref.json
+++ b/tests/cases/checker/05_control_flow.py.ref.json
@@ -42,5 +42,215 @@
      },
      "message": "Mixed return types: [BaseType(name='int'), BaseType(name='str')]"
    }
+  ],
+  "judgments": [
+    {
+      "location": {
+        "from": "L2:11",
+        "to": "L2:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L2:15",
+        "to": "L2:16"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L5:7",
+        "to": "L5:8"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L5:11",
+        "to": "L5:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L6:15",
+        "to": "L6:16"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L6:19",
+        "to": "L6:20"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L8:15",
+        "to": "L8:16"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L8:19",
+        "to": "L8:20"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L15:7",
+        "to": "L15:8"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L15:11",
+        "to": "L15:13"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": 10
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L16:15",
+        "to": "L16:16"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L16:19",
+        "to": "L16:21"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": 10
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L22:7",
+        "to": "L22:8"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L22:11",
+        "to": "L22:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L23:15",
+        "to": "L23:16"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L23:19",
+        "to": "L23:20"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "int"
+      }
+    }
  ]
 }
--- a/tests/cases/checker/06_subtyping.py
+++ b/tests/cases/checker/06_subtyping.py
@@ -0,0 +1,12 @@
+v1: int = 3
+v2: float = 4
+
+
+def maximum(a: float, b: float):
+    if b > a:
+        return b
+    return a
+
+
+v3 = maximum(v1, v2)
+v3 = v1 + v2
--- a/tests/cases/checker/06_subtyping.py.ref.json
+++ b/tests/cases/checker/06_subtyping.py.ref.json
@@ -0,0 +1,193 @@
+{
+  "diagnostics": [],
+  "judgments": [
+    {
+      "location": {
+        "from": "L1:10",
+        "to": "L1:11"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": 3
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L2:12",
+        "to": "L2:13"
+      },
+      "expr": {
+        "_type": "LiteralExpr",
+        "value": 4
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L6:7",
+        "to": "L6:8"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "b"
+      },
+      "type": {
+        "name": "float"
+      }
+    },
+    {
+      "location": {
+        "from": "L6:11",
+        "to": "L6:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "a"
+      },
+      "type": {
+        "name": "float"
+      }
+    },
+    {
+      "location": {
+        "from": "L11:5",
+        "to": "L11:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "maximum"
+      },
+      "type": {
+        "name": "maximum",
+        "pos_args": [],
+        "args": [
+          {
+            "pos": 0,
+            "name": "a",
+            "type": {
+              "name": "float"
+            },
+            "required": true
+          },
+          {
+            "pos": 1,
+            "name": "b",
+            "type": {
+              "name": "float"
+            },
+            "required": true
+          }
+        ],
+        "kw_args": [],
+        "returns": {
+          "name": "float"
+        }
+      }
+    },
+    {
+      "location": {
+        "from": "L11:13",
+        "to": "L11:15"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "v1"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L11:17",
+        "to": "L11:19"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "v2"
+      },
+      "type": {
+        "name": "float"
+      }
+    },
+    {
+      "location": {
+        "from": "L11:5",
+        "to": "L11:20"
+      },
+      "expr": {
+        "_type": "CallExpr",
+        "callee": {
+          "_type": "VariableExpr",
+          "name": "maximum"
+        },
+        "arguments": [
+          {
+            "_type": "VariableExpr",
+            "name": "v1"
+          },
+          {
+            "_type": "VariableExpr",
+            "name": "v2"
+          }
+        ],
+        "keywords": {}
+      },
+      "type": {
+        "name": "float"
+      }
+    },
+    {
+      "location": {
+        "from": "L12:5",
+        "to": "L12:7"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "v1"
+      },
+      "type": {
+        "name": "int"
+      }
+    },
+    {
+      "location": {
+        "from": "L12:10",
+        "to": "L12:12"
+      },
+      "expr": {
+        "_type": "VariableExpr",
+        "name": "v2"
+      },
+      "type": {
+        "name": "float"
+      }
+    },
+    {
+      "location": {
+        "from": "L12:5",
+        "to": "L12:12"
+      },
+      "expr": {
+        "_type": "BinaryExpr",
+        "left": {
+          "_type": "VariableExpr",
+          "name": "v1"
+        },
+        "operator": "+",
+        "right": {
+          "_type": "VariableExpr",
+          "name": "v2"
+        }
+      },
+      "type": {
+        "name": "float"
+      }
+    }
+  ]
+}
--- a/tests/cases/midas-parser/01_simple_types.midas.ref.json
+++ b/tests/cases/midas-parser/01_simple_types.midas.ref.json
@@ -2385,7 +2385,7 @@
              "_type": "NamedType",
              "name": "Difference"
            },
-            "params": [
+            "args": [
              {
                "_type": "NamedType",
                "name": "GeoLocation"
@@ -2416,7 +2416,7 @@
                "_type": "NamedType",
                "name": "Difference"
              },
-              "params": [
+              "args": [
                {
                  "_type": "NamedType",
                  "name": "Latitude"
@@ -2433,7 +2433,7 @@
                "_type": "NamedType",
                "name": "Difference"
              },
-              "params": [
+              "args": [
                {
                  "_type": "NamedType",
                  "name": "Longitude"
@@ -2464,7 +2464,7 @@
              "_type": "NamedType",
              "name": "Difference"
            },
-            "params": [
+            "args": [
              {
                "_type": "NamedType",
                "name": "Latitude"
@@ -2494,7 +2494,7 @@
              "_type": "NamedType",
              "name": "Difference"
            },
-            "params": [
+            "args": [
              {
                "_type": "NamedType",
                "name": "Longitude"
@@ -2638,7 +2638,7 @@
                "_type": "NamedType",
                "name": "Optional"
              },
-              "params": [
+              "args": [
                {
                  "_type": "ConstraintType",
                  "type": {
--- a/tests/checker.py
+++ b/tests/checker.py
@@ -1,19 +1,19 @@
-import ast
 import json
 from dataclasses import asdict, dataclass, field
 from pathlib import Path

 import midas.ast.python as p
-from midas.checker.checker import Checker
+from midas.checker.checker import TypeChecker
 from midas.checker.diagnostic import Diagnostic
-from midas.parser.python import PythonParser
-from midas.resolver.resolver import Resolver
+from midas.checker.types import Type
 from tests.base import Tester
+from tests.serializer.python import PythonAstJsonSerializer


@dataclass
 class CaseResult:
    diagnostics: list[dict] = field(default_factory=list)
+    judgments: list = field(default_factory=list)

    def dumps(self) -> str:
        return json.dumps(asdict(self), indent=2)
@@ -33,23 +33,16 @@ class CheckerTester(Tester):
        if not path.is_file():
            raise TypeError(f"Test '{path}' is not a file")

-        types_paths: list[Path] = []
+        result: CaseResult = CaseResult()
+
+        checker = TypeChecker()
        types_path: Path = path.with_suffix(".midas")
        if types_path.exists():
-            types_paths.append(types_path)
-        source: str = path.read_text()
-        tree: ast.Module = ast.parse(source, filename=path)
-        parser = PythonParser()
-        stmts: list[p.Stmt] = parser.parse_module(tree)
-        resolver = Resolver()
-        resolver.resolve(*stmts)
-        result: CaseResult = CaseResult()
-        checker = Checker(
-            resolver.locals,
-            source_path=path,
-            types_paths=types_paths,
-        )
-        diagnostics: list[Diagnostic] = checker.check(stmts)
+            checker.import_midas(types_path)
+
+        checker.type_check(path)
+
+        diagnostics: list[Diagnostic] = checker.diagnostics
        for diagnostic in diagnostics:
            result.diagnostics.append(
                {
@@ -68,6 +61,21 @@ class CheckerTester(Tester):
                }
            )

+        judgements: list[tuple[p.Expr, Type]] = checker.python_typer.judgements
+        serializer = PythonAstJsonSerializer()
+        for expr, type in judgements:
+            loc = expr.location
+            result.judgments.append(
+                {
+                    "location": {
+                        "from": f"L{loc.lineno}:{loc.col_offset}",
+                        "to": f"L{loc.end_lineno}:{loc.end_col_offset}",
+                    },
+                    "expr": expr.accept(serializer),
+                    "type": asdict(type),
+                }
+            )
+
        return result


--- a/tests/serializer/midas.py
+++ b/tests/serializer/midas.py
@@ -6,6 +6,7 @@ from midas.ast.midas import (
    ConstraintType,
    Expr,
    ExtendStmt,
+    FunctionType,
    GenericType,
    GetExpr,
    GroupingExpr,
@@ -17,6 +18,7 @@ from midas.ast.midas import (
    PropertyStmt,
    Stmt,
    Type,
+    TypeParam,
    TypeStmt,
    UnaryExpr,
    VariableExpr,
@@ -46,13 +48,11 @@ class MidasAstJsonSerializer(
        return {
            "_type": "TypeStmt",
            "name": stmt.name.lexeme,
-            "params": [
-                self._serialize_type_stmt_template_param(param) for param in stmt.params
-            ],
+            "params": [self._serialize_type_param(param) for param in stmt.params],
            "type": stmt.type.accept(self),
        }

-    def _serialize_type_stmt_template_param(self, param: TypeStmt.Param) -> dict:
+    def _serialize_type_param(self, param: TypeParam) -> dict:
        return {
            "name": param.name.lexeme,
            "bound": self._serialize_optional(param.bound),
@@ -150,7 +150,7 @@ class MidasAstJsonSerializer(
        return {
            "_type": "GenericType",
            "type": type.type.accept(self),
-            "params": self._serialize_list(type.params),
+            "args": self._serialize_list(type.args),
        }

    def visit_constraint_type(self, type: ConstraintType) -> dict:
@@ -165,3 +165,18 @@ class MidasAstJsonSerializer(
            "_type": "ComplexType",
            "properties": self._serialize_list(type.properties),
        }
+
+    def visit_function_type(self, type: FunctionType) -> dict:
+        return {
+            "_type": "FunctionType",
+            "pos_args": [self._serialize_func_arg(arg) for arg in type.pos_args],
+            "kw_args": [self._serialize_func_arg(arg) for arg in type.kw_args],
+            "returns": type.returns.accept(self),
+        }
+
+    def _serialize_func_arg(self, arg: FunctionType.Argument) -> dict:
+        return {
+            "name": arg.name,
+            "type": arg.type.accept(self),
+            "required": arg.required,
+        }
--- a/tests/serializer/python.py
+++ b/tests/serializer/python.py
@@ -16,11 +16,11 @@ from midas.ast.python import (
    Function,
    GetExpr,
    IfStmt,
+    ListExpr,
    LiteralExpr,
    LogicalExpr,
    MidasType,
    ReturnStmt,
-    SetExpr,
    Stmt,
    TernaryExpr,
    TypeAssign,
@@ -232,14 +232,6 @@ class PythonAstJsonSerializer(
            "right": expr.right.accept(self),
        }

-    def visit_set_expr(self, expr: SetExpr) -> dict:
-        return {
-            "_type": "SetExpr",
-            "object": expr.object.accept(self),
-            "name": expr.name,
-            "value": expr.value.accept(self),
-        }
-
    def visit_cast_expr(self, expr: CastExpr) -> dict:
        return {
            "_type": "CastExpr",
@@ -254,3 +246,9 @@ class PythonAstJsonSerializer(
            "if_true": expr.if_true.accept(self),
            "if_false": expr.if_false.accept(self),
        }
+
+    def visit_list_expr(self, expr: ListExpr) -> dict:
+        return {
+            "_type": "ListExpr",
+            "items": [item.accept(self) for item in expr.items],
+        }
Author	SHA1	Message	Date
LordBaryhobal	c6ead886ec	feat: add function type to midas syntax	2026-06-09 23:48:06 +02:00
LordBaryhobal	9de03bf2b5	feat(types): add type params to extend statement	2026-06-09 23:40:57 +02:00
LordBaryhobal	a26b9293be	refactor(types): extract TypeParams also rename generic type params to type args (when calling a generic)	2026-06-09 15:30:45 +02:00
LordBaryhobal	efa5454776	feat(types): add human-friendly string rep add `__str__` methods on type structures to improve readability of diagnostics	2026-06-09 12:59:36 +02:00
LordBaryhobal	b8bb8190c4	fix(resolver): define variable on assignment if a variable is not already defined when an assignment is visited, it is then defined in the current scope	2026-06-09 08:06:46 +02:00
LordBaryhobal	a4f5db7ece	fix(checker): use reduce_types to infer return type	2026-06-09 08:05:31 +02:00
LordBaryhobal	fc67f01f34	refactor(checker): extract reduce_types function	2026-06-09 08:04:45 +02:00
LordBaryhobal	0a748a36a3	feat(types): WIP add AppliedType	2026-06-08 18:26:11 +02:00
LordBaryhobal	89fdd1b47e	feat(checker): WIP add lists	2026-06-08 18:25:37 +02:00
LordBaryhobal	0cde53ac6e	feat(types): add name to generic type	2026-06-08 18:21:40 +02:00
LordBaryhobal	f3ec3606c2	fix: avoid circular import in builtins.py	2026-06-08 13:48:46 +02:00
LordBaryhobal	67ec029529	refactor(resolver): move resolver to checker module	2026-06-08 13:45:48 +02:00
LordBaryhobal	e2aef7a811	refactor(checker): unify builtins definitions	2026-06-08 13:44:26 +02:00
LordBaryhobal	86ba4e658a	refactor(checker): restructure around shared registry restructure the type checker with a shared TypesRegistry used by MidasTyper and PythonTyper this commit also relocates some methods in more appropriate places, such as is_subtype and apply_generic (now in TypesRegistry)	2026-06-08 13:41:42 +02:00
LordBaryhobal	7eccf59558	feat(checker): add reporter class	2026-06-08 13:38:35 +02:00
LordBaryhobal	9dd7801d2d	feat(resolver): handle generic application	2026-06-08 10:59:01 +02:00
LordBaryhobal	154cb8b314	refactor(checker): move is_subtype to resolver	2026-06-08 10:57:50 +02:00
LordBaryhobal	c64ab434b5	refactor(checker): move unfold_type to types.py	2026-06-08 10:56:27 +02:00
LordBaryhobal	25e6410546	feat(resolver): handle generics definition	2026-06-08 10:55:15 +02:00
LordBaryhobal	8a22acc17c	feat(checker): add generic type structure	2026-06-08 10:52:34 +02:00
LordBaryhobal	e0179bc442	feat(checker): handle assignments to attributes	2026-06-07 17:50:56 +02:00
LordBaryhobal	e665d03533	fix: remove unused SetExpr	2026-06-07 17:48:31 +02:00
LordBaryhobal	b8cb2b4273	feat(checker): handle attribute getter	2026-06-07 15:07:24 +02:00
LordBaryhobal	d278dc5f5b	tests: update tests with operation overloads	2026-06-07 14:28:36 +02:00
LordBaryhobal	59e73f0fd9	fix(checker): invert property subtype check	2026-06-07 14:00:02 +02:00
LordBaryhobal	3e0dc60283	fix(checker): only unfold alias on subtype	2026-06-07 13:59:27 +02:00
LordBaryhobal	c24eb5125e	feat(checker): resolve operation overloads with subtypes	2026-06-07 13:43:43 +02:00
LordBaryhobal	25bd895dde	feat(cli): improve diagnostic printing	2026-06-07 13:42:15 +02:00
LordBaryhobal	bccd75317e	tests: add subtyping test	2026-06-06 16:59:49 +02:00
LordBaryhobal	f0e3f7574f	feat(tests): add judgements to test results add type judgements to checker test results and update all tests (including the new subtyping rules)	2026-06-06 16:58:13 +02:00
LordBaryhobal	5d44081847	feat(checker): implement function subtyping the logic for checking function subtypes is a WIP and has not been fully tested, there may be some errors and unhandled edge cases Claude helped lay out and verify the overall steps Co-authored-by: Claude <noreply@anthropic.com>	2026-06-06 16:53:52 +02:00
LordBaryhobal	2a2bb0aec7	feat(checker): store function param position	2026-06-06 16:50:42 +02:00
LordBaryhobal	67c40a3909	feat(checker): add is_subtype method	2026-06-06 16:30:04 +02:00
LordBaryhobal	1c30188122	feat(checker): record type judgements	2026-06-06 16:25:33 +02:00
LordBaryhobal	82a0f13242	feat(cli): add verbose flag to compile	2026-06-05 14:17:24 +02:00