diff --git a/midas/checker/types.py b/midas/checker/types.py
index 1e2f149..d9d91b7 100644
--- a/midas/checker/types.py
+++ b/midas/checker/types.py
@@ -9,9 +9,9 @@ class BaseType:
 
 
 @dataclass(frozen=True, kw_only=True)
-class SimpleType:
+class AliasType:
     name: str
-    base: BaseType | SimpleType
+    type: Type
 
 
 @dataclass(frozen=True, kw_only=True)
@@ -39,4 +39,16 @@ class Function:
         required: bool
 
 
-Type = BaseType | SimpleType | UnknownType | UnitType | Function
+@dataclass(frozen=True, kw_only=True)
+class ComplexType:
+    properties: dict[str, Type]
+
+
+@dataclass(frozen=True, kw_only=True)
+class UnionType:
+    alternatives: list[Type]
+
+
+Type = (
+    BaseType | AliasType | UnknownType | UnitType | Function | ComplexType | UnionType
+)
diff --git a/midas/resolver/midas.py b/midas/resolver/midas.py
index d57ca70..ff97dbc 100644
--- a/midas/resolver/midas.py
+++ b/midas/resolver/midas.py
@@ -1,11 +1,15 @@
 from typing import Optional
 
 import midas.ast.midas as m
-from midas.checker.types import BaseType, SimpleType, Type
+from midas.checker.types import (
+    Type,
+    UnionType,
+    UnknownType,
+)
 from midas.resolver.builtin import define_builtins
 
 
-class MidasResolver(m.Stmt.Visitor[None], m.Expr.Visitor[Type]):
+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:
@@ -94,20 +98,12 @@ class MidasResolver(m.Stmt.Visitor[None], m.Expr.Visitor[Type]):
         for stmt in stmts:
             stmt.accept(self)
 
-    def visit_simple_type_stmt(self, stmt: m.SimpleTypeStmt) -> None:
-        # TODO generics, optional, constraint
-        base: Type = self.get_type(stmt.base.name.lexeme)
-        match base:
-            case BaseType() | SimpleType():
-                type = SimpleType(
-                    name=stmt.name.lexeme,
-                    base=base,
-                )
-                self.define_type(type.name, type)
-            case _:
-                raise TypeError(f"Invalid base {base} for simple type")
-
-    def visit_complex_type_stmt(self, stmt: m.ComplexTypeStmt) -> None: ...
+    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)
+        self.define_type(stmt.name.lexeme, type)
 
     def visit_property_stmt(self, stmt: m.PropertyStmt) -> None: ...
 
@@ -127,27 +123,44 @@ class MidasResolver(m.Stmt.Visitor[None], m.Expr.Visitor[Type]):
 
     def visit_predicate_stmt(self, stmt: m.PredicateStmt) -> None: ...
 
-    def visit_simple_type_expr(self, expr: m.SimpleTypeExpr) -> Type:
-        return self.get_type(expr.name.lexeme)
+    def visit_logical_expr(self, expr: m.LogicalExpr) -> None: ...
 
-    def visit_logical_expr(self, expr: m.LogicalExpr) -> Type: ...
+    def visit_binary_expr(self, expr: m.BinaryExpr) -> None: ...
 
-    def visit_binary_expr(self, expr: m.BinaryExpr) -> Type: ...
+    def visit_unary_expr(self, expr: m.UnaryExpr) -> None: ...
 
-    def visit_unary_expr(self, expr: m.UnaryExpr) -> Type: ...
+    def visit_get_expr(self, expr: m.GetExpr) -> None: ...
 
-    def visit_get_expr(self, expr: m.GetExpr) -> Type: ...
+    def visit_variable_expr(self, expr: m.VariableExpr) -> None: ...
 
-    def visit_variable_expr(self, expr: m.VariableExpr) -> Type: ...
-
-    def visit_grouping_expr(self, expr: m.GroupingExpr) -> Type:
+    def visit_grouping_expr(self, expr: m.GroupingExpr) -> None:
         return expr.expr.accept(self)
 
-    def visit_literal_expr(self, expr: m.LiteralExpr) -> Type: ...
+    def visit_literal_expr(self, expr: m.LiteralExpr) -> None: ...
 
-    def visit_wildcard_expr(self, expr: m.WildcardExpr) -> Type: ...
+    def visit_wildcard_expr(self, expr: m.WildcardExpr) -> None: ...
 
-    def visit_template_expr(self, expr: m.TemplateExpr) -> Type: ...
+    def visit_named_type(self, type: m.NamedType) -> Type:
+        return self.get_type(type.name.lexeme)
 
-    def visit_type_expr(self, expr: m.TypeExpr) -> Type:
-        return self.get_type(expr.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_union_type(self, type: m.UnionType) -> Type:
+        types: list[Type] = [type_.accept(self) for type_ in type.types]
+        return UnionType(alternatives=types)
+
+    def visit_complex_type(self, type: m.ComplexType) -> Type:
+        for prop in type.properties:
+            prop.accept(self)
+        # TODO
+        return UnknownType()