diff --git a/midas/checker/checker.py b/midas/checker/checker.py
index bc0d323..dcc7188 100644
--- a/midas/checker/checker.py
+++ b/midas/checker/checker.py
@@ -8,13 +8,17 @@ from midas.ast.location import Location
 from midas.checker.diagnostic import Diagnostic, DiagnosticType
 from midas.checker.environment import Environment
 from midas.checker.operators import OPERATOR_METHODS
-from midas.checker.types import Type, UnknownType
+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
 
 
+class ReturnException(Exception):
+    pass
+
+
 class Checker(
     p.Stmt.Visitor[None],
     p.Expr.Visitor[Type],
@@ -63,6 +67,16 @@ class Checker(
     def evaluate(self, expr: p.Expr) -> Type:
         return expr.accept(self)
 
+    def evaluate_block(self, block: list[p.Stmt], env: Environment) -> None:
+        previous_env: Environment = self.env
+        self.env = env
+        for stmt in block:
+            try:
+                stmt.accept(self)
+            except ReturnException:
+                break
+        self.env = previous_env
+
     def check(self, statements: list[p.Stmt]) -> list[Diagnostic]:
         self.diagnostics = []
         for stmt in statements:
@@ -105,7 +119,69 @@ class Checker(
     def visit_expression_stmt(self, stmt: p.ExpressionStmt) -> None:
         self.evaluate(stmt.expr)
 
-    def visit_function(self, stmt: p.Function) -> None: ...
+    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 None:
+                return UnknownType()
+            return arg.type.accept(self)
+
+        for arg in stmt.posonlyargs:
+            pos_args.append(
+                Function.Argument(
+                    name=arg.name,
+                    type=eval_arg_type(arg),
+                )
+            )
+        for arg in stmt.args:
+            args.append(
+                Function.Argument(
+                    name=arg.name,
+                    type=eval_arg_type(arg),
+                )
+            )
+        for arg in stmt.kwonlyargs:
+            kw_args.append(
+                Function.Argument(
+                    name=arg.name,
+                    type=eval_arg_type(arg),
+                )
+            )
+
+        for arg in pos_args + args + kw_args:
+            env.define(arg.name, arg.type)
+
+        self.evaluate_block(stmt.body, env)
+        inferred_return: Type = UnknownType()
+        if len(env.return_types) == 1:
+            inferred_return = list(env.return_types)[0]
+        elif len(env.return_types) > 1:
+            self.error(
+                stmt.location,
+                f"Mixed return types: {env.return_types}",
+            )
+        returns: Type = UnknownType()
+        if stmt.returns is not None:
+            returns = stmt.returns.accept(self)
+            if returns != inferred_return:
+                self.error(
+                    stmt.returns.location,
+                    f"Return type mismatch, annotated {returns} but returns {inferred_return}",
+                )
+        else:
+            returns = inferred_return
+
+        function: Function = Function(
+            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
@@ -132,6 +208,11 @@ class Checker(
                         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.add(type)
+        raise ReturnException()
+
     def visit_binary_expr(self, expr: p.BinaryExpr) -> Type:
         method: Optional[str] = OPERATOR_METHODS.get(expr.operator.__class__)
         if method is None:
diff --git a/midas/checker/environment.py b/midas/checker/environment.py
index dc5cac8..fa6dfe2 100644
--- a/midas/checker/environment.py
+++ b/midas/checker/environment.py
@@ -9,6 +9,7 @@ class Environment:
     def __init__(self, enclosing: Optional[Environment] = None) -> None:
         self.enclosing: Optional[Environment] = enclosing
         self.values: dict[str, Type] = {}
+        self.return_types: set[Type] = set()
 
     def define(self, name: str, value: Type):
         self.values[name] = value
diff --git a/midas/checker/types.py b/midas/checker/types.py
index ea94b45..2afd0ae 100644
--- a/midas/checker/types.py
+++ b/midas/checker/types.py
@@ -19,4 +19,22 @@ class UnknownType:
     pass
 
 
-Type = BaseType | SimpleType | UnknownType
+@dataclass(frozen=True, kw_only=True)
+class UnitType:
+    pass
+
+
+@dataclass(frozen=True, kw_only=True)
+class Function:
+    pos_args: list[Argument]
+    args: list[Argument]
+    kw_args: list[Argument]
+    returns: Type
+
+    @dataclass(frozen=True, kw_only=True)
+    class Argument:
+        name: str
+        type: Type
+
+
+Type = BaseType | SimpleType | UnknownType | UnitType | Function