fix(checker): early define fully-typed function

to handle simple recursion cases where the function has an explicit return type hint, the function must be defined before evaluating its body

examples/01_simple_type_checking/03_control_flow.py

@@ -7,3 +7,8 @@ def minimum(x: int, y: int):
 a = 15
 b = 72
 c = minimum(a, b)
+
+def factorial(n: int) -> int:
+    if n <= 1:
+        return 1
+    return n * factorial(n - 1)

midas/checker/checker.py

@@ -224,6 +224,19 @@ class Checker(
         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.evaluate_block(stmt.body, env)
         inferred_return: Type = UnknownType()
         if not returned:
@@ -236,9 +249,11 @@ class Checker(
                 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_hint is not None:
+            assert stmt.returns is not None
+            returns = returns_hint
             if returns != inferred_return:
                 self.error(
                     stmt.returns.location,