refactor(checker): split type computation and judgement

midas/checker/python.py

@@ -78,17 +78,37 @@ class PythonTyper(
 
         return TypedAST(stmts=stmts, judgements=self.judgements)
 
-    def type_of(self, expr: p.Expr) -> Type:
+    def judge(self, expr: p.Expr, type: Type):
+        """Record a typing judgement
+
+        Args:
+            expr (p.Expr): the judged expression
+            type (Type): the type of the expression
+        """
+        self.judgements.append((expr, type))
+
+    def compute_type(self, expr: p.Expr) -> Type:
         """Evaluate the type of an expression
 
+        Args:
+            expr (p.Expr): the expression to type
+
+        Returns:
+            Type: the type of the given expression
+        """
+        return expr.accept(self)
+
+    def type_of(self, expr: p.Expr) -> Type:
+        """Evaluate the type of an expression and record the judgement
+
         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))
+        type: Type = self.compute_type(expr)
+        self.judge(expr, type)
         return type
 
     def resolve_type_expr(self, expr: p.MidasType) -> Type:
@@ -337,10 +357,14 @@ class PythonTyper(
     def visit_for_stmt(self, stmt: p.ForStmt) -> None:
         item_type: Optional[Type] = self._get_iterator_type(stmt.iterator)
         if item_type is None:
-            self.reporter.error(stmt.iterator.location, "Iterator is not an iterator")
+            iterator_type: Type = self.compute_type(stmt.iterator)
+            self.reporter.error(
+                stmt.iterator.location, f"{iterator_type} is not iterable"
+            )
             item_type = UnknownType()
 
         self._assign(stmt.location, stmt.target, item_type)
+        self.judge(stmt.target, item_type)
         env: Environment = Environment(self.env)
         body_returned: bool = self.process_block(stmt.body, env)
         if body_returned:
@@ -906,9 +930,7 @@ class PythonTyper(
             return None
 
         index: p.Expr = p.LiteralExpr(location=expr.location, value=0)
-        index_type: Type = index.accept(
-            self
-        )  # skip type_of() to avoid recording judgement
+        index_type: Type = self.compute_type(index)
         result: Optional[Type] = self._get_call_result(
             location=expr.location,
             callee=getitem,