From 3ab0dda6820140d066430d71c9c634f6d2dd86a9 Mon Sep 17 00:00:00 2001
From: LordBaryhobal <lordbaryhobal@gmail.com>
Date: Sun, 19 Jul 2026 15:51:46 +0200
Subject: [PATCH 1/2] fix(resolver): check undefined varaible in parent scopes

---
 midas/checker/resolver.py | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/midas/checker/resolver.py b/midas/checker/resolver.py
index 70747e9..71603d4 100644
--- a/midas/checker/resolver.py
+++ b/midas/checker/resolver.py
@@ -97,6 +97,21 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
                 return True
         return False
 
+    def is_defined(self, name: str) -> bool:
+        """Check whether the given variable is defined
+
+        Args:
+            name (str): the name of the variable
+
+        Returns:
+            bool: `True` if the variable is defined in the closest scope where it
+                is declared, `False` otherwise
+        """
+        for scope in reversed(self.scopes):
+            if name in scope:
+                return scope[name]
+        return False
+
     def resolve_function(self, function: p.Function) -> None:
         """Resolve a function definition
 
@@ -230,7 +245,7 @@ class Resolver(p.Stmt.Visitor[None], p.Expr.Visitor[None]):
         pass
 
     def visit_variable_expr(self, expr: p.VariableExpr) -> None:
-        if len(self.scopes) != 0 and self.scopes[-1].get(expr.name) is False:
+        if self.is_declared(expr.name) and not self.is_defined(expr.name):
             self.reporter.error(
                 expr.location,
                 f"Variable '{expr.name}' is declared but may not be defined",

From e37432ad571ecf3ed2bbc32607367d6ec3aeba0f Mon Sep 17 00:00:00 2001
From: LordBaryhobal <lordbaryhobal@gmail.com>
Date: Fri, 24 Jul 2026 00:03:26 +0200
Subject: [PATCH 2/2] fix(checker): correct return type for column aggregation

---
 .../checker/frames/column_groupby_methods.py  |  5 +-
 midas/checker/frames/column_methods.py        | 10 ++--
 tests/cases/checker/09_frame_ops.py.ref.json  | 48 +++++--------------
 3 files changed, 17 insertions(+), 46 deletions(-)

diff --git a/midas/checker/frames/column_groupby_methods.py b/midas/checker/frames/column_groupby_methods.py
index c92b08e..2cd1c8e 100644
--- a/midas/checker/frames/column_groupby_methods.py
+++ b/midas/checker/frames/column_groupby_methods.py
@@ -13,7 +13,6 @@ from midas.checker.types import (
     Function,
     ParamSpec,
     Type,
-    UnknownType,
 )
 
 if TYPE_CHECKING:
@@ -97,11 +96,9 @@ class ColumnGroupByMethodRegistry(MethodRegistry[Call]):
             positional=[],
             keywords={},
         )
-        if not isinstance(returns, ColumnType):
-            returns = ColumnType(type=UnknownType())
         signature = Function(
             params=ParamSpec(mixed=real_params),
-            returns=returns,
+            returns=ColumnType(type=returns),
         )
 
         result: CallResult = self.dispatcher.get_result(
diff --git a/midas/checker/frames/column_methods.py b/midas/checker/frames/column_methods.py
index e957a55..3aa0136 100644
--- a/midas/checker/frames/column_methods.py
+++ b/midas/checker/frames/column_methods.py
@@ -364,13 +364,11 @@ class ColumnMethodRegistry(MethodRegistry[Call]):
             Type: the result type
         """
 
-        returns: Type = ColumnType(type=TopType())
+        returns: Type = TopType()
         if formula:
-            returns = ColumnType(
-                type=self._resolve_formula_type(
-                    call,
-                    formula(call.column.type),
-                )
+            returns = self._resolve_formula_type(
+                call,
+                formula(call.column.type),
             )
         signature = Function(
             params=ParamSpec(
diff --git a/tests/cases/checker/09_frame_ops.py.ref.json b/tests/cases/checker/09_frame_ops.py.ref.json
index 2e6c448..c48c697 100644
--- a/tests/cases/checker/09_frame_ops.py.ref.json
+++ b/tests/cases/checker/09_frame_ops.py.ref.json
@@ -4305,9 +4305,7 @@
         "arguments": [],
         "keywords": {}
       },
-      "type": {
-        "type": {}
-      }
+      "type": {}
     },
     {
       "location": {
@@ -4342,9 +4340,7 @@
         "arguments": [],
         "keywords": {}
       },
-      "type": {
-        "type": {}
-      }
+      "type": {}
     },
     {
       "location": {
@@ -4380,9 +4376,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4419,9 +4413,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "float"
-        }
+        "name": "float"
       }
     },
     {
@@ -4458,9 +4450,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4497,9 +4487,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4536,9 +4524,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4575,9 +4561,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4614,9 +4598,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4652,9 +4634,7 @@
         "arguments": [],
         "keywords": {}
       },
-      "type": {
-        "type": {}
-      }
+      "type": {}
     },
     {
       "location": {
@@ -4690,9 +4670,7 @@
         "keywords": {}
       },
       "type": {
-        "type": {
-          "name": "int"
-        }
+        "name": "int"
       }
     },
     {
@@ -4728,9 +4706,7 @@
         "arguments": [],
         "keywords": {}
       },
-      "type": {
-        "type": {}
-      }
+      "type": {}
     },
     {
       "location": {