diff --git a/midas/checker/checker.py b/midas/checker/checker.py
index 3100587..0c54fa4 100644
--- a/midas/checker/checker.py
+++ b/midas/checker/checker.py
@@ -6,13 +6,10 @@ from typing import Optional
 import midas.ast.midas as m
 import midas.ast.python as p
 from midas.ast.location import Location
-from midas.checker.builtins import BUILTIN_SUBTYPES
 from midas.checker.diagnostic import Diagnostic, DiagnosticType
 from midas.checker.environment import Environment
 from midas.checker.operators import COMPARATOR_METHODS, OPERATOR_METHODS
 from midas.checker.types import (
-    AliasType,
-    BaseType,
     ComplexType,
     Function,
     Operation,
@@ -180,149 +177,7 @@ class Checker(
         self.ctx.resolve(stmts)
 
     def is_subtype(self, type1: Type, type2: Type) -> bool:
-        """Check whether `type1` is a subtype of `type2`
-
-        For more details on the rules checked here, see TAPL Chap. 15-16-17
-
-        Args:
-            type1 (Type): the potential subtype
-            type2 (Type): the potential supertype
-
-        Returns:
-            bool: whether `type1` is a subtype of `type2`
-        """
-
-        if type1 == type2:
-            return True
-
-        match (type1, type2):
-            case (AliasType(type=base1), _):
-                return self.is_subtype(base1, type2)
-
-            case (BaseType(name=name1), BaseType(name=name2)):
-                return name1 in BUILTIN_SUBTYPES.get(name2, set())
-
-            case (ComplexType(properties=props1), ComplexType(properties=props2)):
-                for k, t in props2.items():
-                    if k not in props1:
-                        return False
-                    if not self.is_subtype(props1[k], t):
-                        return False
-                return True
-
-            case (Function(returns=return1), Function(returns=return2)):
-                if not self.is_func_subtype(type1, type2):
-                    return False
-                if not self.is_subtype(return1, return2):
-                    return False
-                return True
-
-        return False
-
-    # TODO: verify the logic in here
-    def is_func_subtype(self, func1: Function, func2: Function) -> bool:
-        """Check whether a function is a subtype of another
-
-        Args:
-            func1 (Function): the potential function subtype
-            func2 (Function): the potential function supertype
-
-        Returns:
-            bool: whether `func1` is a subtype of `func2`
-        """
-        if not self.is_subtype(func1.returns, func2.returns):
-            return False
-
-        pos1: list[Function.Argument] = func1.pos_args
-        mixed1: list[Function.Argument] = func1.args
-        kw1: dict[str, Function.Argument] = {a.name: a for a in func1.kw_args}
-        pos2: list[Function.Argument] = func2.pos_args
-        mixed2: list[Function.Argument] = func2.args
-        kw2: dict[str, Function.Argument] = {a.name: a for a in func2.kw_args}
-
-        mixed_by_pos: dict[int, Function.Argument] = {arg.pos: arg for arg in mixed2}
-        mixed_by_name: dict[str, Function.Argument] = {arg.name: arg for arg in mixed2}
-
-        def is_arg_subtype(sub: Function.Argument, sup: Function.Argument) -> bool:
-            if not self.is_subtype(sub.type, sup.type):
-                return False
-            if not sup.required and sub.required:
-                return False
-            return True
-
-        for arg1 in pos1:
-            arg2: Function.Argument
-            if arg1.pos < len(pos2):
-                arg2 = pos2[arg1.pos]
-            elif arg1.pos in mixed_by_pos:
-                arg2 = mixed_by_pos[arg1.pos]
-            elif not arg1.required:
-                continue
-            else:
-                return False
-            if not is_arg_subtype(arg2, arg1):
-                return False
-
-        for name, arg1 in kw1.items():
-            arg2: Function.Argument
-            if name in kw2:
-                arg2 = kw2[name]
-            elif name in mixed_by_name:
-                arg2 = mixed_by_name[name]
-            elif not arg1.required:
-                continue
-            else:
-                return False
-            if not is_arg_subtype(arg2, arg1):
-                return False
-
-        for arg1 in mixed1:
-            pos_arg2: Optional[Function.Argument] = None
-            kw_arg2: Optional[Function.Argument] = None
-            if arg1.name in kw2:
-                kw_arg2 = kw2[arg1.name]
-            elif arg1.name in mixed_by_name:
-                kw_arg2 = mixed_by_name[arg1.name]
-            if arg1.pos < len(pos2):
-                pos_arg2 = pos2[arg1.pos]
-            elif arg1.pos in mixed_by_pos:
-                pos_arg2 = mixed_by_pos[arg1.pos]
-
-            # No match in func2 and arg is required
-            if pos_arg2 is None and kw_arg2 is None and arg1.required:
-                return False
-
-            # Matching keyword argument
-            if kw_arg2 is not None and not is_arg_subtype(kw_arg2, arg1):
-                return False
-
-            # Matching positional argument
-            if pos_arg2 is not None and not is_arg_subtype(pos_arg2, arg1):
-                return False
-
-        mixed_positions: set[int] = {a.pos for a in mixed1}
-        mixed_names: set[str] = {a.name for a in mixed1}
-        for arg2 in pos2:
-            if not arg2.required:
-                continue
-            if arg2.pos >= len(pos1) and arg2.pos not in mixed_positions:
-                return False
-
-        for name, arg2 in kw2.items():
-            if not arg2.required:
-                continue
-            if name not in kw1 and name not in mixed_names:
-                return False
-
-        for arg2 in mixed2:
-            if arg2.required:
-                continue
-            pos_match: bool = arg2.pos < len(pos1) or arg2.pos in mixed_positions
-            kw_match: bool = arg2.name in kw1 or arg2.name in mixed_names
-            if not pos_match or not kw_match:
-                return False
-
-        return True
+        return self.ctx.is_subtype(type1, type2)
 
     def visit_expression_stmt(self, stmt: p.ExpressionStmt) -> None:
         self.type_of(stmt.expr)
diff --git a/midas/resolver/midas.py b/midas/resolver/midas.py
index 063a752..c7b168d 100644
--- a/midas/resolver/midas.py
+++ b/midas/resolver/midas.py
@@ -1,9 +1,12 @@
 from typing import Optional
 
 import midas.ast.midas as m
+from midas.checker.builtins import BUILTIN_SUBTYPES
 from midas.checker.types import (
     AliasType,
+    BaseType,
     ComplexType,
+    Function,
     GenericType,
     Operation,
     Type,
@@ -198,3 +201,144 @@ class MidasResolver(m.Stmt.Visitor[None], m.Expr.Visitor[None], m.Type.Visitor[T
                 prop.name.lexeme: prop.type.accept(self) for prop in type.properties
             }
         )
+
+    def is_subtype(self, type1: Type, type2: Type) -> bool:
+        """Check whether `type1` is a subtype of `type2`
+
+        For more details on the rules checked here, see TAPL Chap. 15-16-17
+
+        Args:
+            type1 (Type): the potential subtype
+            type2 (Type): the potential supertype
+
+        Returns:
+            bool: whether `type1` is a subtype of `type2`
+        """
+
+        if type1 == type2:
+            return True
+
+        match (type1, type2):
+            case (AliasType(type=base1), _):
+                return self.is_subtype(base1, type2)
+
+            case (BaseType(name=name1), BaseType(name=name2)):
+                return name1 in BUILTIN_SUBTYPES.get(name2, set())
+
+            case (ComplexType(properties=props1), ComplexType(properties=props2)):
+                for k, t in props2.items():
+                    if k not in props1:
+                        return False
+                    if not self.is_subtype(props1[k], t):
+                        return False
+                return True
+
+            case (Function(), Function()):
+                return self.is_func_subtype(type1, type2)
+
+        return False
+
+    # TODO: verify the logic in here
+    def is_func_subtype(self, func1: Function, func2: Function) -> bool:
+        """Check whether a function is a subtype of another
+
+        Args:
+            func1 (Function): the potential function subtype
+            func2 (Function): the potential function supertype
+
+        Returns:
+            bool: whether `func1` is a subtype of `func2`
+        """
+        if not self.is_subtype(func1.returns, func2.returns):
+            return False
+
+        pos1: list[Function.Argument] = func1.pos_args
+        mixed1: list[Function.Argument] = func1.args
+        kw1: dict[str, Function.Argument] = {a.name: a for a in func1.kw_args}
+        pos2: list[Function.Argument] = func2.pos_args
+        mixed2: list[Function.Argument] = func2.args
+        kw2: dict[str, Function.Argument] = {a.name: a for a in func2.kw_args}
+
+        mixed_by_pos: dict[int, Function.Argument] = {arg.pos: arg for arg in mixed2}
+        mixed_by_name: dict[str, Function.Argument] = {arg.name: arg for arg in mixed2}
+
+        def is_arg_subtype(sub: Function.Argument, sup: Function.Argument) -> bool:
+            if not self.is_subtype(sub.type, sup.type):
+                return False
+            if not sup.required and sub.required:
+                return False
+            return True
+
+        for arg1 in pos1:
+            arg2: Function.Argument
+            if arg1.pos < len(pos2):
+                arg2 = pos2[arg1.pos]
+            elif arg1.pos in mixed_by_pos:
+                arg2 = mixed_by_pos[arg1.pos]
+            elif not arg1.required:
+                continue
+            else:
+                return False
+            if not is_arg_subtype(arg2, arg1):
+                return False
+
+        for name, arg1 in kw1.items():
+            arg2: Function.Argument
+            if name in kw2:
+                arg2 = kw2[name]
+            elif name in mixed_by_name:
+                arg2 = mixed_by_name[name]
+            elif not arg1.required:
+                continue
+            else:
+                return False
+            if not is_arg_subtype(arg2, arg1):
+                return False
+
+        for arg1 in mixed1:
+            pos_arg2: Optional[Function.Argument] = None
+            kw_arg2: Optional[Function.Argument] = None
+            if arg1.name in kw2:
+                kw_arg2 = kw2[arg1.name]
+            elif arg1.name in mixed_by_name:
+                kw_arg2 = mixed_by_name[arg1.name]
+            if arg1.pos < len(pos2):
+                pos_arg2 = pos2[arg1.pos]
+            elif arg1.pos in mixed_by_pos:
+                pos_arg2 = mixed_by_pos[arg1.pos]
+
+            # No match in func2 and arg is required
+            if pos_arg2 is None and kw_arg2 is None and arg1.required:
+                return False
+
+            # Matching keyword argument
+            if kw_arg2 is not None and not is_arg_subtype(kw_arg2, arg1):
+                return False
+
+            # Matching positional argument
+            if pos_arg2 is not None and not is_arg_subtype(pos_arg2, arg1):
+                return False
+
+        mixed_positions: set[int] = {a.pos for a in mixed1}
+        mixed_names: set[str] = {a.name for a in mixed1}
+        for arg2 in pos2:
+            if not arg2.required:
+                continue
+            if arg2.pos >= len(pos1) and arg2.pos not in mixed_positions:
+                return False
+
+        for name, arg2 in kw2.items():
+            if not arg2.required:
+                continue
+            if name not in kw1 and name not in mixed_names:
+                return False
+
+        for arg2 in mixed2:
+            if arg2.required:
+                continue
+            pos_match: bool = arg2.pos < len(pos1) or arg2.pos in mixed_positions
+            kw_match: bool = arg2.name in kw1 or arg2.name in mixed_names
+            if not pos_match or not kw_match:
+                return False
+
+        return True