diff --git a/midas/checker/checker.py b/midas/checker/checker.py
index dcc7188..b14c406 100644
--- a/midas/checker/checker.py
+++ b/midas/checker/checker.py
@@ -1,3 +1,4 @@
+from dataclasses import dataclass
 import logging
 from pathlib import Path
 from typing import Optional
@@ -19,6 +20,13 @@ class ReturnException(Exception):
     pass
 
 
+@dataclass(frozen=True, kw_only=True)
+class MappedArgument:
+    expr: p.Expr
+    type: Type
+    argument: Function.Argument
+
+
 class Checker(
     p.Stmt.Visitor[None],
     p.Expr.Visitor[Type],
@@ -126,15 +134,18 @@ class Checker(
         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)
+            if arg.type is not None:
+                return arg.type.accept(self)
+            if arg.default is not None:
+                return arg.default.accept(self)
+            return UnknownType()
 
         for arg in stmt.posonlyargs:
             pos_args.append(
                 Function.Argument(
                     name=arg.name,
                     type=eval_arg_type(arg),
+                    required=arg.default is None,
                 )
             )
         for arg in stmt.args:
@@ -142,6 +153,7 @@ class Checker(
                 Function.Argument(
                     name=arg.name,
                     type=eval_arg_type(arg),
+                    required=arg.default is None,
                 )
             )
         for arg in stmt.kwonlyargs:
@@ -149,6 +161,7 @@ class Checker(
                 Function.Argument(
                     name=arg.name,
                     type=eval_arg_type(arg),
+                    required=arg.default is None,
                 )
             )
 
@@ -175,7 +188,9 @@ class Checker(
         else:
             returns = inferred_return
 
+        # TODO: handle *args and **kwargs sinks
         function: Function = Function(
+            name=stmt.name,
             pos_args=pos_args,
             args=args,
             kw_args=kw_args,
@@ -240,11 +255,18 @@ class Checker(
             self.import_midas(path)
             return UnknownType()
         callee: Type = self.evaluate(expr.callee)
-        arguments: list[Type] = [self.evaluate(arg) for arg in expr.arguments]
-        keywords: dict[str, Type] = {
-            name: self.evaluate(arg) for name, arg in expr.keywords.items()
-        }
-        return UnknownType()
+        if not isinstance(callee, Function):
+            self.error(expr.callee.location, "Callee is not a function")
+            return UnknownType()
+        function: Function = callee
+        mapped: list[MappedArgument] = self.map_call_arguments(function, expr)
+        for arg in mapped:
+            if arg.type != arg.argument.type:
+                self.error(
+                    arg.expr.location,
+                    f"Wrong type for argument '{arg.argument.name}', expected {arg.argument.type}, got {arg.type}",
+                )
+        return function.returns
 
     def visit_get_expr(self, expr: p.GetExpr) -> Type: ...
 
@@ -277,3 +299,87 @@ class Checker(
     def visit_frame_column(self, node: p.FrameColumn) -> Type: ...
 
     def visit_frame_type(self, node: p.FrameType) -> Type: ...
+
+    def map_call_arguments(
+        self, function: Function, call: p.CallExpr
+    ) -> list[MappedArgument]:
+        positional: list[tuple[p.Expr, Type]] = [
+            (arg, self.evaluate(arg)) for arg in call.arguments
+        ]
+        keywords: dict[str, tuple[p.Expr, Type]] = {
+            name: (arg, self.evaluate(arg)) for name, arg in call.keywords.items()
+        }
+        set_args: set[str] = set()
+
+        required_positional: set[str] = {
+            arg.name for arg in function.pos_args + function.args if arg.required
+        }
+        required_keyword: set[str] = {
+            arg.name for arg in function.kw_args if arg.required
+        }
+
+        mapped: list[MappedArgument] = []
+
+        pos_params: list[Function.Argument] = list(function.pos_args)
+        mixed_params: list[Function.Argument] = list(function.args)
+        kw_params: dict[str, Function.Argument] = {
+            arg.name: arg for arg in function.kw_args
+        }
+
+        # TODO: handle *args and **kwargs sinks
+        for arg in positional:
+            param: Function.Argument
+            if len(pos_params) != 0:
+                param = pos_params.pop(0)
+            elif len(mixed_params) != 0:
+                param = mixed_params.pop(0)
+            else:
+                self.error(arg[0].location, "Too many positional arguments")
+                break
+            required_positional.discard(param.name)
+            required_keyword.discard(param.name)
+            set_args.add(param.name)
+            mapped.append(
+                MappedArgument(
+                    expr=arg[0],
+                    type=arg[1],
+                    argument=param,
+                )
+            )
+
+        kw_params.update({arg.name: arg for arg in mixed_params})
+        for name, arg in keywords.items():
+            param: Function.Argument
+            if name not in kw_params:
+                if name in set_args:
+                    self.error(
+                        arg[0].location, f"Multiple values for argument '{name}'"
+                    )
+                else:
+                    self.error(arg[0].location, f"Unknown keyword argument '{name}'")
+                continue
+            param = kw_params.pop(name)
+            required_positional.discard(name)
+            required_keyword.discard(name)
+            set_args.add(name)
+            mapped.append(
+                MappedArgument(
+                    expr=arg[0],
+                    type=arg[1],
+                    argument=param,
+                )
+            )
+
+        if len(required_positional) != 0:
+            self.error(
+                call.location,
+                f"Missing required positional arguments: {required_positional}",
+            )
+
+        if len(required_keyword) != 0:
+            self.error(
+                call.location,
+                f"Missing required keyword arguments: {required_keyword}",
+            )
+
+        return mapped
diff --git a/midas/checker/types.py b/midas/checker/types.py
index 2afd0ae..1e2f149 100644
--- a/midas/checker/types.py
+++ b/midas/checker/types.py
@@ -26,6 +26,7 @@ class UnitType:
 
 @dataclass(frozen=True, kw_only=True)
 class Function:
+    name: str
     pos_args: list[Argument]
     args: list[Argument]
     kw_args: list[Argument]
@@ -35,6 +36,7 @@ class Function:
     class Argument:
         name: str
         type: Type
+        required: bool
 
 
 Type = BaseType | SimpleType | UnknownType | UnitType | Function