diff --git a/src/interpreter.c b/src/interpreter.c
index 5e6ac0a..fe76d0c 100644
--- a/src/interpreter.c
+++ b/src/interpreter.c
@@ -1753,7 +1753,8 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
                     if (in->args.args[i + 1].type != VALUE) {
                         runtimeError(ARG_TYPE_MISMATCH, "Expecting a Value", in, currentInstruction);
                     }
-                    if (in->args.args[i + 1].value.value.type != function->args[i].type) {
+                    //if (in->args.args[i + 1].value.value.type != function->args[i].type) {
+                    if (!checkFnTypes(&in->args.args[i + 1].value.value, &function->args[i])) {
                         runtimeError(ARG_TYPE_MISMATCH, "Mismatched function argument types", in, currentInstruction);
                     }
                     appendToList(&argsList, in->args.args[i + 1].value.value);
@@ -1781,7 +1782,8 @@ GroundValue interpretGroundInstruction(GroundInstruction inst, GroundScope* scop
                     if (in->args.args[i + 1].type != VALUE) {
                         runtimeError(ARG_TYPE_MISMATCH, "Expecting a Value", in, currentInstruction);
                     }
-                    if (in->args.args[i + 1].value.value.type != function->args[i].type) {
+                    //if (in->args.args[i + 1].value.value.type != function->args[i].type) {
+                    if (!checkFnTypes(&in->args.args[i + 1].value.value, &function->args[i])) {
                         runtimeError(ARG_TYPE_MISMATCH, "Mismatched function argument types", in, currentInstruction);
                     }
                     addVariable(newScope.variables, function->args[i].name, in->args.args[i + 1].value.value);
diff --git a/src/types.c b/src/types.c
index 01becb4..4a55807 100644
--- a/src/types.c
+++ b/src/types.c
@@ -582,3 +582,43 @@ GroundError createGroundError(char* what, char* type, GroundInstruction* where,
 
     return ge;
 }
+
+bool checkFnTypes(GroundValue* left, GroundFunctionArgs* right) {
+    if (left->type != right->type) {
+        return false;
+    }
+    if (left->type == CUSTOM) {
+        if (left->customType->size != right->customType->size) {
+            return false;
+        }
+        for (size_t i = 0; i < left->customType->size; i++) {
+            if (strcmp(left->customType->fields[i].id, right->customType->fields[i].id) != 0) {
+                return false;
+            }
+            if (!checkTypes(&left->customType->fields[i].value, &right->customType->fields[i].value)) {
+                return false;
+            }
+        }
+    }
+    return true;
+}
+
+bool checkTypes(GroundValue* left, GroundValue* right) {
+    if (left->type != right->type) {
+        return false;
+    }
+    if (left->type == CUSTOM) {
+        if (left->customType->size != right->customType->size) {
+            return false;
+        }
+        for (size_t i = 0; i < left->customType->size; i++) {
+            if (strcmp(left->customType->fields[i].id, right->customType->fields[i].id) != 0) {
+                return false;
+            }
+            if (!checkTypes(&left->customType->fields[i].value, &right->customType->fields[i].value)) {
+                return false;
+            } 
+        }
+    }
+    return true;
+}
diff --git a/src/types.h b/src/types.h
index 0b3c903..a24ab32 100644
--- a/src/types.h
+++ b/src/types.h
@@ -62,6 +62,7 @@ typedef struct GroundError {
  */
 typedef struct GroundValue {
     GroundValueType type;
+    struct GroundStruct* customType;
     union {
         int64_t intVal;
         double doubleVal;
@@ -125,6 +126,7 @@ typedef struct GroundProgram {
  */
 typedef struct GroundFunctionArgs {
     GroundValueType type;
+    struct GroundStruct* customType;
     char* name;
 } GroundFunctionArgs;
 
@@ -276,6 +278,11 @@ void freeGroundObject(GroundObject* object);
 // Creates a GroundError.
 GroundError createGroundError(char* what, char* type, GroundInstruction* where, size_t* line);
 
+// Compares types of a value and function args. 
+bool checkFnTypes(GroundValue* left, GroundFunctionArgs* arg);
+
+// Compares types of two values.
+bool checkTypes(GroundValue* left, GroundValue* right);
 
 
 #endif