add check

instructor/distil.py

@@ -69,6 +69,9 @@ def is_return_type_base_model_or_instance(func: Callable[..., Any]) -> bool:
     :return: True if the return type is a pydantic BaseModel or an instance of it.
     """
     return_type = inspect.signature(func).return_annotation
+    assert (
+        return_type != inspect.Signature.empty
+    ), "Must have a return type hint that is a pydantic BaseModel"
     return inspect.isclass(return_type) and issubclass(return_type, BaseModel)
 
 

tests/test_distil.py

@@ -1,4 +1,4 @@
-from pyexpat import model
+import pytest
 import openai
 from pydantic import BaseModel
 from instructor.distil import (
@@ -19,6 +19,22 @@ class SimpleModel(BaseModel):
     data: int
 
 
+def test_must_have_hint():
+    with pytest.raises(AssertionError):
+
+        @instructions.distil
+        def test_func(x: int):
+            return SimpleModel(data=x)
+
+
+def test_must_be_base_model():
+    with pytest.raises(AssertionError):
+
+        @instructions.distil
+        def test_func(x) -> int:
+            return SimpleModel(data=x)
+
+
 def test_is_return_type_base_model_or_instance():
     def valid_function() -> SimpleModel:
         return SimpleModel(data=1)