From 14918130535c2806b47c05895ed778e43639bbda Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Tue, 2 Jan 2024 10:21:17 -0500
Subject: [PATCH] Assume `str` return annotation if none provided

---
 pyproject.toml                        |  1 -
 src/marvin/_mappings/types.py         |  3 +++
 src/marvin/components/prompt/fn.py    |  5 ++++-
 tests/components/test_ai_functions.py | 24 ++++++++++++++++++++++++
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/pyproject.toml b/pyproject.toml
index 732677241..8b9c3e2bc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -36,7 +36,6 @@ dev = [
     "ipython",
     "mkdocs-autolinks-plugin~=0.7",
     "mkdocs-awesome-pages-plugin~=2.8",
-    "mkdocs-livereload",
     "mkdocs-markdownextradata-plugin~=0.2",
     "mkdocs-jupyter>=0.24.1",
     "mkdocs-material>=9.1.17",
diff --git a/src/marvin/_mappings/types.py b/src/marvin/_mappings/types.py
index e68e0bf49..a32472e04 100644
--- a/src/marvin/_mappings/types.py
+++ b/src/marvin/_mappings/types.py
@@ -25,6 +25,9 @@ def cast_type_to_model(
     else:
         metadata = FieldInfo(description=field_description)
 
+    if _type is None:
+        raise ValueError("No type provided; unable to create model for casting.")
+
     return create_model(
         model_name,
         __doc__=model_description,
diff --git a/src/marvin/components/prompt/fn.py b/src/marvin/components/prompt/fn.py
index b6637fdde..1dfa70cb8 100644
--- a/src/marvin/components/prompt/fn.py
+++ b/src/marvin/components/prompt/fn.py
@@ -192,9 +192,12 @@ def wrapper(func: Callable[P, Any], *args: P.args, **kwargs_: P.kwargs) -> Self:
             signature = inspect.signature(func)
             params = signature.bind(*args, **kwargs_)
             params.apply_defaults()
+            _type = inspect.signature(func).return_annotation
+            if _type is inspect._empty:
+                _type = str
 
             toolset = cast_type_to_toolset(
-                _type=inspect.signature(func).return_annotation,
+                _type=_type,
                 model_name=model_name,
                 model_description=model_description,
                 field_name=field_name,
diff --git a/tests/components/test_ai_functions.py b/tests/components/test_ai_functions.py
index 070a0b97d..79845ee08 100644
--- a/tests/components/test_ai_functions.py
+++ b/tests/components/test_ai_functions.py
@@ -41,6 +41,30 @@ async def list_fruit(n: int) -> list[str]:
             assert len(result) == 3
 
     class TestAnnotations:
+        def test_no_annotations(self):
+            @ai_fn
+            def f(x):
+                """returns x + 1"""
+
+            result = f(3)
+            assert result == "4"
+
+        def test_arg_annotations(self):
+            @ai_fn
+            def f(x: int):
+                """returns x + 1"""
+
+            result = f(3)
+            assert result == "4"
+
+        def test_return_annotations(self):
+            @ai_fn
+            def f(x) -> int:
+                """returns x + 1"""
+
+            result = f("3")
+            assert result == 4
+
         def test_list_fruit_with_generic_type_hints(self):
             @ai_fn
             def list_fruit(n: int) -> List[str]: