Add better error message when using @function in class

docs/changelog.md

@@ -6,6 +6,7 @@ _This project uses semantic versioning_
 
 - Fix pretty printing of lambda functions
 - Add support for subsuming rewrite generated by default function and method definitions
+- Add better error message when using @function in class (thanks @shinawy)
 
 ## 8.0.1 (2024-10-24)
 

python/egglog/egraph.py

@@ -577,23 +577,25 @@ def _generate_class_decls(  # noqa: C901,PLR0912
             decl = FunctionDecl(special_function_name, builtin=True, egg_name=egg_fn)
             decls.set_function_decl(ref, decl)
             continue
-
-        _, add_rewrite = _fn_decl(
-            decls,
-            egg_fn,
-            ref,
-            fn,
-            locals,
-            default,
-            cost,
-            merge,
-            on_merge,
-            mutates,
-            builtin,
-            ruleset=ruleset,
-            unextractable=unextractable,
-            subsume=subsume,
-        )
+        try:
+            _, add_rewrite = _fn_decl(
+                decls,
+                egg_fn,
+                ref,
+                fn,
+                locals,
+                default,
+                cost,
+                merge,
+                on_merge,
+                mutates,
+                builtin,
+                ruleset=ruleset,
+                unextractable=unextractable,
+                subsume=subsume,
+            )
+        except ValueError as e:
+            raise ValueError(f"Error processing {cls_name}.{method_name}: {e}") from e
 
         if not builtin and not isinstance(ref, InitRef) and not mutates:
             add_default_funcs.append(add_rewrite)
@@ -721,6 +723,9 @@ def _fn_decl(
     """
     Sets the function decl for the function object and returns the ref as well as a thunk that sets the default callable.
     """
+    if isinstance(fn, RuntimeFunction):
+        msg = "Inside of classes, wrap methods with the `method` decorator, not `function`"
+        raise ValueError(msg)  # noqa: TRY004
     if not isinstance(fn, FunctionType):
         raise NotImplementedError(f"Can only generate function decls for functions not {fn}  {type(fn)}")
 

python/tests/test_high_level.py

@@ -762,3 +762,15 @@ def test_inserting_map(self):
 
     def test_creating_map(self):
         EGraph().simplify(Map[String, i64].empty(), 1)
+
+
+def test_helpful_error_function_class():
+    class E(Expr):
+        @function(cost=10)
+        def __init__(self) -> None: ...
+
+    with pytest.raises(
+        ValueError,
+        match="Error processing E.__init__: Inside of classes, wrap methods with the `method` decorator, not `function`",
+    ):
+        E()