Fix lookup of value in cost model

saulshanabrook · saulshanabrook · commit b18d3e70a9b2 · 2026-03-03T14:01:50.000-08:00
diff --git a/docs/changelog.md b/docs/changelog.md
@@ -13,6 +13,7 @@ _This project uses semantic versioning_
   - Improve doctest support, teaching expressions about their `__module__`, `__dir__`, and special methods.
   - Surface original Python exceptions from the runtime and tighten pretty-printing of values that cannot be re-parsed to make debugging e-graph executions easier.
   - Update the bundled Egglog crate, visualizer, and related dev dependencies (including `ipykernel`) to pick up the latest backend fixes.
+- Fix lookup of cost model based on value (see [zulip for issue](https://egraphs.zulipchat.com/#narrow/channel/375765-egg.2Fegglog/topic/Cost.20function.3A.20using.20function.20values.20of.20subtrees/near/577062352))
 
 ## 11.4.0 (2025-10-02)
 
diff --git a/python/egglog/egraph_state.py b/python/egglog/egraph_state.py
@@ -683,6 +683,8 @@ def _generate_callable_egg_name(self, ref: CallableRef) -> str:
                 assert_never(ref)
 
     def typed_expr_to_value(self, typed_expr: TypedExprDecl) -> bindings.Value:
+        if isinstance(typed_expr.expr, ValueDecl):
+            return typed_expr.expr.value
         egg_expr = self.typed_expr_to_egg(typed_expr, False)
         return self.egraph.eval_expr(egg_expr)[1]
 
diff --git a/python/tests/test_high_level.py b/python/tests/test_high_level.py
@@ -1568,3 +1568,37 @@ def __radd__(self, other: object) -> tuple[X, X]: ...
 
     assert X(1) + 10 == (X(1), 10)
     assert 10 + X(1) == (X(10), X(1))
+
+
+def test_custom_cost_model_size():
+    """
+    https://egraphs.zulipchat.com/#narrow/channel/375765-egg.2Fegglog/topic/Cost.20function.3A.20using.20function.20values.20of.20subtrees/near/577062352
+    """
+
+    class KAT(Expr):
+        @classmethod
+        def eps(cls) -> KAT: ...
+
+        @classmethod
+        def emp(cls) -> KAT: ...
+
+        def func(self, other: KAT) -> KAT: ...
+
+        def size(self) -> i64: ...
+
+    eps, emp = KAT.eps(), KAT.emp()
+
+    eg = EGraph()
+    q0 = eg.let("q0", KAT.func(eps, emp))
+
+    eg.register(set_(eps.size()).to(1))
+    eg.register(set_(emp.size()).to(0))
+
+    def conv_cost(eg, expr, child_costs):
+        if isinstance(expr, KAT):
+            args = get_callable_args(expr)
+            return sum(int(eg.lookup_function_value(cast("KAT", a).size())) for a in args)
+
+        return 2
+
+    assert eg.extract(q0, include_cost=True, cost_model=conv_cost) == (KAT.eps().func(KAT.emp()), 1)
