adityamukho · Oct 25, 2026 · Oct 25, 2026
@@ -54,7 +54,7 @@ Worktree `.worktrees/180-udf-ffi` (reused), branch `feat/180-fallible-udfs` from
 Branch `feat/180-udf-ffi` from `minigraf-binding-template` `v3`.
 
 14. **Pin** `minigraf` to PR 1's merge commit (git rev, as the other v3 pins).
-15. **FFI code** (spec §6) in `src/lib.rs`: `MiniGrafUdfError` with `From<uniffi::UnexpectedUniFFICallbackError>`, the three traits under `#[uniffi::export]`, and `MiniGrafDb::register_predicate`/`register_aggregate` over `try_register_*` with `Acc = Arc<dyn MiniGrafAccumulator>`. A `Ref` from `finish` that is not a UUID becomes `UdfError` with `to_core`'s message. The handle lock change (spec §6.1 "Handle locks"): every `MiniGrafDb` method locks only to clone the `Minigraf`. Rust tests with Rust implementations of the traits: a predicate, an aggregate in `:find` and `:over`, a failing `step` → `MiniGrafError::Query` containing `QRY-014`, a bad `Ref` from `finish`, and a predicate that queries the same `MiniGrafDb`.
+15. **FFI code** (spec §6) in `src/lib.rs`: `MiniGrafUdfError` with `From<uniffi::UnexpectedUniFFICallbackError>`, the three traits under `#[uniffi::export(with_foreign)]`, and `MiniGrafDb::register_predicate`/`register_aggregate` over `try_register_*` with `Acc = Arc<dyn MiniGrafAccumulator>`. A `Ref` from `finish` that is not a UUID becomes `UdfError` with `to_core`'s message. The handle lock change (spec §6.1 "Handle locks"): every `MiniGrafDb` method locks only to clone the `Minigraf`. Rust tests with Rust implementations of the traits: a predicate, an aggregate in `:find` and `:over`, a failing `step` → `MiniGrafError::Query` containing `QRY-014`, a bad `Ref` from `finish`, and a predicate that queries the same `MiniGrafDb`.
 16. **Bindgen check**: generate Python, Kotlin and Swift from the built cdylib (README step 5) and confirm that each has an implementable interface for all three traits (Python: a class to subclass; Kotlin: an `interface`; Swift: a `protocol`), that throwing `MiniGrafUdfError` is declared, and note Swift's `Sendable` requirement if the protocol carries one. Put the findings in the README's §6.1 section (lifecycle, threading, cost, re-entry), which every binding copies.
 
 ## PRs 3–6: one per binding (python, java, android, swift)

@@ -108,18 +108,18 @@ pub enum MiniGrafUdfError {
 }
 impl From<uniffi::UnexpectedUniFFICallbackError> for MiniGrafUdfError { /* Failed { msg: reason } */ }
 
-#[uniffi::export]
+#[uniffi::export(with_foreign)]
 pub trait MiniGrafPredicate: Send + Sync {
     fn test(&self, value: MiniGrafValue) -> Result<bool, MiniGrafUdfError>;
 }
 
-#[uniffi::export]
+#[uniffi::export(with_foreign)]
 pub trait MiniGrafAggregate: Send + Sync {
     /// A new, empty accumulator: one per group, or per window partition.
     fn accumulator(&self) -> Result<Arc<dyn MiniGrafAccumulator>, MiniGrafUdfError>;
 }
 
-#[uniffi::export]
+#[uniffi::export(with_foreign)]
 pub trait MiniGrafAccumulator: Send + Sync {
     fn step(&self, value: MiniGrafValue) -> Result<(), MiniGrafUdfError>;
     /// `count`: values stepped so far.
@@ -133,7 +133,7 @@ impl MiniGrafDb {
 }
 ```
 
-- The traits are exported with plain `#[uniffi::export]`: UniFFI 0.31 lets foreign code implement an exported trait taken as `Arc<dyn Trait>`, and `with_foreign` is a deprecated alias for that. The plan checks that each target language (Python, Kotlin, Swift) generates an implementable interface.
+- The traits are exported with `#[uniffi::export(with_foreign)]`. Without it UniFFI 0.31.1 generates no callback interface: Python, Kotlin and Swift get only the Rust-backed class, and `register_*` accepts nothing else. (An earlier draft of this spec said plain `#[uniffi::export]` was enough. The template's bindgen check, plan step 16, showed it was not.) With it, Python subclasses `MiniGrafPredicate`, Kotlin implements `interface MiniGrafPredicate` (the Rust-backed class is `MiniGrafPredicateImpl`), and Swift conforms to `protocol MiniGrafPredicate: AnyObject, Sendable`.
 - The binding implements these with `try_register_predicate`/`try_register_aggregate`. `Acc` is `Arc<dyn MiniGrafAccumulator>`. `MiniGrafUdfError::Failed { msg }` becomes `UdfError::new(msg)`.
 - A `finish` that returns a `Ref` whose value is not a UUID is a failing call: ``ref value "<s>" is not a UUID`` (the same text `to_core` gives).
 - `count` is `i64`, following the bindings' rule that counts cross the FFI signed (Kotlin's unsigned types cannot be called from Java).