Function Values v2: generic collection callbacks

Status: implemented private callback profile; HOSTED GREEN under the v0.4.0 release baseline.

Audience: language users, compiler contributors, collection-adapter authors, and backend implementers.

This additive prerequisite advances reusable collection adapters. It does not introduce iterator objects, consuming next, captures, or a public callable ABI. Those implemented iterator and scalar-closure additions retain their separate contracts rather than changing this callback profile.

Scoped callable signature

The existing private, single-parameter compiler collection profile may receive value parameters of type fn(T) -> T, fn(T) -> bool, or another signature of zero through eight scalar leaves. Each leaf is an existing Copy scalar or the same declaration's sole type parameter. The function must otherwise satisfy the existing Box<T>/Vec<T> collection profile. Callable return slots and callable generic argument substitutions remain closed.

Every concrete substitution independently checks the exact scalar callable signature from Function Values v1. References still identify monomorphic, effect-free local declarations; generic target references remain unsupported. Callables carry no environment or owner in this profile. Invocation evaluates arguments once, left to right, and propagates the selected target's ordinary checked failures.

Reusable adapters

A private map<T>(own Vec<T>, fn(T)->T)->Vec<T> can snapshot length, allocate one bounded output vector, and call the callback once per element in ascending index order. filter<T>(own Vec<T>, fn(T)->bool)->Vec<T> preserves accepted order. fold<T>(own Vec<T>, T, fn(T,T)->T)->T threads an accumulator from left to right. These are ordinary source functions assembled from existing vector operations, not new intrinsic names or authenticated standard-library aliases.

The collection template profile additionally preserves mutable local assignment and bounded while statements through generic materialization. Template replay checks exact scoped types, statement paths and target identities. Each concrete instance then undergoes ordinary HIR, loop, ownership, cleanup and backend checks. A failed callback retains its status and settles input/output owners using the existing canonical cleanup plan. Empty input executes no callback.

Projections and boundaries

Canonical source retains authored function types and generic calls. Materialized HIR retains genuine invocation operands and exact concrete signatures; Graph v36 includes the generic instance and compatible callable target facts together. Unmaterialized callable templates also select Graph v36. The runtime target universe scans ordinary functions and materialized instance bodies, returning only monomorphic reference targets; a template is not an additional executable instance. Candidate cycle replay uses exact instance identities. Native uses typed context/status/output function pointers. The Wasm aggregate lane uses its existing status/output-pointer ABI in the indirect-call table and snapshots the callable and each argument before evaluating the next operand. No new prelude operation, host import, vector layout, public descriptor, or workspace export is introduced. Function Values v1 programs retain their existing representation.

Focused language evidence lives in function_values_generic; all-scalar adapter execution and ownership settlement belong to the owned-data generic collection harness. Success, empty input, callback failure and hostile template mutation have hosted-green release evidence. Native allocation accounting and Wasm generation tracking require zero live owners after repeated success and callback failure. Earlier local executions retain their original scope.

Public package use and general iterator interfaces remain unfinished. Scalar captures and generic/loop closure construction are implemented in Closures v1 and v2; consuming traversal and generic iterator operations likewise have their own versioned profiles.

SemanticProgram v4 also retains checked source graphs for unused generic callback templates omitted from executable reachability. Its focused workspace check covers exact replay and comment-only semantic stability; the canonical workspace contract owns that additive projection. It grants no runtime reachability or authority.