sx/current/PLAN-RACE.md

# PLAN-RACE — Stream B2/A1: `race` over the M:1 fiber scheduler

Carved from the async roadmap ([../design/execution-evolution-roadmap.md](../design/execution-evolution-roadmap.md)
§4.5, §4.6, §7 step 3). The headline A1 feature still missing after Stream B1: **`race`** — start N
async tasks, return when the FIRST completes, and **structurally** cancel + join the losers before
returning. The result is a synthesized tagged-union mirroring the input named tuple's labels.

```sx
fa := s.go(() -> A => read_a(conn));      // *Task(A)
fb := s.go(() -> B => read_b(conn));      // *Task(B)
winner := s.race((a: fa, b: fb));         // RaceResult = enum { a: A; b: B }
if winner == {
    case .a: (v) { handle_a(v); }         // v : A   (fb cancelled + joined)
    case .b: (v) { handle_b(v); }         // v : B   (fa cancelled + joined)
}
// positional form: s.race((fa, fb)) → tags ._0 / ._1
```

## Design decisions (grounded against the tree, 2026-06-26)

- **Built over the M:1 `Task` layer, NOT `context.io`/`Future`.** The suspending async is
  `sched.go`/`wait`/`cancel` over `*Task($R)` (B1.4a); `context.io.async`→`Future` is the BLOCKING
  impl (workers run inline → racing is meaningless there). The roadmap's "Future" maps to our
  `*Task`. `race` is a `Scheduler`/`Task` UFCS function in `library/modules/std/sched.sx`.
- **The result is a comptime-synthesized nominal tagged-union** (`RaceResult`), one variant per input
  tuple element: variant NAME = the tuple label (positional → `_0`/`_1`), payload = the task's result
  type. Synthesis uses the proven `declare`/`define`/`make_enum` + `field_count`/`field_name`/
  `field_type` reflection (examples 0619–0623, 0646). The input arrives as an inferred type PARAMETER
  `$T` (a named tuple of `*Task(..)`), which reflects correctly (issue 0195 fixed; the tuple-*alias*
  gap is issue 0196, NOT on this path).
- **Cancellation rides the existing cooperative `Task.cancel`** (sets the flag + `.canceled` state).
  `race` cancels every loser, then `wait`s each (joins) so no loser fiber outlives the `race` call —
  structured. Reuses `suspend_self`/`wake`; no new scheduler machinery.

## The one net-new compiler primitive (step 1)

**`pointee($P: Type) -> Type #builtin`** — given a pointer type `*X`, return `X`. This is the only
missing reflection capability: `race` must project each tuple element `*Task(A)` to its result type
`A`, and there is currently NO way to get a pointer's target type at comptime (`field_count(*X)`=0,
`type_info` has no pointer variant). With it the projection is pure sx:
```sx
TaskResult :: ($P: Type) -> Type {      // P = *Task(A)  →  A
    return field_type(pointee(P), 0);   // pointee → Task(A); field 0 = `value: A`
}
```
Small + generally useful (reflection is currently complete for aggregates but blind to pointer
targets). Mirror `field_type`'s `#builtin` plumbing (`src/ir/lower/call.zig` + `src/ir/calls.zig`),
backed by the pointer TypeInfo's pointee TypeId (`src/ir/types.zig`). Lock with a comptime example.

## Steps (each: implement → lock with an example → `zig build test` green → both platforms)

1. **`pointee` reflection builtin.** Add `pointee($P: Type) -> Type` (core.sx + compiler). Example:
   `pointee(*i64)` = `i64`, `field_type(pointee(*Task(i64)), 0)` = the task value type. (worker+review)
2. **`RaceResult($T) -> Type` synthesis.** Type-fn: reflect the named-tuple `$T` of `*Task(..)`,
   project each element via `TaskResult`, mint the tagged-union (labels → variants). Comptime-only
   example asserting the minted type's `field_count`/`field_name` match the input tuple.
3. **`Task.Value` projection + result construction.** Confirm a winner's value can be boxed into the
   minted variant by label/index (uses the existing variant-construction path).
4. **Runtime `race(tasks: $T) -> RaceResult(T)`.** Suspend the caller until the first task is
   `.ready`; build the winner variant; then cancel + `wait`-join every loser before returning.
   Single-winner (first by completion order; FIFO tiebreak). Example: 2 tasks, deterministic winner
   via `sleep` ordering (like 1817), asserting the loser is cancelled + joined.
5. **Positional tuple form** (`._0`/`._1`) + edge cases (already-ready task → immediate, single-task
   race, all-cancelled). Examples.
6. **Validate** every new example byte-identical on aarch64-macOS host AND aarch64-linux container;
   full `zig build test` green; adversarially review each step.

## Status

Prereqs DONE (each committed + adversarially reviewed + suite-green):
- **issue 0195** (tuple/array/vector field reflection) fixed (`8ac6c573`). Tuple reflection works on
  inline + `$T`-param forms. Issue 0196 (tuple *alias*) filed, not on the critical path.
- **`pointee($P) -> Type`** builtin added (`f1d29876`) — projects `*Task(A)` → `Task(A)`.
- **`field_count`/`size_of`/`align_of` fold as comptime constants** (`2a6ef398`) — so a generic
  `($T) -> Type` builder can `inline for 0..field_count(T)` and size `[field_count(T)]EnumVariant`.
  Verified: the variable-arity loop + array dim now work inside `RaceResult`.

**NEXT BLOCKER (step 2, not yet resolved): nested comptime-type-call composition.** Building each
variant payload needs `*Task(A) → A`, i.e. `field_type(pointee(field_type(T, i)), 0)`. Today:
- passing a `field_type(...)` RESULT as the type-arg to a generic (`TaskResult(field_type(T, i))`) →
  *"cannot infer generic type parameter 'P'"*;
- the same nested inline (`field_type(pointee(field_type(T, i)), 0)`) → *"cannot infer 'T' for
  field_type"*;
- a `::` alias bound to a comptime-type-call result (`P0 :: field_type(NT, 0)`) → *"unresolved type
  'P0'"* (kin to issue 0196).
So a comptime-type-call's Type result isn't usable as a generic type-arg / `::` alias / nested
type-call arg. This composition gap is the next thing to fix (or design around) before the variable-
arity `RaceResult` payloads can be built. Everything up to the payload projection works.