dea96bd66a
Plan the `race` async deliverable (roadmap A1): a structured first-wins over the M:1 `Task` layer, returning a comptime-synthesized tagged-union mirroring the input named tuple's labels. Identifies the one net-new compiler primitive needed — `pointee($P: Type) -> Type` (project `*Task(A)` → `A`); everything else (tuple reflection, union synthesis, the suspending runtime) is already in place. Issue 0196: a named-tuple type ALIAS (`NT :: Tuple(a: i64, b: bool)`) loses its structure — field access and reflection both fail on the alias, while the inline and `$T`-type-parameter forms work. Not on the race critical path (race reflects a tuple type parameter), filed for tracking.
74 lines
4.7 KiB
Markdown
74 lines
4.7 KiB
Markdown
# PLAN-RACE — Stream B2/A1: `race` over the M:1 fiber scheduler
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Carved from the async roadmap ([../design/execution-evolution-roadmap.md](../design/execution-evolution-roadmap.md)
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§4.5, §4.6, §7 step 3). The headline A1 feature still missing after Stream B1: **`race`** — start N
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async tasks, return when the FIRST completes, and **structurally** cancel + join the losers before
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returning. The result is a synthesized tagged-union mirroring the input named tuple's labels.
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```sx
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fa := s.go(() -> A => read_a(conn)); // *Task(A)
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fb := s.go(() -> B => read_b(conn)); // *Task(B)
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winner := s.race((a: fa, b: fb)); // RaceResult = enum { a: A; b: B }
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if winner == {
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case .a: (v) { handle_a(v); } // v : A (fb cancelled + joined)
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case .b: (v) { handle_b(v); } // v : B (fa cancelled + joined)
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}
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// positional form: s.race((fa, fb)) → tags ._0 / ._1
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```
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## Design decisions (grounded against the tree, 2026-06-26)
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- **Built over the M:1 `Task` layer, NOT `context.io`/`Future`.** The suspending async is
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`sched.go`/`wait`/`cancel` over `*Task($R)` (B1.4a); `context.io.async`→`Future` is the BLOCKING
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impl (workers run inline → racing is meaningless there). The roadmap's "Future" maps to our
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`*Task`. `race` is a `Scheduler`/`Task` UFCS function in `library/modules/std/sched.sx`.
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- **The result is a comptime-synthesized nominal tagged-union** (`RaceResult`), one variant per input
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tuple element: variant NAME = the tuple label (positional → `_0`/`_1`), payload = the task's result
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type. Synthesis uses the proven `declare`/`define`/`make_enum` + `field_count`/`field_name`/
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`field_type` reflection (examples 0619–0623, 0646). The input arrives as an inferred type PARAMETER
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`$T` (a named tuple of `*Task(..)`), which reflects correctly (issue 0195 fixed; the tuple-*alias*
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gap is issue 0196, NOT on this path).
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- **Cancellation rides the existing cooperative `Task.cancel`** (sets the flag + `.canceled` state).
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`race` cancels every loser, then `wait`s each (joins) so no loser fiber outlives the `race` call —
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structured. Reuses `suspend_self`/`wake`; no new scheduler machinery.
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## The one net-new compiler primitive (step 1)
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**`pointee($P: Type) -> Type #builtin`** — given a pointer type `*X`, return `X`. This is the only
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missing reflection capability: `race` must project each tuple element `*Task(A)` to its result type
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`A`, and there is currently NO way to get a pointer's target type at comptime (`field_count(*X)`=0,
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`type_info` has no pointer variant). With it the projection is pure sx:
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```sx
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TaskResult :: ($P: Type) -> Type { // P = *Task(A) → A
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return field_type(pointee(P), 0); // pointee → Task(A); field 0 = `value: A`
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}
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```
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Small + generally useful (reflection is currently complete for aggregates but blind to pointer
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targets). Mirror `field_type`'s `#builtin` plumbing (`src/ir/lower/call.zig` + `src/ir/calls.zig`),
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backed by the pointer TypeInfo's pointee TypeId (`src/ir/types.zig`). Lock with a comptime example.
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## Steps (each: implement → lock with an example → `zig build test` green → both platforms)
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1. **`pointee` reflection builtin.** Add `pointee($P: Type) -> Type` (core.sx + compiler). Example:
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`pointee(*i64)` = `i64`, `field_type(pointee(*Task(i64)), 0)` = the task value type. (worker+review)
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2. **`RaceResult($T) -> Type` synthesis.** Type-fn: reflect the named-tuple `$T` of `*Task(..)`,
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project each element via `TaskResult`, mint the tagged-union (labels → variants). Comptime-only
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example asserting the minted type's `field_count`/`field_name` match the input tuple.
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3. **`Task.Value` projection + result construction.** Confirm a winner's value can be boxed into the
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minted variant by label/index (uses the existing variant-construction path).
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4. **Runtime `race(tasks: $T) -> RaceResult(T)`.** Suspend the caller until the first task is
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`.ready`; build the winner variant; then cancel + `wait`-join every loser before returning.
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Single-winner (first by completion order; FIFO tiebreak). Example: 2 tasks, deterministic winner
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via `sleep` ordering (like 1817), asserting the loser is cancelled + joined.
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5. **Positional tuple form** (`._0`/`._1`) + edge cases (already-ready task → immediate, single-task
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race, all-cancelled). Examples.
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6. **Validate** every new example byte-identical on aarch64-macOS host AND aarch64-linux container;
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full `zig build test` green; adversarially review each step.
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## Status
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- **Step 0 (prereq) DONE:** issue 0195 (tuple/array/vector field reflection) fixed + committed
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(`8ac6c573`). Tuple reflection works on inline + `$T`-param forms. Issue 0196 (tuple *alias*) filed,
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not on the critical path.
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- **NEXT: Step 1 — the `pointee` builtin.**
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