94 lines
6.4 KiB
Markdown
94 lines
6.4 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: $T) -> RaceResult(T)`.** Suspend the caller until the first task is
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`.ready`; build the winner variant; then cancel + `wait`-join every loser before returning.
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Single-winner (first by completion order; FIFO tiebreak). Example: 2 tasks, deterministic winner
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via `sleep` ordering (like 1817), asserting the loser is cancelled + joined.
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5. **Positional tuple form** (`._0`/`._1`) + edge cases (already-ready task → immediate, single-task
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race, all-cancelled). Examples.
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6. **Validate** every new example byte-identical on aarch64-macOS host AND aarch64-linux container;
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full `zig build test` green; adversarially review each step.
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## Status
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Prereqs DONE (each committed + adversarially reviewed + suite-green):
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- **issue 0195** (tuple/array/vector field reflection) fixed (`8ac6c573`). Tuple reflection works on
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inline + `$T`-param forms. Issue 0196 (tuple *alias*) filed, not on the critical path.
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- **`pointee($P) -> Type`** builtin added (`f1d29876`) — projects `*Task(A)` → `Task(A)`.
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- **`field_count`/`size_of`/`align_of` fold as comptime constants** (`2a6ef398`) — so a generic
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`($T) -> Type` builder can `inline for 0..field_count(T)` and size `[field_count(T)]EnumVariant`.
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Verified: the variable-arity loop + array dim now work inside `RaceResult`.
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- **comptime type-call composition** fixed (`eb18bbc6`) — a `field_type(...)`/`pointee(...)` result is
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now usable as a `Type`-typed struct-field value, a generic `$P: Type` arg, and a nested type-call
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arg (incl. with an `inline for` loop-var index). The **variable-arity `RaceResult` synthesis works
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end-to-end** (proven by `examples/comptime/0649-comptime-typecall-composition.sx`: reflect a named
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tuple of `*Box(..)` handles → mint a tagged-union with the tuple's labels, projecting `*Box(A)`→`A`).
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**All comptime prerequisites are now in place — the rest is pure-sx library work in sched.sx.**
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**NEXT (step 2–4): the actual `race` in `library/modules/std/sched.sx`.**
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- `RaceResult :: ($T) -> Type` over `*Task(..)` (the 0649 shape, with `Task` instead of `Box`).
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- `race :: ufcs (self: *Scheduler, tasks: $T) -> RaceResult(T)`: suspend the caller until the FIRST
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task is `.ready`; build the winner variant (construct the minted union by the winning index/label —
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watch for a possible variant-construction-by-dynamic-index gap); then `cancel` + `wait`-join every
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loser before returning (structured). Reuses `suspend_self`/`wake` + `Task.cancel`/`wait`.
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- Lock with a 2/3-task example (deterministic winner via `sleep` ordering, asserting losers cancelled).
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- Validate byte-identical on aarch64-macOS host AND aarch64-linux container; full `zig build test`.
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