Parser: spawn, select, and emit productions (§8.2, §8.6, §8.9, §11.1, §11.3, §16)

[StreamDemon]

Summary

spawn, select, and emit are reserved keywords with zero parse productions — every actor spawn (§8.2/§8.7), multiplexed receive (§8.6), non-actor task (§8.9), and event emission (§11.1/§11.3) fails today, including §4.6's own spawn move || { ... } example. This blocks #67 and #68: round-trip validation and corpus growth cover the spec's §8 examples, and nearly all of them spawn actors. priority/high is deliberate — this is milestone 1's exit criterion, not a peer of the remaining medium issues.

Grammar basis (§16, verbatim — no spec amendment needed)

expr        = ... | "spawn" expr | "spawn" "async" block | ... | select_expr | ... ;
select_expr = "select" "{" { select_arm } "}" ;
select_arm  = pattern "=" expr "=>" ( expr "," | block ) ;
emit_stmt   = "emit" IDENT "{" field_inits "}" ";" ;

Design (adversarially reviewed; see PR for the review-folded changes)

AST: ExprKind::Spawn(Box<Expr>), SpawnAsync { body: Block } (raw block like Loop — it's a block by production), Select { arms: Vec<SelectArm> } with SelectArm { pattern, source, body } (unboxed, source names the receive-source expr; body block-wrapped per MatchArm); Stmt::Emit { event: String, fields: Vec<(String, Option<Expr>)> } reusing field_inits shorthand.

Parser: spawn_expr() — operand via plain expr(0), not cond_expr() (spawn is not a block-head-restricted position; spawn Worker { count: 0 } must parse); select_expr() modeled on match_expr() (same arm loop, recovery, comma discipline; arms defer pattern? exactly like match_arm); emit as a block_inner() statement arm next to return (reserved keyword, unambiguous head). can_begin_expr gains Spawn/Select only — send spawn … / send select {…} open send-statements whose operands fail the method-call check (same consequence class as closures, pinned by test). Emit is statement-only: no prefix() arm.

Span discipline (explicit, per the #92 lesson): all new expressions anchor at their keyword token (expect_keyword span, never prev_span()-before-consume), select ends at its closing }, Stmt::Emit spanless per the Stmt enum convention. Pinned by a *_spans_anchor_correctly test.

Documented decisions:

• Parse-only — actor runtime semantics, deterministic select ordering, JoinHandle, and "emit only inside onchain mod" are milestone 3/7 (emit parses anywhere a statement goes, like break/continue outside loops today).
• spawn x |> f parses greedily as spawn (x |> f) (consistent with match scrutinees); §16 derives both groupings and no example disambiguates — noted on Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89.
• §8.6's _ = timeout(5000) => return Err(AppError::Timeout), is Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89 item 1 (return-as-expression) again — corpus adapts that arm to a block body; Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89 records the additional occurrences (§8.6, §8.8).

Scope / acceptance criteria

• All three productions + AST nodes, with the span/test decisions above.
• Unit tests (~16): structural asserts for all five node shapes; §8.2/§8.7/§8.2a spawn chains inside map; §4.6 spawn move || verbatim; §8.9 spawn async with typed let; §8.6 select (adapted return arm; wildcard + timeout(5000); mixed bodies); §11.1/§11.3 emit with to shorthand; send-head consequence; pipe pins (spawn (x |> f) greedy reading, select {…} |> f, pipe in arm body); struct-literal pins (spawn operand, select source); negatives (spawn alone, spawn async x, select arm missing =/=>, emit missing braces/semi, emit in expression position, unclosed select); error-recovery count; degenerate spawn { block }; non-tail ; behavior.
• Corpus fixture tests/corpus/spawn_select_emit.sp from §8.2/§8.2a/§8.6/§8.7/§8.9/§11.1/§11.3 examples (return arms adapted, noted).
• crates/AGENTS.md sync: the three keywords move to Accepted with parse-only caveats; four now-stale sentences updated (patterns-wired-into list gains select arms; send-head list gains spawn/select; non-tail-; set gains select/spawn async; the Spec drift: §5.2/§3.7 examples do not derive from the §16 grammar (return-as-expression, ref field-pat shorthand) #89 rejection note covers select arm bodies).

Filed from an adversarially-reviewed design draft (12 findings folded in, APPROVE-WITH-CHANGES).