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Component: syntax

Status: tested Last updated: 2026-08-02 · S48 B2 (review release blocker): a guard block mixed with statements and/or a tail is never filtered into a clean StageKind::Guard again — parse_classified_block retains the complete block as a StmtBlock (P0006 per mixed statement, P0006 now also names the tail case, P0115 out-of-Core), and lower rejects the stage loudly (L1000). The B2 probe { -true-> 1; -false-> 2; 999 } keeps its 999. · S29 time builtin: () now PARSES to ExprKind::Unit — the wire-less chain head, sole sanctioned use () -> time — replacing the P0001 "empty parentheses" rejection; the parser records the shape only, every other position is lower's L1301. · S22 ADR-0031: the ADR-0029 call-expression carve REMOVED — P0108 rejects every call expression uniformly again (iota(...)/fill(...) get the arrow-form teaching message: "write n -> iota / (x, n) -> fill"); ExprKind::Call remains as the rejection-recovery node only. S20 (carve, superseded) · S13 (ADR-0021 element-update bind c[i] <- x; P0013–P0015) Spec references: user-guide.md §3 (Syntax reference) + ADR-0005 (a flow is a statement; E4) + ADR-0009 (map/fold postfix block) + ADR-0010 (guard-arrow lexing) + ADR-0011 (Core loops are loop only) as amended by ADR-0012 (labeled blocks :label { … } / jumps -> :label;; Ident { is always a struct literal) + ADR-0019 (seq { … } is a statement block StageKind::SeqBlock, not a fanout kind; P0117) + docs/components/lower/plans/plan-time-builtin.md (() → ExprKind::Unit, the () -> time head). Supporting: architecture.md §2.2.1–§2.2.2 (lexer + recursive-descent parser, minimal parse tree), ADR-0008 (error recovery + structured diagnostics, binding), ADR-0006 (type keyword, category reserved). Depends on: (none) Depended on by: lower, cli

What works

  • P1 frontend complete: lexer + parser. lex(source) -> LexOutput and parse(source) -> ParseOutput (total, pure, error-recovering; merged span-sorted diagnostics).
  • Lexer (Session 02): full v0.2-surface token set, spans + LineIndex, L0001–L0008 with recovery, ADR-0010 single-lexeme guard arrows.
  • Parser (Session 03): two-tier grammar per ADR-0005 (expressions 1–7; ->/<- chains at statement level), thin spanned parse tree (DESIGN §15, Expr::Hole for operator shorthand), guard blocks/fanout/map/fold postfix blocks (ADR-0009), loop statements with -> loop; back-edges, struct/array/tuple literals, typed mut bindings both directions.
  • seq statement block (ADR-0019, Session 11): seq { … } is StageKind::SeqBlock(Block) — the ordinary block production (statements + optional tail), no longer a FanoutKind::Seq fanout. Rebinds/loops inside seq are first-class (no silent drop); the old bare-chain branch form still parses; guard arms in seq are stray guards (P0004).
  • Unit literal () (S29, time builtin): parse_paren_or_tuple builds ExprKind::Unit where it used to emit P0001 + Expr::Error. () -> time -> t0; parses with zero diagnostics and a Unit chain head. The parser does not police where () may appear — that is a chain-context question it cannot answer locally, so () as a value or heading anything but time is lower's L1301 (no wire).
  • Element-update bind c[i] <- x (ADR-0021, Session 13): BindStmt gained index: Option<Expr> — the optional [' expr '] after the name (parse_bind_stmt). Some marks the indexed (element-write) form; mut/type-annotation/nested-[…] on it draw P0013/P0014/P0015 with recovery. The desugar to Update is lower's job, not syntax's.
  • Diagnostics: P0001–P0015 (syntax/recovery incl. ADR-0010's two guard hints with machine-applicable fixes; P0013–P0015 guard the ADR-0021 element-update bind c[i] <- x — mut on an indexed target / a type annotation on one / a nested c[i][j] target, all recovered; P0001 no longer covers () — S29 made it the Unit literal; its other triggers, (e,)/[]/missing-;/…, are untouched) + P0101–P0117 (out-of-Core rejections, each naming construct + horizon; P0117 is a structural reject — a non-chain statement dropped from a fanout void block, ADR-0019). All eight examples/*.mapal parse with zero diagnostics; full_surface.mapal yields exactly {P0101×2, P0102×2, P0103×2, P0107, P0109, P0111, P0112, P0113} and zero L-codes — the C8 story end-to-end.
  • Recovery: panic-mode with sync sets, diagnostic cooldown, shared depth guard (128) over expressions/blocks/types, progress lemma — parse is total on adversarial input (≈490K-case proptest run during review).

What does not / known issues

  • Parser-level imprecision, deliberate (DESIGN §16/§17): expression-position generics surface as P0007/P0001 not P0103 (W16); filter recovery renders as map in the tree (P0114 diagnostic carries the name); stage-position P0006 statements are reported then dropped from the arm-only Guard node.
  • Semantic/scope checks deferred to mapal-check by design (C10): scalar-type validity, string-as-data, mut/arity/exhaustiveness, print placement, recursion, named-param partial application (W21).
  • Char literals still unlexed (' → L0001) — corpus-empty soft spot (DESIGN §10).

Invariants enforced (and where in code)

  • Lexer I1–I5 — unchanged (see Session 02 entry; lexer.rs, proptests).
  • J1 parse total: depth guard (P0011, limit 128, shared incl. type recursion) + progress lemma debug_asserts — parser.rs; proptests over arbitrary strings/unicode/flow-soup.
  • J2 span sanity (child ⊆ parent, in-bounds): debug_asserts in node constructors + recursive walkers in unit & property tests (incl. Bind.ty, MapFold params, arm/discr spans).
  • J3 zero diagnostics ⟹ no Error nodes / rejected-kept forms: walkers in goldens + proptests.
  • J4 acceptance: eight examples parse with zero diagnostics (tests/golden_trees.rs; zip_demo + vector_add added with ADR-0018's zip form).
  • J5 presentation-free: no Display in crate (C3); render helpers live in tests/support/.
  • J6 lex-diagnostic preservation: parse(s).diagnostics ⊇ lex(s).diagnostics — unit + proptest.

Test coverage (golden / property / differential / skipped+why)

201 tests, all green (cargo test -p mapal-syntax): 160 lib (102 parser units — precedence/§3.6 verbatim, W10–W24 ledger, F-matrix payloads, ADR-0011 scan, classification, the ADR-0019 seq statement-block suite [stmt-form/headless-compat/rebind+loop-kept/tail/empty/optional-;/guard-arm-P0004/fanout-P0117 + each-drop-P0117/unterminated-tail-branch-kept/seq-arm-mixed-P0006 regressions], the ADR-0021 element-update-bind suite [index_bind_top_level/index_bind_in_loop_body/index_bind_computed_index accept + index_bind_mut_rejected_p0013/index_bind_type_annotation_rejected_p0014/index_bind_nested_rejected_p0015], the ADR-0031 uniform-P0108 test iota_fill_calls_are_p0108_rejected_with_teaching_text — every call expression rejects, iota/fill with the arrow-form teaching text, and the arrow forms (4 -> iota, (1.0, 4) -> fill) parse clean, every P-code, design-review regression pins; 58 lexer) · 9 golden token streams · 10 golden parse trees (zero diags asserted; +zip_demo +vector_add, ADR-0018; +tree_unit_time_head, S29 — () -> time -> t0; parses clean and the chain head is asserted ExprKind::Unit before the snapshot, so a wrong-but-stable golden cannot hide a regression to P0001/Error) · 2+3+6 diagnostics/error/out-of-Core fixtures · 3 coverage · 3+3 proptests (lexer/parser, 2048 cases each). Golden trees were verified by independent re-derivation (one reviewer per example, node-by-node against source + grammar); implementation passed 2 adversarial reviews + a fix round (stack-overflow totality fix, P0007 climber path).

Performance notes (numbers + bench name + date; regressions flagged)

benches/lex_parse.rs (criterion 0.5.1), 2026-06-12, Apple-silicon dev machine: parse abs 1.24 µs · pipeline 1.07 µs · fanout 1.51 µs · sum_to_n 1.79 µs · fir 3.03 µs · sepia 7.45 µs · ~100× sepia synthetic 740 µs (≈100× single sepia ⇒ linear, no superlinear blowup from the ADR-0011 scan on real shapes). No baseline regressions to flag (first recording).

Open questions (→ ADR candidates)

  • ADR-0012 (decided with Sapir, Session 03): labeled blocks :label { … }, jumps -> :label;, enclosing-targets-only; Core+1 lifts P0110. Break-to-after-a-loop deliberately undecided — the Core+1 ADR must decide or re-defer.
  • P0115 scope reading: anonymous block stages (user-guide §8.3 fanout form, §5.2 seq branches) rejected as out-of-Core — HANDOFF §4.1 silence read as default-reject; flip = lift P0115.
  • W15 unary-minus/! precedence: §3.6 table omits unary; bound tighter than *, looser than postfix (standard). Spec gap, no ADR.
  • W23: ? parsed as expression postfix (per exhibits) not §3.6 rank 9 — final call belongs to the Core+1 error-handling ADR.
  • Lexer items unchanged (string escape set; Core+1 pattern guards).