First iteration of the mut-local milestone (foundation step on the
Stateful-islands roadmap path). Lands the schema + surface tier:
Term::Mut, Term::Assign, and the nested MutVar struct become
first-class AST nodes that round-trip cleanly through Form A.
Typecheck and codegen recognition are deferred to mut.2 and mut.3
per the spec's out-of-iteration boundary; reaching either dispatch
entry point with these variants produces CheckError::Internal /
CodegenError::Internal with a 'deferred to iter mut.{2,3}' message.
Concretely:
- crates/ailang-core/src/ast.rs: two new Term variants behind
#[serde(tag = 't')]; pub struct MutVar { name, ty, init } adjacent
to Arm. Two canonical-bytes pin tests for the explicit-empty-vars
serialisation and the assign round-trip.
- ~25 substantive Term-walker arms across ailang-core/desugar,
ailang-core/workspace, ailang-check (lib + lift + linearity + mono
+ pre_desugar_validation + reuse_shape + uniqueness),
ailang-codegen (escape + lambda + lib), ailang-prose, and
crates/ail/src/main.rs. Universal policy: substantive recurse-into-
children at every site; only the two dispatch entry points
(synth in ailang-check, lower_term in ailang-codegen) stub with
Internal-error. One test-side walker arm in
crates/ail/tests/codegen_import_map_fallback_pin.rs not
enumerated by the plan was added as well (defensive recursion).
- ailang-surface: parse_mut + parse_assign helpers; Term::Mut
body desugared from a flat statement sequence into a right-folded
Term::Seq chain inside the JSON-AST. Print arms in print.rs match
the parser convention. EBNF prologue + crates/ailang-core/specs/
form_a.md productions updated. Four new parser pin tests cover
the empty-mut, single-var, body-required, and vars-only-no-body
cases.
- Drift + coverage tests extended: design_schema_drift.rs adds two
exemplars + match arms; schema_coverage.rs adds two VariantTag
entries + EXPECTED_VARIANTS + visit_term arms; spec_drift.rs adds
two exemplars + match arms. DESIGN.md §'Term (expression)' gets
jsonc-blocked schemas for the two new variants.
- examples/mut.ail: six-fn round-trip fixture exercising empty mut,
single-var, two-var, nested-shadow, and the four supported scalar
return types (Int, Float, Bool, Unit). The round_trip auto-glob
and schema_coverage corpus walker both pick it up.
Plan deviation: the plan named lib.rs:2572 as the typecheck
dispatch stub site, but that line is actually verify_tail_positions
(substantive walker). The real dispatch is synth (3403-area, stub
at 3489); the orchestrator routed correctly.
Tests: 564 → 579 green; cargo build green; round-trip green for
the new fixture; all drift + coverage tests green.
Journal: docs/journals/2026-05-15-iter-mut.1.md.
Refs: docs/specs/2026-05-15-mut-local.md, docs/plans/2026-05-15-iter-mut.1.md.
Brummel
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