# iter mut.2 — typecheck recognition of `Term::Mut` and `Term::Assign`

**Date:** 2026-05-15
**Started from:** 3a5b1d33472e5c35c4917702e30fa5a12b749e31
**Status:** DONE
**Tasks completed:** 7 of 7

## Summary

Replaces the mut.1 dispatch stubs in `synth` (`ailang-check`) for
`Term::Mut` and `Term::Assign` with real typecheck logic. Three new
`CheckError` variants ship (`MutAssignOutOfScope`,
`AssignTypeMismatch`, `UnsupportedMutVarType`) with kebab-coded
`code()` arms and structured `ctx()` payloads, following the
cli-diag-human bracketed-`[code]` convention. A new
`mut_scope_stack: &mut Vec<IndexMap<String, Type>>` parameter
threads through every `synth` call site (24 sites across lib.rs +
mono.rs:712 / mono.rs:1337 + lift.rs:723 + builtins.rs in two test
helpers); each entry-point caller (`check_fn`, `check_const`, mono
re-synth, lift letrec-capture synth) opens a fresh empty stack at
top-of-body. `Term::Var` resolution prepends the mut-scope check
ahead of the existing locals/globals chain (innermost-first wins on
shadowing). `Term::Mut` pushes a frame after gating each
`MutVar.ty` against `is_supported_mut_var_type` (Int / Float /
Bool / Unit), synths inits in the in-progress scope, unifies init
type with declared, and synths the body in the extended scope.
`Term::Assign` resolves the target name through the stack, returns
`MutAssignOutOfScope` (with a flat-deduplicated `available` list)
on miss, compares value type against the declared via
`subst.apply` + equality (manual to preserve the
`AssignTypeMismatch` diagnostic identity), and yields `Type::unit()`
on success. Five `.ail.json` fixtures + a 5-test driver
(`mut_typecheck_pin.rs`) cover each diagnostic plus the positive
nested-shadow case; the carve-out inventory at
`carve_out_inventory.rs` is extended 7 → 12 to accept them.
`examples/mut.ail` (the mut.1 six-fn round-trip fixture) now
typechecks clean via `ail check`. Codegen stubs unchanged —
`examples/mut.ail` still cannot run end-to-end; that lands in mut.3.
Full `cargo test --workspace`: 592 passed / 0 failed (was 579 at
start of mut.2; +13 = 5 driver pins + 7 lib.rs `mod tests` pins +
1 inventory test count unchanged — the inventory entry count went
from `seven` to `twelve` but the test itself was already counted).

## Per-task notes

- iter mut.2.1 — `CheckError` variants + `code()` + `ctx()`:
  Three new variants land before the `Internal` variant with
  thiserror `#[error("[code] message")]` attributes per the
  cli-diag-human convention. `code()` arms return the canonical
  kebab strings (`mut-assign-out-of-scope`, `assign-type-mismatch`,
  `mut-var-unsupported-type`). `ctx()` arms emit the structured
  payloads named in the plan (`{name, available}`,
  `{name, expected, actual}`, `{name, type}`). One in-crate pin
  (`mut_typecheck_diagnostics_have_kebab_codes`) gates the code()
  + ctx() shape; RED → GREEN observed.

- iter mut.2.2 — `synth` signature threads `mut_scope_stack`:
  New parameter `mut_scope_stack: &mut Vec<IndexMap<String, Type>>`
  sits immediately after `locals` (canonical mid-position; matches
  the existing convention for per-fn-body lexical state). 18
  recursive call sites in `lib.rs::synth` updated via
  pattern-by-pattern `replace_all` (each first-arg form had a
  unique-enough wrapper). Entry-point callers in `check_fn:1934`
  and `check_const:2687` declare a fresh empty stack. mono.rs's
  two re-synth sites at 712 + 1337 also declare a fresh empty
  stack (they begin from a top-of-body position). Plan-recon
  missed two more call sites: `lift.rs:723` (letrec-capture
  re-synth) and `builtins.rs` (two test helpers
  `synth_in_builtins_env` + a FloatPatternNotAllowed pin) — both
  required for build green; both get fresh empty stacks. The
  in-test mq.3 call site at `lib.rs:6663` also got the new
  parameter. Build was red structurally (E0061 at 23+ sites) until
  every call site was updated, then green.

- iter mut.2.3 — `Term::Var` prepends mut-scope resolution: A 5-
  line block at the top of the Var arm walks
  `mut_scope_stack.iter().rev()` innermost-first, returning the
  matched type cloned. Mut-vars are monomorphic per spec §"var
  element types" — no Forall instantiation, no FreeFnCall. Two
  pins (`mut_var_resolution_shadows_outer_local`,
  `mut_var_resolution_picks_innermost_frame`) directly populate
  the stack to isolate this change from the Mut-arm work in
  Task 4 (the plan's literal Let { Mut { ... } } shape would have
  hit the still-stubbed Mut arm at this stage; the property
  protected — mut-scope wins over locals + innermost-wins on
  shadowing — is identical). TDD discipline gap: the Var-arm edit
  landed before the pin tests were observed RED on disk; the
  reasoning is analytical (the pre-edit code consults
  `locals.get(name)` first, so with `locals = { x → Int }` the
  Var arm returned Int, failing the assert-Float; same logic for
  the two-frame test). Recorded as Concern.

- iter mut.2.4 — `Term::Mut` typecheck arm: Replaces the stub
  with real logic per plan Step 3. A file-scope helper
  `is_supported_mut_var_type(&Type) -> bool` gates each var's
  declared type to `Int | Float | Bool | Unit` (no args). Per-var
  loop synths the init in the outer scope plus the in-progress
  frame (the in-flight frame is cloned and pushed during the init
  synth, then popped — keeps `mut_scope_stack` truthful at every
  nested call), unifies init type with declared, inserts the
  (name → declared ty) entry into the frame. After all vars: push
  the completed frame, synth body in extended scope, pop frame
  (regardless of body outcome — the `body_result` is captured
  before pop). Two pins:
  `mut_block_typechecks_with_int_var_and_returns_int` (positive,
  initially RED because Assign was still stubbed — flipped GREEN
  by Task 5) and `mut_block_rejects_str_var_with_unsupported_type`
  (negative, GREEN at Task 4 close).

- iter mut.2.5 — `Term::Assign` typecheck arm: Replaces the stub
  with real logic per plan Step 3. Resolves the target name via
  `mut_scope_stack.iter().rev().find_map(|f| f.get(name).cloned())`.
  None case: flattens every name in every frame innermost-first,
  dedup-preserving-order via a manual `contains` check (small N,
  no need for a Set), returns `MutAssignOutOfScope`. Some case:
  synths the value, compares `subst.apply(&declared)` vs
  `subst.apply(&value)` via `!=` (manual equality, NOT `unify` —
  unify would have emitted `TypeMismatch`, and the spec demands
  the distinct `AssignTypeMismatch` code), returns
  `AssignTypeMismatch` with rendered type strings on mismatch or
  `Ok(Type::unit())` on match. Three pins:
  `assign_to_in_scope_mut_var_synths_unit` (positive),
  `assign_to_out_of_scope_name_fires_diagnostic` (None case),
  `assign_with_wrong_value_type_fires_diagnostic` (Some-mismatch
  case). RED → GREEN observed for all three.

- iter mut.2.6 — Five fixtures + driver test: Five `.ail.json`
  fixtures land under `examples/` (carve-out style, per the
  Boss decision in the carrier overriding the spec's literal
  `crates/ailang-check/tests/`-side placement). Each fixture is
  minimal — single-fn `main`, no imports, body shape exactly the
  one the plan named. The driver test
  `crates/ailang-check/tests/mut_typecheck_pin.rs` uses the
  canonical `load_workspace` + `check_workspace` +
  `diagnostics.iter().map(|d| d.code.clone()).collect()` pattern
  from `method_collision_pin.rs` and `typeclass_22b2.rs` — no
  `todo!()` survives. Carve-out inventory extended from seven to
  twelve to accept the five new fixtures (alphabetised; the
  comment block names mut.2 as the lineage). All 5 driver tests
  GREEN; carve-out inventory GREEN.

- iter mut.2.7 — Verify `examples/mut.ail` typechecks clean:
  `cargo run --quiet --bin ail -- check examples/mut.ail` reports
  `ok (26 symbols across 2 modules)` — zero diagnostics across
  all six fns (`mut_empty`, `mut_single_var`, `mut_two_vars`,
  `mut_nested_shadow`, `mut_returns_bool`, `mut_returns_unit`).
  Full `cargo test --workspace`: 592 / 0.

## Concerns

- Task 3 TDD-ordering gap: the `Term::Var` arm prepend landed
  before the two pin tests were observed RED on disk. The
  reasoning that the pre-edit code would have failed both pins
  is analytical (locals-first resolution returns Int when
  `locals = { x → Int }`, contradicting both `assert_eq!(Float)`
  and `assert_eq!(Bool)`). The discipline gap is recorded
  per the implementer's Iron Law ("the test you wrote in RED MUST
  pass; no other test may regress" — and the spirit-not-ritual
  clause that tests-after prove nothing about whether they would
  have caught the regression pre-edit). Subsequent tasks
  (4 + 5) restored the RED-first sequencing; on those, the Mut /
  Assign stubs already emit `CheckError::Internal` so the pins
  were observed RED with the canonical pattern. No correctness
  impact; the pins protect the property correctly post-edit.

- Plan-recon missed three synth call sites beyond the four the
  plan enumerated (lib.rs 1885 + 2634; mono.rs 712 + 1337). The
  build-red structural signal flushed them out via E0061:
  `lib.rs:6663` (in-test mq.3 helper), `lift.rs:723` (letrec-
  capture re-synth), `builtins.rs` (`synth_in_builtins_env` at
  351 + a FloatPatternNotAllowed test pin at 594). Each gets a
  fresh empty stack with the same "begins from top-of-body"
  rationale as mono.rs. No behavioural impact; documentary
  concern for the planner's next plan-recon dispatch.

- The carve-out inventory extension was strictly required (the
  inventory test enforces a closed list of `examples/*.ail.json`
  files) but was not explicitly named in the Boss carrier or the
  plan. Recording here so the planner's recon for future iters
  knows to enumerate the side-effect when fixtures land under
  `examples/`. The boundary call: the fixtures match §C4(a)'s
  precedent (negative typecheck fixtures as `.ail.json` so the
  diagnostic-code assertion is the load-bearing form, not the
  surface). The positive `test_mut_nested_shadow_legal.ail.json`
  is the one stretch — its load-bearing assertion is "zero
  diagnostics", which the form-A `.ail` form could equally
  carry. Landing it as `.ail.json` keeps all five mut.2 fixtures
  on one driver and one extension on the carve-out list; the
  alternative (splitting positive to `.ail`, negatives to
  `.ail.json`) would have produced two drivers or a hybrid
  loader. Conservative call documented for visibility.

## Known debt

- Codegen stubs unchanged in mut.2 per the spec's iteration
  boundary. A `Term::Mut` or `Term::Assign` reaching
  `lower_term` still produces `CodegenError::Internal`. The
  full mut.ail E2E (build + run) lands in mut.3. The spec's
  Acceptance criterion #3 (`mut_counter.ail` + `mut_sum_floats.ail`
  build + run + print expected stdout) is mut.3-scoped.

- The Mut-arm's in-progress-frame clone-on-each-init is O(n²)
  over the var count. n is realistically ≤ 2 (the mut.ail
  fixture's `mut_two_vars` is the largest in the corpus). Not
  worth a more elaborate structure (e.g. pushing a single in-
  progress frame and mutating it across inits); the clone-pop
  shape keeps the stack truthful at every nested call and makes
  the per-var init scope obvious. Recording for visibility.

## Files touched

Modified:
- `crates/ailang-check/src/lib.rs` — 3 new CheckError variants +
  3 code() arms + 3 ctx() arms; `synth` signature gains
  `mut_scope_stack` after `locals`; 18 recursive synth call
  sites updated; 2 entry-point callers (`check_fn`,
  `check_const`) declare fresh empty stacks; in-test synth call
  (mq.3 helper) gets new param; `Term::Var` arm prepends
  mut-scope resolution (5-line block); `Term::Mut` stub replaced
  with real logic (push/pop frame, per-var init, body synth);
  `Term::Assign` stub replaced with real logic
  (find/None-MutAssignOutOfScope/Some-AssignTypeMismatch-or-Unit);
  file-scope helper `is_supported_mut_var_type`; 7 new tests in
  `mod tests` (`mut_typecheck_diagnostics_have_kebab_codes`,
  `mut_var_resolution_shadows_outer_local`,
  `mut_var_resolution_picks_innermost_frame`,
  `mut_block_typechecks_with_int_var_and_returns_int`,
  `mut_block_rejects_str_var_with_unsupported_type`,
  `assign_to_in_scope_mut_var_synths_unit`,
  `assign_to_out_of_scope_name_fires_diagnostic`,
  `assign_with_wrong_value_type_fires_diagnostic`) plus the
  `synth_standalone` helper.
- `crates/ailang-check/src/mono.rs` — 2 re-synth call sites
  (712, 1337) each get a fresh `mut_scope_stack`.
- `crates/ailang-check/src/lift.rs` — letrec-capture re-synth at
  :723 gets a fresh `mut_scope_stack`.
- `crates/ailang-check/src/builtins.rs` — 2 in-test synth calls
  (synth_in_builtins_env + FloatPatternNotAllowed pin) get a
  fresh `mut_scope_stack`.
- `crates/ailang-core/tests/carve_out_inventory.rs` — EXPECTED
  list 7 → 12, comment-block updated naming mut.2 as the
  lineage of the 5 new entries.

New:
- `crates/ailang-check/tests/mut_typecheck_pin.rs` — 5-test
  driver via canonical load_workspace + check_workspace +
  collect-codes pattern.
- `examples/test_mut_assign_out_of_scope.ail.json`
- `examples/test_mut_assign_outside_mut.ail.json`
- `examples/test_mut_assign_type_mismatch.ail.json`
- `examples/test_mut_nested_shadow_legal.ail.json`
- `examples/test_mut_var_unsupported_type.ail.json`

## Stats

bench/orchestrator-stats/2026-05-15-iter-mut.2.json
