# iter it.1 — loop/recur additive (parse/print/prose/check/codegen)

**Date:** 2026-05-15
**Started from:** 7381a4233b0ea34b23871c59e3a3e802441751ef
**Status:** DONE
**Tasks completed:** 8 of 8

## Summary

First of three iterations in the iteration-discipline milestone.
Adds `Term::Loop` / `Term::Recur` / `LoopBinder` as first-class
additive AST nodes end-to-end — parse, print, prose projection,
canonical-JSON serde + round-trip, schema/spec-drift lockstep,
typecheck (binder typing, recur arity/type unification via a
`loop_stack: &mut Vec<Vec<Type>>` threaded exactly as mut.2 threaded
`mut_scope_stack`, recur tail-position via a new private
`verify_loop_body` helper with `verify_tail_positions`'s public
signature unchanged), and codegen (loop-header block with one phi
per binder, `recur` as a back-edge `br` with a NEW parallel
`block_terminated = true` setter). The change is **strictly
additive**: zero deletions touch the existing `tail-app`/`tail-do`
paths, `verify_tail_positions`' tail-app role, or any of the seven
existing codegen `block_terminated` sites — the 32 within-iter
deletion-lines are exclusively the Task-1 `Internal(...)` stubs and
the Task-1 `verify_tail_positions` structural-passthrough being
replaced by their real Task-5/Task-7 implementations (DD-3
stub-then-fill). This additive discipline is what makes the
destructive it.3 safe later. Full `cargo test --workspace` green
(63 ok result groups, 0 failed); `loop_counter.ail` builds and
prints `55`; the four `Recur*` diagnostics fire on their negatives;
`tail-app` fixtures (`bench_compute_intsum`, `list_map_poly`) run
byte-identically; zero IR/prose snapshot drift.

## Per-task notes

- iter it.1.1: AST variants + every walker arm + serde/schema
  lockstep. `Term::Loop { binders: Vec<LoopBinder>, body: Box<Term> }`,
  `Term::Recur { #[serde(default)] args: Vec<Term> }`, and
  `LoopBinder { name, #[serde(rename="type")] ty, init: Box<Term> }`
  (DD-2 derives = `Debug, Clone, Serialize, Deserialize`, no
  PartialEq; note `init` is `Box<Term>` per the plan's literal code
  block, whereas `MutVar.init` is bare `Term` — a small divergence
  from "mirrors MutVar exactly", recorded under Concerns). ~40
  walker arms across desugar (incl. 3 in-test `any_*` helpers not
  enumerated by the plan), workspace, all of ailang-check
  (lib/lift/linearity/mono/pre_desugar_validation/reuse_shape/
  uniqueness), codegen (escape×3/lambda/lib), prose, surface
  parse/print, ail main + the test-side
  `codegen_import_map_fallback_pin.rs` walker the plan flagged
  (mut.1 missed its analogue; did not miss it here). Typecheck +
  codegen *dispatch* stubbed `Internal(...)` (tuple variant —
  the plan pseudo-code's `Internal { msg }` struct form does not
  exist; copied mut.1's verbatim tuple shape). `synth_with_extras`
  got real arms (loop → body's type, recur → unit) per plan.
  schema_coverage left RED on "variant never observed" until
  Task 3 (plan-expected); every other test green.

- iter it.1.2: Form-A parse. `parse_loop` / `parse_recur` modelled
  on `parse_mut` / `parse_assign`; the binder list is an explicitly
  parenthesised group `(loop ((var ...)*) BODY+)` (form_a.md
  production added in Task 1.10 lockstep). Plan pseudo-code named
  non-existent helpers (`parse_term_str`, `parse_body_seq`,
  `parse_loop_binders`); substituted the real harness (module-level
  `parse_term`) and inlined the per-`(var …)` decode exactly as
  `parse_mut` does — the plan explicitly forbade introducing a new
  body-folding helper. EBNF prologue + unknown-head error list
  extended. RED→GREEN confirmed.

- iter it.1.3: Form-A print + positive fixtures. Print arms landed
  early in Task 1 as a build-unblock necessity (documented mut.1
  precedent — print arms are exhaustive-match neighbours of the
  synth/lower_term dispatchers). `loop_smoke.ail` /
  `loop_nested_in_lambda.ail` written in real Form-A matching
  `mut_counter.ail`'s verbatim surface spellings (`(con Int)`,
  `(app == …)`, `(app + …)`, `(lam (params (typed …)) (ret …)
  (body …))`) — the plan's fixture pseudo-code (`(fn count_to :
  Int -> Int …)`) is not valid Form-A and the plan's prose
  explicitly directed matching mut_counter.ail verbatim.
  parse→print→parse byte-idempotent; schema_coverage now GREEN
  (Task 1.11 deferred RED closed).

- iter it.1.4: Prose projection lockstep. Step 4.1's literal
  instruction ("render to prose and back, asserting AST equality")
  is **not expressible** — AILang has no Form-B→AST parser by
  design (CLAUDE.md / `crates/ail/src/main.rs:207` "no parser
  exists for the prose surface"). Recorded as a plan defect (see
  Concerns). The substantive work (render/free-var/subst arms,
  landed in Task 1) is correct and verified by the full prose
  suite; added `loop_and_recur_project_to_prose`, a render-assertion
  pin using the same `render_term` harness the nearest existing
  prose tests (`not_with_tail_flag_keeps_prefix_form`) use — the
  feasible equivalent of the plan's intent. This is the documented
  mut.1-class plan-pseudo-vs-real-API substitution, not a silent
  swap. Status DONE_WITH_CONCERNS.

- iter it.1.5: Real typecheck (DD-1). Four `CheckError` variants
  (`RecurOutsideLoop` / `RecurArityMismatch` / `RecurTypeMismatch`
  / `RecurNotInTailPosition`) — **no `span` field** because no
  `Span` type is in scope in this crate and no existing CheckError
  variant carries one (the plan pseudo-code's `span: Option<Span>`
  references a non-existent-here type; mirrored the real
  `MutAssignOutOfScope` field-shape exactly as the plan's prose
  instructed). `loop_stack: &mut Vec<Vec<Type>>` threaded through
  `synth`'s signature + every call site (signature + 22 single-line
  recursive + 3 multi-line recursive + 2 production entry points +
  lift.rs + 2 mono.rs + builtins.rs test helpers + 8 in-source
  `#[cfg(test)]` calls), exactly the mut.2 pattern. Real Loop arm
  binds binder names into `self`-… `locals` (save/restore like
  `Term::Let` — the correct precedent for *immutable* bindings,
  not the mut-var mechanism) and pushes the binder type-vector onto
  `loop_stack`. **`Term::Recur` synth returns a fresh metavar
  (`Subst::fresh(counter)`), not the plan's flagged-risky
  `Type::unit()`** — see Concerns; this resolves the plan's own
  self-review "Open risk" correctly (a fresh metavar is the
  codebase-idiomatic bottom that unifies anywhere, which is what
  makes `(if c then (recur …))` typecheck — `Type::unit()` would
  have broken `loop_smoke`'s if-branch unification). `verify_loop_body`
  added as a new private helper; `verify_tail_positions`' public
  signature unchanged (it.3 reworks it). The diagnostic-doc list in
  `diagnostic.rs` was NOT extended — the mut codes were never added
  there (false plan premise; followed the actual mut.2 precedent).
  RED→GREEN: loop_smoke clean, four negatives each return their
  exact code. `loop_stack` threading caused zero regressions.

- iter it.1.6: Carve-out inventory. EXPECTED extended with the four
  `test_recur_*.ail.json` negatives, 13→17, in an it.1 group
  (the const is milestone-grouped not alphabetical; the test uses
  a BTreeSet so order is functionally irrelevant). GREEN.

- iter it.1.7: Real codegen (loop-header + phi + recur back-edge +
  lambda boundary). `loop_frames: Vec<LoopFrame>` field added (mut.3
  analogue to `mut_var_allocas` — init/clear/lambda-save-reset-
  restore symmetric). Loop lowers to `loop.header.<id>` with one
  `phi` per binder; binder phi SSA pushed into `self.locals` so
  `Term::Var` resolves to it via the existing lookup. Each phi's
  back-edge incoming list is emitted as a unique `/*RECUR_EDGES…*/`
  LLVM-comment placeholder, then `replacen(…, 1)`-patched once the
  body and all its recur sites are lowered (the plan's
  `patch_loop_phis` mechanism; the placeholder is a comment so an
  unpatched one would be inert IR, not a syntax error). `Term::Recur`
  records `(pred_block, [arg_ssa])`, emits the back-edge `br`, sets
  `block_terminated = true` — a NEW parallel setter; the seven
  existing tail-driven setters are untouched (verified: zero
  deletions touch them). Reused `start_block`/`fresh_ssa`/`fresh_id`/
  `self.current_block` instead of adding the plan-pseudo's sprawl of
  single-use helper methods (plan explicitly: "REUSE … grep before
  adding"). `loop_counter.ail` builds + prints `55`.

- iter it.1.8: tail-app coexistence + IR snapshots + acceptance.
  `bench_compute_intsum.ail` → `3500003500000`,
  `list_map_poly.ail` → `2\n3\n…` (byte-identical to pre-it.1; no
  existing codegen path modified). Zero IR/prose snapshot drift (no
  blanket regen). All it.1 acceptance bullets verified — the spec's
  "five Recur diagnostics" is four in it.1 (the fifth,
  `NonStructuralRecursion`, is it.2; resolved per plan Step 8.3).

- Phase 3 (E2E): added `loop_in_lambda_e2e.ail` + e2e
  `loop_in_lambda_runs_and_prints_49` — a `loop` inside a `lam`
  body invoked via a returned closure, proving the lambda-boundary
  `loop_frames` save/restore is codegen-sound (the no-`main`
  `loop_nested_in_lambda.ail` parse/print fixture cannot reach
  codegen via the CLI; this runnable fixture closes that gap).

## Concerns

- `LoopBinder.init` is `Box<Term>` whereas the analogous
  `MutVar.init` is bare `Term`. Per the plan's literal Step 1.1
  code block (which specified `init: Box<Term>`), not implementer
  drift. DD-2 says "mirrors MutVar exactly" — the divergence is in
  the plan's own pseudo-code vs. its own DD-2 prose. Functional
  impact: none (consistent `(*b.init).clone()` / `Box::new(...)` /
  `&mut b.init` handling everywhere). A future iter that wants
  strict parity could unbox; no it.1 test depends on the boxing.

- `Term::Recur` synth result: chose `Subst::fresh(counter)` (a
  fresh metavar) over the plan's named fallback `Type::unit()`.
  The plan's self-review flagged this exact point as an "Open
  risk" with `Type::unit()` as the fallback "because recur is
  always in tail position so its value is never consumed". But
  `recur` frequently appears as an `if` branch (`loop_smoke`:
  `(if (== i n) i (recur …))`), where the branch types MUST
  unify — `unify(Int, unit)` fails, so `Type::unit()` would have
  broken `loop_smoke`. A fresh metavar unifies with any type
  (codegen-idiomatic bottom; the same mechanism `Subst::fresh`
  serves everywhere). This is the correct resolution of the plan's
  own flagged risk, not a deviation from its intent.

- Step 4.1 plan defect: it asks for a prose round-trip asserting
  AST equality; AILang has no Form-B parser by design. Substituted
  a feasible render-assertion pin (mut.1-class plan-pseudo-vs-real
  substitution). The plan's *substance* (prose lockstep for the new
  variants) is fully met. Boss may want to tighten the planner
  template so it does not script prose round-trips.

- Step 5.2 plan premise defect: "Add the four code strings to the
  diagnostic-doc list … same list the mut codes were added to" —
  the mut codes were never added to `diagnostic.rs`'s doc list.
  Followed the actual mut.2 precedent (not extended); the
  authoritative registry is `CheckError::code()`, which has all
  four.

- The plan's recon line numbers (e.g. `desugar.rs:349/559/…`,
  `lib.rs:2590/2613/3593/3614`, `parse.rs:1212`) had drifted
  vs. the live tree; drove every site off `cargo build`'s
  E0004 enumeration as Step 1.2 instructs. Site *count* also
  differed (6 desugar E0004 sites vs. plan's 9; resolved via the
  build, which is authoritative). No behavioural impact —
  same class as mut.1's documented recon misindexing.

## Known debt

- Prose-side `(loop …)` / `(recur …)` rendering is a
  minimal-correctness shape (`loop { var n = init; …; body }` /
  `recur(a, b)`), explicitly mirroring the deliberately-placeholder
  mut-block prose shape. The prose surface for loops is not yet
  designed; a follow-on prose iter refines it once the LLM-author
  signal arrives. Not drift — recorded for visibility.

- `loop_nested_in_lambda.ail` has no `main`, so it is a
  parse/print/typecheck fixture only (CLI codegen needs a `main`).
  Codegen-of-loop-in-lambda is instead proven by the runnable
  Phase-3 `loop_in_lambda_e2e.ail`. No gap remains; recorded so a
  future reader does not mistake the no-main fixture for dead
  coverage.

## Files touched

AST/core: `crates/ailang-core/src/ast.rs`,
`crates/ailang-core/src/desugar.rs`,
`crates/ailang-core/src/workspace.rs`,
`crates/ailang-core/specs/form_a.md`.
Drift/coverage tests: `crates/ailang-core/tests/{design_schema_drift,
schema_coverage,spec_drift,carve_out_inventory}.rs`.
Check: `crates/ailang-check/src/{lib,lift,linearity,mono,
pre_desugar_validation,reuse_shape,uniqueness,builtins}.rs`,
`crates/ailang-check/tests/loop_recur_pin.rs` (new).
Codegen: `crates/ailang-codegen/src/{lib,escape,lambda}.rs`.
Surface/prose: `crates/ailang-surface/src/{parse,print}.rs`,
`crates/ailang-prose/src/lib.rs`.
CLI: `crates/ail/src/main.rs`,
`crates/ail/tests/{e2e,codegen_import_map_fallback_pin}.rs`.
Spec: `docs/DESIGN.md`.
Fixtures (new): `examples/loop_smoke.ail`,
`examples/loop_nested_in_lambda.ail`, `examples/loop_counter.ail`,
`examples/loop_in_lambda_e2e.ail`,
`examples/test_recur_{outside_loop,arity_mismatch,type_mismatch,
not_in_tail_position}.ail.json`.

## Stats

bench/orchestrator-stats/2026-05-15-iter-it.1.json
