# iter 24.3 — fn print polymorphic free fn + E2E + DESIGN.md sync; milestone 24 close

**Date:** 2026-05-13
**Started from:** c04c07f
**Status:** DONE
**Tasks completed:** 8 of 8

## Summary

Ships `fn print : forall a. Show a => (a borrow) -> () !IO` in
`examples/prelude.ail.json` with explicit-let body
`\x -> let s = show x in do io/print_str s`. The let-binder is
structurally pinned by `crates/ail/tests/print_mono_body_shape.rs`
(post-mono `print__Int.body` is `Term::Let → Term::App(show__Int) →
Term::Do(io/print_str)`). Three new E2E fixtures + tests verify the
full path: `show_print_smoke` (4 primitives) and `show_user_adt`
(`data IntBox` + `instance prelude.Show IntBox` + `print (MkIntBox 7)`)
both compile, run, and produce expected stdout; `show_no_instance`
fires a Show-aware `NoInstance` diagnostic with the canonical
DESIGN.md cross-reference. DESIGN.md §"Prelude (built-in) classes"
flips `print` from "ships in 24.3" to "shipped in iter 24.3" with the
explicit-let body + pin reference. DESIGN.md §"Float semantics" gains
a Show-Float NaN-spelling cross-reference paragraph. Roadmap P1
"Post-22 Prelude — Show + print rewire" entry checked off; new P2
entry "Retire `io/print_int|bool|float` effect-ops + migrate example
corpus to `print`" inserted at top of P2. **Three plan-defects-fixed-
inline** surfaced when running the user-ADT E2E: (a) `MonoTarget::
FreeFn::type_args` were carrying bare type-cons references that broke
the post-mono walk's registry lookup when the synthesised body lived
in a different module from the type — fixed by normalising via
`workspace_registry.normalize_type_for_lookup` at two
`mono.rs` collection sites; (b) codegen's cross-module fn resolution
went through `import_map`, but post-mono synthesised bodies may
reference modules their source template didn't import — fixed by
falling back to direct `module_user_fns` lookup at three codegen
sites (`resolve_top_level_fn`, `lower_app` cross-module arm,
`synth_with_extras` Var arm); (c) the synth FreeFnCall arm didn't push
residuals for declared constraints, so `Show (Int -> Int)` never
reached the typecheck-time discharge logic — fixed by walking
`Type::Forall.constraints` at the FreeFnCall site and pushing one
residual per constraint with the fresh-metavar substitution. Full
`cargo test --workspace` 556 passed / 0 failed (was 552 post-24.2,
+4 new = 1 print_mono_body_shape + 2 show_print_e2e + 1
show_no_instance_e2e). `bench/cross_lang.py` clean; `bench/compile_check.py`
+ `bench/check.py` exit 1 with documented noise-class metrics per the
audit-cma lineage envelope (baseline pristine, 8th consecutive
audit-grade observation). Milestone 24 closes structurally; only the
audit pipeline remains.

## Per-task notes

- iter 24.3.1: Appended `fn print` to `examples/prelude.ail.json`
  after `fn ge` (last def pre-edit). 20 defs total (was 19). Body
  is the literal `Term::Let { name: "s", value: App(show, [x]), body:
  Do(io/print_str, [s]) }` with the constraint `Show a` bare per
  mq.1 same-module canonical form. 552 tests stay green.
- iter 24.3.2: `crates/ail/tests/print_mono_body_shape.rs` asserts
  post-mono `print__Int.body` shape. Initial test asserted outer
  `Term::Lam` wrapper — corrected after the panic surfaced that
  `FnDef.body: Term` is the *inner* function body for top-level fns
  (params carried separately on `FnDef.params`). The test now asserts
  directly `Term::Let → App(show__Int, [x]) → Do(io/print_str, [s])`,
  faithful to the plan's intent (preserve the explicit-let-binder
  discipline) while matching the actual AST shape.
- iter 24.3.3: `examples/show_print_smoke.ail.json` written with
  nested `Term::Seq` of four `App(print, [lit])` calls (Int 42, Bool
  true, Str "hello", Float bits=40091eb851eb851f for 3.14). `ail
  build` clean; stdout: `42\ntrue\nhello\n3.14` — io/print_str
  appends a newline via `@puts`.
- iter 24.3.4: `examples/show_user_adt.ail.json` written + 2-test
  `crates/ail/tests/show_print_e2e.rs`. Surfaced three substantive
  plan-defects-fixed-inline (Concerns below); after fix, both E2E
  tests pass (`print (MkIntBox 7)` → stdout "7").
- iter 24.3.5: Extended NoInstance Float-aware arm in
  `crates/ailang-check/src/lib.rs:770-779` with a parallel
  `class == "prelude.Show"` branch. The arm fires for ANY type
  lacking a Show instance (function types, user types without
  instance) — cross-references DESIGN.md §"Prelude (built-in)
  classes" verbatim. Negative fixture `examples/show_no_instance.ail.json`
  (let-bound `f : Int -> Int` + `print f`) and pin test in
  `crates/ail/tests/show_no_instance_e2e.rs` (asserts code,
  Show-substring, Prelude-(built-in)-classes substring). After
  the constraint-residual-push fix in synth (Concerns below), the
  diagnostic fires at typecheck.
- iter 24.3.6: DESIGN.md §"Prelude (built-in) classes" milestone-24
  paragraph amended: `print` flipped to past tense ("shipped in iter
  24.3"), body shape + pin file reference added, retirement
  follow-up named. DESIGN.md §"Float semantics" gained a new
  paragraph in the "Unspecified" block cross-referencing `show 1.5`
  / `show nan` / `show inf` to `instance Show Float` via
  `float_to_str`. ~10 lines added net (2697 → 2707).
- iter 24.3.7: Roadmap P1 "Post-22 Prelude — Show + print rewire"
  entry flipped to `[x]` with a closing-summary block naming all
  three shipped iters (24.1 f38bad8, 24.2 iter-24.2, 24.3 this
  iter). New P2 entry "Retire `io/print_int|bool|float` effect-ops
  + migrate example corpus to `print`" inserted at top of P2,
  before the existing operator-routing-through-Eq/Ord entry.
- iter 24.3.8: Full workspace 556 passed / 0 failed. `prelude_free_fns`
  5/5 green; `mono_hash_stability` 2/2 green (Eq/Ord hashes
  unchanged, Show hashes from 24.2 unchanged). `bench/cross_lang.py`
  exit 0 (25/25 stable). `bench/compile_check.py` exit 1 with 1
  noise-class regression on a `check_ms.*` metric (regressed metric
  identity migrates between runs — pattern consistent with the
  documented noise envelope per audit-cma lineage, 8th consecutive
  observation). `bench/check.py` exit 1 with 2 improvements beyond
  tolerance on `latency.explicit_at_rc.*` — same envelope, same
  conservative call. Baseline pristine.

## Concerns

- **Three plan-defects-fixed-inline surfaced during Task 4 (user-ADT
  E2E).** All three are necessary repairs to make the spec's stated
  user-ADT trajectory (§"Data flow / print x at user type IntBox"
  lines 297-310) work end-to-end. The plan did not anticipate them
  because the test architecture envisioned (Tasks 3+4 sharing one
  test file) didn't actually exercise the cross-module mono synthesis
  before this iter; primitive-only smoke worked through the existing
  pipeline.
  - (a) **Bare type-cons in `MonoTarget::FreeFn::type_args`.** At
    mono target-collection (`collect_mono_targets` + `collect_residuals_ordered`
    free-fn arms), `subst.apply(meta)` returns the unification result
    in whatever qualification the caller-module's view used. For a
    user-defined type referenced from `main` (in `show_user_adt`),
    `IntBox` came out bare. The downstream `synthesise_mono_fn_for_free_fn`
    then substituted bare `IntBox` everywhere in the synthesised body,
    and Phase 3 rewrite of the synthesised body (run in PRELUDE
    caller-module context, where `IntBox` is not a known TypeDef) saw
    `Type::Con{IntBox}` that didn't normalise to the registry-keyed
    `show_user_adt.IntBox`. Lookup missed → no `show__IntBox` target
    scheduled → bare `Var "show"` persisted post-mono. Fix:
    `mono.rs` lines 685-696 + 1267-1278 normalise via
    `env.workspace_registry.normalize_type_for_lookup(module_name, &resolved)`
    before pushing the type into `MonoTarget::FreeFn::type_args`. Same
    invariant applies symmetric to the existing
    `r_ty_norm = normalize_type_for_lookup(...)` calls already in
    `collect_mono_targets`'s class-method arm.
  - (b) **Codegen cross-module references via import_map.** Post
    fix-(a), the synthesised body now correctly references
    `show_user_adt.show__cb5eb497` from inside `prelude.print__cb5eb497`.
    But `prelude` does not have `show_user_adt` in its `import_map`
    (prelude is the base; user modules import IT, not the other way
    around). Three codegen sites raised UnknownVar:
    `resolve_top_level_fn:2200`, `lower_app` cross-module arm:1981,
    `synth_with_extras` Var arm:2777. All three now fall back to a
    direct `module_user_fns.contains_key(prefix)` (or
    `module_def_ail_types.contains_key(prefix)`) lookup when the
    prefix isn't in the current module's import_map. This is the
    post-mono synthesised-body invariant: cross-module references in
    synthesised bodies may bypass the source template's imports
    (both ends were independently typechecked under their own module
    contexts before mono ran; the cross-module reference is created
    by mono, not by the source). Doc comments cross-reference all
    three sites.
  - (c) **Poly-free-fn constraint residuals not pushed at synth.**
    Pre-iter-24.3, the FreeFnCall arm at `lib.rs:2820` had
    `constraints: _` — declared constraints of the poly free fn were
    ignored. For all milestone-23 fixtures (`ne`/`lt`/`le`/`gt`/`ge`
    at primitive types), this was latent because every call site had
    a registry entry; the mono pass silently scheduled the
    corresponding `eq__T`/`compare__T` targets, and codegen succeeded.
    But for negative cases — `print f` where `f : Int -> Int` has no
    Show instance — the constraint never reached the typecheck-time
    `NoInstance` discharge (lib.rs:1880-1905). The build would emit
    a confusing post-mono `unknown variable: show` from codegen
    rather than the right typecheck-phase diagnostic. Fix: at
    `lib.rs:2820+`, walk `Type::Forall.constraints` at FreeFnCall
    sites, substitute the rigid vars with their fresh metavars, and
    push one `ResidualConstraint` per declared constraint. The
    discharge loop then fires the correct NoInstance with full
    diagnostic context (including the Show-aware addendum from
    Task 5). Pre-existing milestone-23 tests stay green because
    primitive-type Show/Eq/Ord registry entries discharge silently.
- **bench/compile_check.py + check.py exit 1.** Both report
  noise-class metrics (1 regression / 2 improvements respectively).
  Metric identity migrates between consecutive runs — pattern
  consistent with the documented `latency.implicit_at_rc.*` /
  `check_ms.*` noise envelope per audit-cma + audit-ms + audit-eob +
  audit-ct-tidy + audit-mq + iter-24.2 lineage. 8th consecutive
  observation. Baseline pristine per conservative-call convention.
- **Plan deviation on Task 2 (IR-shape pin).** The plan's test stub
  asserted `Term::Lam { body: Term::Let { ... } }` for `print__Int`.
  The actual `FnDef.body: Term` shape for top-level fns is the
  *inner* function body (params separate); no outer `Term::Lam`. The
  test was corrected to assert `Term::Let { ... }` directly. The
  load-bearing property (explicit let-binder for heap-Str RC
  discipline) is identical between the plan's intent and the
  shipped assertion; only the wrapper assumption changed. Doc
  comment in the test file explains the FnDef.body shape contract.

## Known debt

- **Milestone 24 closes structurally with this iter.** The standard
  `audit` pipeline (architect drift review + bench regression
  attribution) runs next. The three plan-defects-fixed-inline above
  may merit architect commentary on whether the fix sites belong as
  permanent invariants or are themselves transitional. Specifically:
  fix (a) implies that `MonoTarget::FreeFn::type_args` carries
  CANONICAL types post-collection — this is a strengthening of the
  pre-iter invariant which was "subst.apply result"; future planner
  work that touches the mono pass should keep that strengthening
  visible.
- **P2 entry "Retire io/print_int|bool|float effect-ops" is queued.**
  The new entry's `~86 fixtures affected` count is from a rough
  grep against `do io/print_<T>`; precise count + migration plan is
  the planner's job when the milestone opens.
- **Negative-fixture coverage is single-shape.** `show_no_instance.ail.json`
  uses `f : Int -> Int`. Other NoInstance shapes (user type without
  instance, `print x` where `x : SomeUnsupportedClass`) are not
  separately tested — the NoInstance arm fires for any non-prelude-Show
  type by construction, so the single fixture is structurally
  representative.

## Files touched

**Created (6):**
- `crates/ail/tests/print_mono_body_shape.rs`
- `crates/ail/tests/show_print_e2e.rs`
- `crates/ail/tests/show_no_instance_e2e.rs`
- `examples/show_print_smoke.ail.json`
- `examples/show_user_adt.ail.json`
- `examples/show_no_instance.ail.json`

**Modified (6):**
- `examples/prelude.ail.json` (+1 def: `fn print`)
- `crates/ailang-check/src/lib.rs` (NoInstance Show-aware branch +
  FreeFnCall constraint-residual push)
- `crates/ailang-check/src/mono.rs` (FreeFn type_args canonical-form
  normalisation, 2 sites)
- `crates/ailang-codegen/src/lib.rs` (cross-module reference fallback
  for post-mono synthesised bodies, 3 sites: `resolve_top_level_fn`,
  `lower_app`'s cross-module call arm, `synth_with_extras`'s Var arm)
- `docs/DESIGN.md` (§"Prelude (built-in) classes" milestone-24
  paragraph + §"Float semantics" Show-Float cross-reference)
- `docs/roadmap.md` (P1 Show+print → [x]; new P2 entry at top)

## Stats

`bench/orchestrator-stats/2026-05-13-iter-24.3.json`