# Iteration ct.3: Codegen + Mono Cleanup — Implementation Plan

> **Parent spec:** `docs/specs/2026-05-10-canonical-type-names.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement`
> to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Delete the codegen-side `lookup_ctor_by_type` imports-
fallback (iter 23.1.4) and narrow `apply_per_module_ctor_index_overlay`
in mono.rs to types-only — its `env.ctor_index` half became
decorative once ct.2.2's Pattern::Ctor lookup stopped consulting
the index. Also refresh two stale doc-comments left behind by ct.2.

**Architecture:** Three tasks across two crates. Task 1 simplifies
`Codegen::lookup_ctor_by_type` to two-branch direct lookup
(qualified or local; bare-non-local fails) — symmetric to ct.2.3's
Term::Ctor synth fix. Task 2 narrows the mono overlay so it scopes
`env.types` per-module (still needed for Term::Ctor bare lookup
when re-running synth in the mono pass) but drops the
`env.ctor_index` clear/rebuild (no consumer left). Task 3 is a
small doc-comment refresh in `crates/ail/src/main.rs` retiring the
"parity with typechecker `ambiguous-type` diagnostic" paragraph
that became obsolete in ct.2.3.

**Tech Stack:** `crates/ailang-codegen/src/lib.rs` (lookup fn),
`crates/ailang-check/src/mono.rs` (overlay fn + two call sites),
`crates/ail/src/main.rs` (doc-comment only).

---

## Files this plan creates or modifies

- Modify: `crates/ailang-codegen/src/lib.rs:1690-1766` — replace
  `lookup_ctor_by_type`'s three-branch lookup with two-branch
  (qualified or local). Refresh the docstring at line 1690 and
  the lockstep-rationale comment at line 380.
- Modify: `crates/ailang-check/src/mono.rs:386-434` — rename
  `apply_per_module_ctor_index_overlay` to
  `apply_per_module_types_overlay`; drop the
  `env.ctor_index.clear()` + insertion lines; rewrite the
  module-level rationale comment.
- Modify: `crates/ailang-check/src/mono.rs:160-167, 228-237` —
  the two call sites (in `monomorphise_workspace` and
  `collect_residuals_ordered`); rename the calls.
- Modify: `crates/ail/src/main.rs:228-250` — drop the obsolete
  paragraph mentioning `ambiguous-type` parity.

No files are created. No files are deleted.

---

## Task 1: Delete codegen `lookup_ctor_by_type` imports-fallback

The codegen-side helper at `crates/ailang-codegen/src/lib.rs:1698`
currently has three branches:

1. Qualified `module.Type`: resolve via `import_map.get(prefix)` →
   `module_ctor_index[target_module][ctor_name]`, validate
   `cref.type_name == suffix`. (Clean, stays.)
2. Local hit (bare `type_name`): look up
   `module_ctor_index[self.module_name][ctor_name]`; accept if
   `cref.type_name == type_name`. (Stays — bare-canonical means
   local.)
3. Imports-walk fallback (iter 23.1): scan every other module's
   ctor_index for `(ctor_name)` with `cref.type_name == type_name`,
   first hit wins. **Delete.** Post-ct.1+ct.2, every Term::Ctor
   reaching codegen carries a canonical `type_name` (bare = local,
   qualified = explicit cross-module); the imports-walk is
   structurally unreachable.

The fail-closed path: when the local hit's `type_name` does not
match the requested `type_name`, return
`CodegenError::Internal("unknown ctor `{ctor_name}` for type
`{type_name}` in module `{self.module_name}`")`. The lookup
function returns `Err` for any non-canonical input.

**Files:**
- Modify: `crates/ailang-codegen/src/lib.rs:1690-1766` —
  `lookup_ctor_by_type` body + its docstring.
- Modify: `crates/ailang-codegen/src/lib.rs:378-385` — the
  comment block at the top of `build_check_env`-related code that
  cites "lockstep with Term::Ctor typecheck / lookup_ctor_by_type
  codegen" needs a tiny touch-up to remove the "imports-fallback"
  framing.

- [ ] **Step 1: Run the current test suite to confirm baseline**

Run: `cargo test --workspace`

Expected: all pass (this is the post-ct.2 baseline; the test
count is 451).

- [ ] **Step 2: Apply the production fix — collapse to two branches**

Edit `crates/ailang-codegen/src/lib.rs:1698-1766`. Replace the
function body with:

```rust
    pub(crate) fn lookup_ctor_by_type(
        &self,
        type_name: &str,
        ctor_name: &str,
    ) -> Result<CtorRef> {
        if type_name.matches('.').count() == 1 {
            let (prefix, suffix) = type_name.split_once('.').expect("checked");
            let target_module = self.import_map.get(prefix).cloned().ok_or_else(|| {
                CodegenError::Internal(format!(
                    "qualified ctor `{type_name}/{ctor_name}`: prefix `{prefix}` not in import map"
                ))
            })?;
            let cref = self
                .module_ctor_index
                .get(&target_module)
                .and_then(|m| m.get(ctor_name))
                .cloned()
                .ok_or_else(|| {
                    CodegenError::Internal(format!(
                        "qualified ctor `{type_name}/{ctor_name}` not in module `{target_module}`"
                    ))
                })?;
            if cref.type_name != suffix {
                return Err(CodegenError::Internal(format!(
                    "ctor `{ctor_name}` belongs to `{}`, not `{type_name}`",
                    cref.type_name
                )));
            }
            Ok(cref)
        } else {
            // ct.3 Task 1: bare type_name is canonical (post-ct.1
            // validator) ⇒ local. Hit the current module's ctor
            // table directly; non-match is a hard error. No
            // imports-walk — the typechecker has already pinned
            // type_name to the owning module.
            let cref = self
                .module_ctor_index
                .get(self.module_name)
                .and_then(|m| m.get(ctor_name))
                .cloned()
                .ok_or_else(|| {
                    CodegenError::Internal(format!(
                        "unknown ctor `{ctor_name}` for type `{type_name}` in module `{}`",
                        self.module_name
                    ))
                })?;
            if cref.type_name != type_name {
                return Err(CodegenError::Internal(format!(
                    "ctor `{ctor_name}` belongs to local type `{}`, not `{type_name}`; \
                     cross-module ctor refs require qualified type_name",
                    cref.type_name
                )));
            }
            Ok(cref)
        }
    }
```

- [ ] **Step 3: Refresh the lookup_ctor_by_type docstring at lib.rs:1690**

Edit `crates/ailang-codegen/src/lib.rs:1690-1697`. Replace the
existing docstring (which references the imports-fallback +
`AmbiguousType` that no longer exist) with:

```rust
    /// Resolves a `Term::Ctor.type_name` (canonical post-ct.1: bare
    /// = local TypeDef, qualified `<owner>.<type>` = explicit
    /// cross-module) to the codegen-side `CtorRef`. Qualified names
    /// route through `import_map`; bare names hit the current
    /// module's `module_ctor_index` directly. No imports-walk
    /// fallback — the typechecker (post-ct.2) and the workspace
    /// validator (post-ct.1) have already pinned canonical form.
```

- [ ] **Step 4: Refresh the lockstep-rationale comment at lib.rs:378-385**

Edit `crates/ailang-codegen/src/lib.rs` at the comment block that
cites "lockstep with Term::Ctor typecheck / lookup_ctor_by_type
codegen". Find the block (search for `Term::Ctor typecheck /
lookup_ctor_by_type` near line 380) and replace it with one that
names the post-ct.3 reality: every Type::Con name reaching
codegen is canonical; the lockstep is now structural, not
fallback-based. The exact rewrite depends on the surrounding
context — preserve the function-level comment's intent, drop only
the bits that cite the now-removed mechanism. Keep the comment
≤ 6 lines.

- [ ] **Step 5: Build and run the full workspace test suite**

Run: `cargo test --workspace`

Expected: all pass.

Rationale for skipping a dedicated RED test: this task is a pure
deletion of unreachable code. The existing E2E suite — in
particular `crates/ail/tests/mono_xmod_ctor_pattern.rs` (the
iter-23.2 cross-module ctor regression) and the
prelude-using examples in `examples/` — exercises every code path
the deletion touches. If the deletion accidentally removes a live
path, those tests would fail. A new RED test would only re-pin
what the existing tests already pin.

- [ ] **Step 6: Commit**

```bash
git add crates/ailang-codegen/src/lib.rs
git commit -m "iter ct.3.1: codegen lookup_ctor_by_type direct lookup; delete imports-fallback"
```

---

## Task 2: Narrow mono overlay to types-only

The function `apply_per_module_ctor_index_overlay` at
`crates/ailang-check/src/mono.rs:418-434` exists to scope
`env.types` AND `env.ctor_index` to the current module's local
TypeDefs before the mono pass re-runs `synth`. The rationale,
spelled out at mono.rs:386-413, has two halves:

- Half A: scope `env.types` per-module so a bare-name `Term::Ctor`
  lookup in mono's re-run synth resolves to the current module's
  local TypeDef (NOT some workspace-wide bare-name collision).
  **Still load-bearing.** Term::Ctor synth at lib.rs:1982 still
  consults `env.types.get(type_name)` for bare canonical names.
- Half B: scope `env.ctor_index` per-module so the Pattern::Ctor
  `env.ctor_index.get(ctor)` lookup found local hits first and the
  imports-walk fallback could produce qualified type names.
  **Obsolete post-ct.2.2.** Pattern::Ctor no longer consults
  `env.ctor_index`; it walks `env.module_types` directly from the
  scrutinee's qualified type name.

This task drops Half B and renames the function to reflect Half A
only.

**Files:**
- Modify: `crates/ailang-check/src/mono.rs:386-434` — the
  function + its docstring.
- Modify: `crates/ailang-check/src/mono.rs:160-167` — the call
  site inside `monomorphise_workspace`.
- Modify: `crates/ailang-check/src/mono.rs:228-237` — the call
  site inside `collect_residuals_ordered`.

- [ ] **Step 1: Run the current test suite to confirm baseline**

Run: `cargo test --workspace`

Expected: all pass.

- [ ] **Step 2: Replace the overlay function body and rename it**

Edit `crates/ailang-check/src/mono.rs:418-434`. Replace the
function with the narrowed types-only form:

```rust
fn apply_per_module_types_overlay(env: &mut crate::Env, ws: &Workspace, module_name: &str) {
    env.types.clear();
    if let Some(m) = ws.modules.get(module_name) {
        for d in &m.defs {
            if let Def::Type(td) = d {
                env.types.insert(td.name.clone(), td.clone());
            }
        }
    }
}
```

The body loses the inner `for c in &td.ctors { env.ctor_index.insert(...) }`
loop and the `env.ctor_index.clear()` line.

- [ ] **Step 3: Replace the function docstring at mono.rs:386-417**

Edit the docstring block above the function (lines ~386-417).
Replace it with a shorter rationale that names only the
still-live purpose:

```rust
/// Rebuild `env.types` to contain only the TypeDefs declared in
/// `module_name`'s `Def::Type`s, mirroring `check_in_workspace`'s
/// per-module overlay at `crates/ailang-check/src/lib.rs:1234`.
///
/// The mono pass re-runs `crate::synth` on every fn body to
/// recover residual class constraints. `synth`'s `Term::Ctor` arm
/// looks up bare canonical `type_name`s via `env.types.get(name)`
/// (lib.rs:1982); without this overlay, `env.types` would be the
/// workspace-flat map built by `build_workspace_env`, and a bare
/// `type_name` in module A could resolve to a same-named TypeDef
/// from module B. The overlay restores per-module bare-name
/// scoping.
///
/// Iter ct.3 (this iter): the `env.ctor_index` half of the
/// original 22c overlay is gone. Pattern::Ctor post-ct.2.2 walks
/// `env.module_types` from the scrutinee's qualified `Type::Con`
/// name directly; it no longer consults `env.ctor_index`.
///
/// Caller-contract assumption: the workspace has already
/// typechecked, so duplicate type names within a single module
/// cannot occur.
```

- [ ] **Step 4: Update the two call sites**

Edit `crates/ailang-check/src/mono.rs:160-167`. Replace:

```rust
        // Per-module ctor_index overlay — see
        // [`apply_per_module_ctor_index_overlay`] for rationale.
```

(and however the surrounding code reads — locate the call by
searching for `apply_per_module_ctor_index_overlay(&mut env_mod`)
with:

```rust
        // Per-module env.types overlay — see
        // [`apply_per_module_types_overlay`] for rationale.
```

Then update the function call itself:

```rust
        apply_per_module_types_overlay(&mut env_mod, &ws_owned, mname);
```

Do the same at the second call site (mono.rs:228-237), again
matching the comment header and the function name.

- [ ] **Step 5: Build and run the full workspace test suite**

Run: `cargo test --workspace`

Expected: all pass. In particular, the
`mono_pass_handles_xmod_ctor_pattern` regression test (which
exercises cross-module ctor patterns through mono) must continue
to pass — its scrutinee carries a qualified type and the
Pattern::Ctor lookup walks `env.module_types` directly,
independently of `env.ctor_index`.

- [ ] **Step 6: Commit**

```bash
git add crates/ailang-check/src/mono.rs
git commit -m "iter ct.3.2: mono overlay narrowed to env.types only; rename to apply_per_module_types_overlay"
```

---

## Task 3: Retire the obsolete `migrate-canonical-types` paragraph in ail/main.rs

The doc-comment at `crates/ail/src/main.rs:228-250` (the
`MigrateCanonicalTypes` variant) has a "Note: this is one-shot
dev tooling and intentionally weaker than the typecheck-time
`ambiguous-type` diagnostic shipped in iter 23.1.3" paragraph.

The `ambiguous-type` diagnostic was removed in ct.2.3. The
paragraph now references a defunct diagnostic and frames a
parity argument with a comparand that no longer exists.

**Files:**
- Modify: `crates/ail/src/main.rs:228-250` — drop the paragraph.

- [ ] **Step 1: Confirm the location**

Run: `grep -n "ambiguous-type" crates/ail/src/main.rs`

Expected: a single hit around line 239.

- [ ] **Step 2: Drop the obsolete paragraph**

Edit `crates/ail/src/main.rs`. Find the doc-comment block on
`MigrateCanonicalTypes` (around lines 228-250). Delete the
paragraph that starts with "Note: this is one-shot dev tooling
and intentionally weaker..." and ends with "...enforce the
stricter rule going forward." Keep the rest of the docstring
(the schema-level description, the "first import wins"
disambiguation note).

The resulting docstring should be:

```rust
    /// ct.1 (dev-only): rewrite every `*.ail.json` under `<dir>` so
    /// that bare cross-module `Type::Con` and `Term::Ctor.type_name`
    /// references gain their owning module's qualifier. Idempotent.
    ///
    /// Disambiguation: the first import (in declaration order) that
    /// owns a matching type wins. Ties are NOT diagnosed — the
    /// migration silently picks the first match. Prelude is consulted
    /// as an implicit last-resort fallback when no listed import owns
    /// the name.
    MigrateCanonicalTypes {
        /// Directory containing `*.ail.json` to migrate (typically
        /// `examples/`).
        dir: PathBuf,
    },
```

- [ ] **Step 3: Build to confirm the doc-comment still parses**

Run: `cargo build --workspace`

Expected: clean build, no warnings related to the doc-comment.

- [ ] **Step 4: Commit**

```bash
git add crates/ail/src/main.rs
git commit -m "iter ct.3.3: retire obsolete ambiguous-type paragraph in migrate-canonical-types docstring"
```

---

## Closing checks

After all three tasks land:

- [ ] **Step C1: Run the full test suite once more**

Run: `cargo test --workspace`

Expected: 451+ tests, 0 failed.

- [ ] **Step C2: Confirm the four obsolete mechanisms catalogue is exhausted**

The ct.1 JOURNAL entry named four mechanisms slated for retirement:
- ✅ `Pattern::Ctor` imports-fallback in ailang-check (ct.2.2).
- ✅ `Term::Ctor` synth imports-fallback in ailang-check (ct.2.3).
- ✅ `lookup_ctor_by_type` imports-fallback in ailang-codegen (ct.3.1).
- ✅ `apply_per_module_ctor_index_overlay`'s ctor_index half in
  ailang-check/mono.rs (ct.3.2).

All four are now gone. Iteration ct.3 closes the typechecker +
codegen + mono side of the milestone. Only ct.4 (DESIGN.md
amendments + hash re-baseline + prose round-trip extension +
optional fixture recreation for iter 23.3 Task 4+5) remains
before the canonical-type-names milestone closes.

- [ ] **Step C3: JOURNAL entry**

Append to `docs/JOURNAL.md`:

```markdown
## 2026-05-11 — Iteration ct.3: codegen + mono cleanup

Three tasks landed:

- **ct.3.1**: deleted the `lookup_ctor_by_type` imports-fallback
  in `crates/ailang-codegen/src/lib.rs`. Replaced with two-branch
  direct lookup (qualified via `import_map` →
  `module_ctor_index[target_module]`; bare via
  `module_ctor_index[self.module_name]` with a hard type-name
  match). Symmetric to ct.2.3's `Term::Ctor` synth fix on the
  typecheck side. Docstring and the lockstep-rationale comment
  refreshed.
- **ct.3.2**: narrowed `apply_per_module_ctor_index_overlay` to
  `env.types` only, dropping the `env.ctor_index` half that became
  decorative after ct.2.2 stopped consulting the index from
  Pattern::Ctor. Renamed to `apply_per_module_types_overlay`.
  Updated both call sites and the module-level rationale comment.
- **ct.3.3**: retired the obsolete "parity with `ambiguous-type`
  diagnostic" paragraph in the `MigrateCanonicalTypes` doc-comment
  (`crates/ail/src/main.rs`). The diagnostic it referenced was
  removed in ct.2.3.

All four obsolete mechanisms named in the ct.1 JOURNAL entry are
now gone. Only ct.4 remains in the canonical-type-names milestone:
DESIGN.md amendments, hash-pinned regression test re-baseline,
prose round-trip extension, and the optional
`examples/compare_primitives_smoke.ail.json` recreation that
demonstrates the end-to-end closure of the iter-23.3 Task 4 bug.

Workspace at iter-ct.3 close: full `cargo build --workspace` and
`cargo test --workspace` green (test count unchanged from ct.2's
451; this iter is pure code deletion + doc tidy).
```

- [ ] **Step C4: Commit the JOURNAL entry**

```bash
git add docs/JOURNAL.md
git commit -m "journal: ct.3 codegen + mono cleanup"
```
