# 22-tidy.6 Form-B prose printer arms for ClassDef + InstanceDef — Implementation Plan

> **Parent spec:** `docs/specs/0002-22-typeclasses.md` (milestone 22, closed 2026-05-09)
>
> **Carried-debt anchor:** `docs/JOURNAL.md` 2026-05-09 entry "Iteration 22-tidy" → "Form-B (prose) printer arms for ClassDef / InstanceDef. One-way, not gating."
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement`
> to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** retire the third (and last gating) milestone-22 carried-debt item: replace the placeholder Form-B render of `Def::Class` / `Def::Instance` (currently `// (class Foo a) -- full Form-B projection deferred (post-22)`) with a full Rust-flavoured projection that mirrors the Form-A schema. Includes forall-class-constraint rendering so a class-constrained fn (e.g. `forall<a>{Ord a => fn uses_eq_via_ord(...)}`) survives the projection without semantic loss.

**Architecture:** add two new helpers to `crates/ailang-prose/src/lib.rs` — `write_class_def` and `write_instance_def` — wired into the existing `write_def` dispatch. `write_fn_def` gets a 5-line constraint-render extension after the existing `forall<...>` line so a class-constrained `Type::Forall.constraints` list renders as `where <Class1> <param>, <Class2> <param>, …`. Snapshot tests use the established `tests/snapshot.rs` framework — one new fixture (`test_22b3_default_e2e`) for class-with-default + instance, one (`test_22b2_constraint_declared_via_superclass`) for class-with-superclass + class-constrained-fn.

**Tech Stack:** `crates/ailang-prose` (lib.rs, tests/snapshot.rs); `examples/` (two new `.prose.txt` snapshot files).

**Files this plan creates or modifies:**

- Modify: `crates/ailang-prose/src/lib.rs` — adds `write_class_def`, `write_instance_def`, `write_class_method`, `write_instance_method`, `write_forall_constraints`; rewires `write_def`'s `Def::Class` / `Def::Instance` arms to call them; extends `write_fn_def` to render forall constraints.
- Modify: `crates/ailang-prose/tests/snapshot.rs` — adds two new `#[test] fn snapshot_test_22b3_default_e2e` and `snapshot_test_22b2_constraint_declared_via_superclass`.
- Create: `examples/test_22b3_default_e2e.prose.txt` — snapshot fixture for class-with-default + instance.
- Create: `examples/test_22b2_constraint_declared_via_superclass.prose.txt` — snapshot fixture for superclass + class-constrained fn.
- Modify: `docs/JOURNAL.md` — append iteration entry.

**Form-B projection design:**

Class without superclass:
```
class Show a {
    fn show(x: a) -> Str
}
```

Class with superclass:
```
class Ord a extends Eq {
    fn lt(x: a, y: a) -> Bool
}
```

(Note: the superclass's `type_` field is validated to equal the class's own param, so it's redundant in the projection; render only the superclass *name*.)

Class method with default:
```
class D a {
    fn dval(x: a) -> Int default {
        99
    }
}
```

Instance:
```
instance D Int {
}
```

(No method overrides → empty body. Method-override case shown below.)

Instance with method override:
```
instance Show Int {
    fn show {
        int_to_str(x)
    }
}
```

(InstanceMethod has only `name` + `body` — no signature; the LLM reads the class def for that, per the prose policy "deliberately lossy where the LLM can re-derive from typecheck context".)

Forall with class constraint on fn:
```
forall<a> where Ord a
fn uses_eq_via_ord(x: a, y: a) -> Bool {
    eq(x, y)
}
```

(The `where` line goes between `forall<a>` and `fn ...`. Multiple constraints comma-separated.)

---

## Task 1: `write_class_def` + `write_instance_def` — full Form-B projection

**Files:**
- Modify: `crates/ailang-prose/src/lib.rs` — replace the placeholder arms (lines 88-101); add four new helpers.
- Create: `examples/test_22b3_default_e2e.prose.txt`.
- Modify: `crates/ailang-prose/tests/snapshot.rs` — add one new test.

- [ ] **Step 1: write the expected snapshot at `examples/test_22b3_default_e2e.prose.txt`** (RED-first — file exists before the helpers do, so the test fails because the placeholder output differs)

```text
// module test_22b3_default_e2e

class D a {
    fn dval(x: a) -> Int default {
        99
    }
}

instance D Int {
}

fn main() -> Unit with IO {
    io/print_int(dval(0))
}
```

- [ ] **Step 2: append the snapshot test to `crates/ailang-prose/tests/snapshot.rs`**

At the end of the file (just before the final closing brace if there is one — it appears the existing tests are at top level, so append at end):

```rust
#[test]
fn snapshot_test_22b3_default_e2e() {
    check_snapshot("test_22b3_default_e2e");
}
```

- [ ] **Step 3: run RED — confirm snapshot mismatch**

Run: `cargo test --workspace -p ailang-prose snapshot_test_22b3_default_e2e 2>&1 | tail -15`
Expected: FAIL — `prose snapshot mismatch for test_22b3_default_e2e` (the placeholder render contains `// (class D a) -- full Form-B projection deferred (post-22)` instead of the new full render).

- [ ] **Step 4: replace the placeholder arms in `crates/ailang-prose/src/lib.rs:88-101`**

Locate the `Def::Class` and `Def::Instance` arms in `write_def` (lines 88-101). Replace them with:

```rust
        Def::Class(c) => write_class_def(out, c, level),
        Def::Instance(i) => write_instance_def(out, i, level),
```

And remove the placeholder comment block (lines 80-87) that introduced the deferred-placeholder note — those four lines of comment go too.

- [ ] **Step 5: add the four helpers**

Add these helpers at the end of the `// ---- defs --------` section (just before `// ---- types --------`, currently around line 344):

```rust
fn write_class_def(out: &mut String, c: &ClassDef, level: usize) {
    write_doc(out, &c.doc, level);
    indent(out, level);
    out.push_str("class ");
    out.push_str(&c.name);
    out.push(' ');
    out.push_str(&c.param);
    if let Some(sc) = &c.superclass {
        out.push_str(" extends ");
        out.push_str(&sc.class);
    }
    out.push_str(" {\n");
    for m in &c.methods {
        write_class_method(out, m, level + 1);
    }
    indent(out, level);
    out.push('}');
}

fn write_class_method(out: &mut String, m: &ClassMethod, level: usize) {
    indent(out, level);
    out.push_str("fn ");
    out.push_str(&m.name);
    out.push('(');
    if let Type::Fn { params, param_modes, ret, ret_mode, effects } = &m.ty {
        for (i, p) in params.iter().enumerate() {
            if i > 0 {
                out.push_str(", ");
            }
            // Class methods carry types but no parameter names — name slots
            // as `x`, `x1`, `x2`, … to keep the projection unambiguous
            // without inventing names that could collide with the body.
            if i == 0 {
                out.push_str("x");
            } else {
                out.push_str(&format!("x{i}"));
            }
            out.push_str(": ");
            let mode = param_modes.get(i).copied().unwrap_or(ParamMode::Implicit);
            write_mode_type(out, p, mode);
        }
        out.push_str(") -> ");
        write_mode_type(out, ret, *ret_mode);
        if !effects.is_empty() {
            out.push_str(" with ");
            for (i, e) in effects.iter().enumerate() {
                if i > 0 {
                    out.push_str(", ");
                }
                out.push_str(e);
            }
        }
    } else {
        out.push_str(") -> ");
        write_type(out, &m.ty);
    }
    if let Some(body) = &m.default {
        out.push_str(" default {\n");
        indent(out, level + 1);
        write_term(out, body, level + 1);
        out.push('\n');
        indent(out, level);
        out.push_str("}\n");
    } else {
        out.push('\n');
    }
}

fn write_instance_def(out: &mut String, i: &InstanceDef, level: usize) {
    write_doc(out, &i.doc, level);
    indent(out, level);
    out.push_str("instance ");
    out.push_str(&i.class);
    out.push(' ');
    write_type(out, &i.type_);
    out.push_str(" {\n");
    for m in &i.methods {
        write_instance_method(out, m, level + 1);
    }
    indent(out, level);
    out.push('}');
}

fn write_instance_method(out: &mut String, m: &InstanceMethod, level: usize) {
    indent(out, level);
    out.push_str("fn ");
    out.push_str(&m.name);
    out.push_str(" {\n");
    indent(out, level + 1);
    write_term(out, &m.body, level + 1);
    out.push('\n');
    indent(out, level);
    out.push_str("}\n");
}
```

Also add the four new types to the `use ailang_core::ast::{...}` import line (line 27-30): add `ClassDef, ClassMethod, InstanceDef, InstanceMethod` to the imported names.

- [ ] **Step 6: run GREEN — snapshot passes**

Run: `cargo build --workspace 2>&1 | tail -5`
Expected: builds clean (no unused-import warnings).

Run: `cargo test --workspace -p ailang-prose snapshot_test_22b3_default_e2e 2>&1 | tail -10`
Expected: PASS — `1 passed; 0 failed`.

If the snapshot fails because the actual output differs from the expected `.prose.txt`, examine the `.actual` file the test framework writes to `examples/test_22b3_default_e2e.prose.txt.actual`. If the diff is a presentation-tweak (whitespace at end-of-line, indentation depth), update the expected file to match the actual; otherwise diagnose the helper.

- [ ] **Step 7: full ailang-prose sweep stays green**

Run: `cargo test --workspace -p ailang-prose 2>&1 | grep -E "^test result" | tail -5`
Expected: every line `test result: ok.`; no FAILED. The 4 pre-existing snapshot tests (`rc_own_param_drop`, `rc_match_arm_partial_drop_leak`, `rc_app_let_partial_drop_leak`, `bench_list_sum`) must stay green — none of those fixtures contain class/instance, so they hit no changed code path.

- [ ] **Step 8: clean up any `.actual` debug file the snapshot framework may have written**

Run: `rm -f examples/test_22b3_default_e2e.prose.txt.actual`

- [ ] **Step 9: commit**

```bash
git add crates/ailang-prose/src/lib.rs crates/ailang-prose/tests/snapshot.rs examples/test_22b3_default_e2e.prose.txt
git commit -m "iter 22-tidy.6.1: write_class_def + write_instance_def — full Form-B projection"
```

---

## Task 2: `write_fn_def` extends with forall-constraint rendering — superclass + class-constrained fn

**Files:**
- Modify: `crates/ailang-prose/src/lib.rs` — `write_fn_def` (lines 248-331); adds a `where`-clause render after the `forall<...>` line.
- Create: `examples/test_22b2_constraint_declared_via_superclass.prose.txt`.
- Modify: `crates/ailang-prose/tests/snapshot.rs` — add one more test.

- [ ] **Step 1: write the expected snapshot at `examples/test_22b2_constraint_declared_via_superclass.prose.txt`** (RED-first)

```text
// module test_22b2_constraint_declared_via_superclass

class Eq a {
    fn eq(x: a, x1: a) -> Bool
}

class Ord a extends Eq {
    fn lt(x: a, x1: a) -> Bool
}

forall<a> where Ord a
fn uses_eq_via_ord(x: a, y: a) -> Bool {
    eq(x, y)
}
```

- [ ] **Step 2: append the snapshot test**

In `crates/ailang-prose/tests/snapshot.rs`, append:

```rust
#[test]
fn snapshot_test_22b2_constraint_declared_via_superclass() {
    check_snapshot("test_22b2_constraint_declared_via_superclass");
}
```

- [ ] **Step 3: run RED — confirm snapshot mismatch**

Run: `cargo test --workspace -p ailang-prose snapshot_test_22b2_constraint_declared_via_superclass 2>&1 | tail -15`
Expected: FAIL — `prose snapshot mismatch`. The class+superclass arm now works (Task 1 shipped it), but the constraint on `uses_eq_via_ord`'s forall is dropped — current `write_fn_def` discards `Type::Forall.constraints`.

- [ ] **Step 4: extend `write_fn_def` to render constraints**

Locate the forall-block in `write_fn_def` (lines 261-276 — the `let (forall_vars, fn_ty) = match &fd.ty { … }` block plus the `if let Some(vars) = &forall_vars { … out.push_str(">\n"); }` block).

Replace:

```rust
    let (forall_vars, fn_ty) = match &fd.ty {
        Type::Forall { vars, constraints: _, body } => (Some(vars.clone()), body.as_ref()),
        other => (None, other),
    };

    if let Some(vars) = &forall_vars {
        indent(out, level);
        out.push_str("forall<");
        for (i, v) in vars.iter().enumerate() {
            if i > 0 {
                out.push_str(", ");
            }
            out.push_str(v);
        }
        out.push_str(">\n");
    }
```

with:

```rust
    let (forall_vars, forall_constraints, fn_ty) = match &fd.ty {
        Type::Forall { vars, constraints, body } => {
            (Some(vars.clone()), constraints.clone(), body.as_ref())
        }
        other => (None, Vec::new(), other),
    };

    if let Some(vars) = &forall_vars {
        indent(out, level);
        out.push_str("forall<");
        for (i, v) in vars.iter().enumerate() {
            if i > 0 {
                out.push_str(", ");
            }
            out.push_str(v);
        }
        out.push('>');
        if !forall_constraints.is_empty() {
            out.push_str(" where ");
            for (i, c) in forall_constraints.iter().enumerate() {
                if i > 0 {
                    out.push_str(", ");
                }
                out.push_str(&c.class);
                out.push(' ');
                write_type(out, &c.type_);
            }
        }
        out.push('\n');
    }
```

Also add `Constraint` to the `use ailang_core::ast::{...}` import line at the top of the file.

- [ ] **Step 5: run GREEN — snapshot passes**

Run: `cargo build --workspace 2>&1 | tail -5`
Expected: builds clean.

Run: `cargo test --workspace -p ailang-prose snapshot_test_22b2_constraint_declared_via_superclass 2>&1 | tail -10`
Expected: PASS.

- [ ] **Step 6: full ailang-prose sweep stays green** (the 4 pre-existing snapshots PLUS the new one from Task 1 PLUS this one — 6 tests total)

Run: `cargo test --workspace -p ailang-prose 2>&1 | grep -E "^test result" | tail -5`
Expected: every line `test result: ok.`; no FAILED.

- [ ] **Step 7: clean up `.actual` debug file**

Run: `rm -f examples/test_22b2_constraint_declared_via_superclass.prose.txt.actual`

- [ ] **Step 8: commit**

```bash
git add crates/ailang-prose/src/lib.rs crates/ailang-prose/tests/snapshot.rs examples/test_22b2_constraint_declared_via_superclass.prose.txt
git commit -m "iter 22-tidy.6.2: write_fn_def renders forall class constraints"
```

---

## Task 3: verify + JOURNAL

**Files:**
- Modify: `docs/JOURNAL.md` — append iteration entry.

- [ ] **Step 1: full workspace test sweep**

Run: `cargo test --workspace 2>&1 | grep -c "FAILED"`
Expected: `0`.

- [ ] **Step 2: bench gates**

Run: `python3 bench/check.py 2>&1 | tail -1 && python3 bench/compile_check.py 2>&1 | tail -1 && python3 bench/cross_lang.py 2>&1 | tail -1`
Expected: each line ends `0 regressed`.

- [ ] **Step 3: confirm placeholder retired**

Run: `grep -n "full Form-B projection deferred" crates/`
Expected: zero hits — the placeholder text and its containing comment block are gone from `crates/ailang-prose/src/lib.rs`.

- [ ] **Step 4: append JOURNAL entry**

Append to `docs/JOURNAL.md`:

```markdown
## 2026-05-10 — Iteration 22-tidy.6: Form-B prose printer arms for ClassDef + InstanceDef

Closing the third (and final gating) milestone-22 carried-debt
item: `crates/ailang-prose` previously rendered `Def::Class` /
`Def::Instance` as a one-line placeholder
(`// (class Foo a) -- full Form-B projection deferred (post-22)`).
This iteration replaces the placeholder with a full Rust-flavoured
projection covering class header, optional `extends <SuperClass>`,
methods (signature plus optional `default { … }` body), and
instance header + method bodies.

`write_fn_def` was extended in the same iter to render forall
class constraints (`forall<a> where Ord a`) so a class-constrained
fn signature survives the projection without semantic loss — a
prerequisite for the superclass snapshot to be useful.

Two new snapshot fixtures pin the new render:

- `examples/test_22b3_default_e2e.prose.txt` — class with default
  method body + instance that inherits the default.
- `examples/test_22b2_constraint_declared_via_superclass.prose.txt`
  — class with superclass + abstract method + class-constrained fn.

Tasks (commit subjects):

- 22-tidy.6.1: write_class_def + write_instance_def — full Form-B projection
- 22-tidy.6.2: write_fn_def renders forall class constraints

Acceptance: 2 RED-first snapshot tests then GREEN; full workspace
test sweep green; bench gates 0/0/0; the four pre-existing prose
snapshots (rc_own_param_drop, rc_match_arm_partial_drop_leak,
rc_app_let_partial_drop_leak, bench_list_sum) stay green
(unaffected — none contain class/instance). The placeholder text
and comment are gone from `crates/ailang-prose/src/lib.rs`.

Carried-debt status (after this iter, milestone 22 fully closed):

- ✅ Primitive-name-set consolidation (closed 22-tidy.4).
- ✅ Strict duplicate-clause detection in fn/const/data (closed 22-tidy.5).
- ✅ Form-B (prose) printer arms for ClassDef / InstanceDef (closed
  this iter).
- ✅ Lib.rs gap-related sites at lib.rs:1266 / 1853-1856 / 1978 /
  2314-2316 — audit confirmed defensive (no action needed).

Milestone 22 carried debt is now empty. Next milestone targets
(orchestrator's call): post-22 Prelude (gated on user-author
demand), operator routing through Eq/Ord (gated on bench
re-baselining), or the `types/ctor_index` overlay shape question
queued from `2026-05-10 — Audit close: env-construction unify`.

Out-of-scope observation (not new debt — design choice): instance
methods render as `fn <name> { <body> }` without a signature line.
The signature is recoverable from the class declaration via
`InstanceDef.class` lookup (a typecheck-context-dependent
substitution of `class.method.ty` with `class.param → instance.type_`).
Per the prose policy ("deliberately lossy where the LLM can
re-derive the dropped machinery from typecheck context"), this is
the correct projection — the class def is the authoritative
signature source. If a future audit decides otherwise, the
projection extends with a typecheck-context-aware substitutor.
```

- [ ] **Step 5: commit**

```bash
git add docs/JOURNAL.md
git commit -m "iter 22-tidy.6: journal entry, milestone-22 carried debt empty"
```