# Iter 24.3 — `fn print` polymorphic free fn + E2E + DESIGN.md sync — Implementation Plan

> **Parent spec:** `docs/specs/0024-24-show-print.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement`
> to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Ship `fn print : forall a. Show a => (a borrow) -> () !IO` in the prelude with body `\x -> let s = show x in do io/print_str s`, accompanied by three E2E fixtures (positive 4-prim smoke, user-ADT, negative NoInstance), a mono-symbol IR-shape pin asserting the let-binder discipline survives mono, a Show-aware NoInstance diagnostic, DESIGN.md amendments, and the roadmap update closing milestone 24.

**Architecture:** Iter 24.3 is the second and final half of milestone 24. It adds one new top-level def to `examples/prelude.ail.json` (`fn print`, the first prelude entry to use the intra-prelude bare class ref `"Show"` in a `Constraint.class` field — load-bearing for mq.1 canonical-form), three new E2E fixtures, three new test files (`show_print_e2e.rs`, `show_no_instance_e2e.rs`, `print_mono_body_shape.rs`), and one diagnostic-wording extension in `ailang-check/src/lib.rs` adjacent to the existing Float-aware Eq/Ord NoInstance arm. The mono pass synthesises `print__Int|Bool|Str|Float` per call-site; the IR-shape pin asserts the post-mono `Def::Fn.body` is structurally `Term::Lam → Term::Let → Term::Do`, protecting the explicit let-binder that the heap-Str RC discipline depends on (eob.1 Str carve-out). DESIGN.md §"Prelude (built-in) classes" amends the milestone-24 paragraph to list `print` as the polymorphic helper; §"Float semantics" gains a paragraph cross-referencing the Show-Float NaN-spelling path.

**Tech Stack:** `ailang-check` (mono synthesis, NoInstance diagnostic), `ailang-core` (workspace loader, AST), `ailang-codegen` (eob.1 Str carve-out, RC discipline — unchanged, just exercised), prelude JSON, three new Rust integration tests.

---

## Pre-flight notes (Boss-ratified post-recon)

**Spec Open Commitments resolved as Boss decisions:**

1. **Implicit-let vs explicit-let for `print`'s prelude source form** — ship **explicit-let**. Recon scanned `crates/ailang-core/src/desugar.rs` and `crates/ailang-check/src/uniqueness.rs`: no automatic let-introduction pass for `Term::App` in effect-op-arg position. Explicit-let is the safe default that matches the spec's prescribed body shape (`\x -> let s = show x in do io/print_str s`) and makes the heap-Str RC discipline visible in the source form.

2. **Mono IR-shape pin: AST-level vs LLVM text** — **AST-level** via recursive `Term`-pattern match on post-mono `Def::Fn.body`. Precedent: `crates/ail/tests/mono_hash_stability.rs` inspects `Def::Fn` directly. The existing `crates/ail/tests/ir_snapshot.rs` opens emitted `.ll` text but uses `ail emit-ir` which does NOT run mono (per roadmap P3 line 227-236), so it's unsuitable for `print__Int` shape assertion.

3. **NoInstance Show diagnostic wording** — extend the existing Float-aware Eq/Ord branch at `crates/ailang-check/src/lib.rs:770-779` with a parallel `class == "prelude.Show"` arm. Cross-reference: literal section anchor `"Prelude (built-in) classes"` (the existing section title in DESIGN.md line 1874).

4. **Roadmap P2 insertion order** — insert the new "Retire io/print_int|bool|float effect-ops + migrate example corpus" entry **at the top of P2** (immediately after the P2 header, before the existing `[feature] Operator routing through Eq/Ord` entry). Rationale: corpus migration is near-term mechanical work directly downstream of milestone 24's close; operator routing is still a feature decision with no committed shape.

5. **`show_no_instance.ail.json` shape** — use `let f : Int -> Int = \x -> x in do print f`. Concrete fn-type, no Show instance, fires `NoInstance Show (Int -> Int)`. The spec's `print id` was schematic; `id` is not in the current prelude (verified — `prelude.ail.json` has `not`, `eq`, `compare`, `ne`, `lt`, `le`, `gt`, `ge`, plus the iter-24.2 Show defs, no `id`). The let-bound concrete-typed fn is the closest LLM-natural shape that triggers the failure.

**Prelude insertion point.** New `fn print` def appends at `examples/prelude.ail.json:485` — immediately after the final `fn ge` def (whose closing brace is at line 485 per the post-24.2 file shape, with `ge` starting at line 484-485 and ending before the workspace-closing `]`). The implementer reads the file tail to confirm the exact line number and trailing-comma placement before editing.

**Iter 24.2 baseline.** Closed at commit `3286117`. 552 tests green. `prelude.ail.json` has 19 top-level defs (post-Show: was 14 pre-iter-24.2). `class Show` + 4 primitive instances live at lines 223-onwards. The post-mono prelude module synthesises `show__Int|Bool|Str|Float`.

---

## Files this plan creates or modifies

**Create:**
- `examples/show_print_smoke.ail.json` — positive E2E: `main` calls `print 42`, `print true`, `print "hello"`, `print 3.14`.
- `examples/show_user_adt.ail.json` — user-ADT E2E: `data IntBox = MkIntBox Int` + `instance prelude.Show IntBox` + `main` calls `print (MkIntBox 7)`.
- `examples/show_no_instance.ail.json` — negative E2E: `main` calls `print f` where `f : Int -> Int`.
- `crates/ail/tests/show_print_e2e.rs` — positive + user-ADT E2E tests (build + run + stdout assertions).
- `crates/ail/tests/show_no_instance_e2e.rs` — negative-diagnostic test (typecheck-only; asserts `code == "no-instance"`, message mentions Show + cross-references DESIGN.md).
- `crates/ail/tests/print_mono_body_shape.rs` — IR-shape pin: post-mono `print__Int` `Def::Fn.body` matches `Term::Lam → Term::Let → Term::Do` recursively.

**Modify:**
- `examples/prelude.ail.json:485` — append the new top-level `fn print` def immediately after the `fn ge` entry, before the workspace-closing `]`.
- `crates/ailang-check/src/lib.rs:770-779` — extend the Float-aware Eq/Ord NoInstance arm with a parallel `class == "prelude.Show"` branch yielding a Show-aware diagnostic that cross-references DESIGN.md §"Prelude (built-in) classes".
- `docs/DESIGN.md:1899-1910` — amend §"Prelude (built-in) classes" milestone-24 paragraph to flip `print`-rewire from "deferred to iter 24.3" to "ships in iter 24.3".
- `docs/DESIGN.md:2441-2447` — extend §"Float semantics" paragraph to add a Show-Float NaN-spelling cross-reference paragraph.
- `docs/roadmap.md:64-86` — flip the P1 "Post-22 Prelude — Show + print rewire" entry checkbox to `[x]`.
- `docs/roadmap.md:88-90` — insert new P2 entry "Retire io/print_int|bool|float effect-ops + migrate example corpus to print" at the top of P2 (before existing entries).

---

## Task 1: Add `fn print` to `prelude.ail.json` with explicit-let body

**Files:**
- Modify: `examples/prelude.ail.json:485` — append `fn print` def.

- [ ] **Step 1: Read the prelude tail to confirm insertion point.**

Run: `tail -10 examples/prelude.ail.json`

Expected: a closing `}` (end of `fn ge` body), then `}` (end of `fn ge` def — note trailing comma if `ge` is not the last def, or no comma if it IS the last def), then `]` (closing defs array), then `}` (closing workspace object).

If `ge` ends with `,` (trailing comma after closing `}`), `ge` is not currently the last def — verify by reading further. If `ge` ends with no trailing comma, `ge` is the current last def and the new `print` def is appended with a leading comma after `ge`'s closing `}`.

- [ ] **Step 2: Append the `fn print` def.**

Use Edit tool to add the following entry after the `fn ge` def. The exact insertion depends on Step 1's finding (with or without leading comma):

```jsonc
    {
      "kind": "fn",
      "name": "print",
      "doc": "Polymorphic console-print helper. `print x` ≡ `do io/print_str (show x)` with an explicit let-binder around `show x` for heap-Str RC discipline per eob.1 Str carve-out. Ships in milestone 24 as the second half of the Show prelude.",
      "type": {
        "k": "forall",
        "vars": ["a"],
        "constraints": [
          { "class": "Show",
            "type": { "k": "var", "name": "a" } }
        ],
        "body": {
          "k": "fn",
          "params": [{ "k": "var", "name": "a" }],
          "param_modes": ["borrow"],
          "ret": { "k": "con", "name": "Unit" },
          "effects": ["IO"]
        }
      },
      "params": ["x"],
      "body": {
        "t": "lam",
        "params": ["x"],
        "paramTypes": [{ "k": "var", "name": "a" }],
        "retType": { "k": "con", "name": "Unit" },
        "body": {
          "t": "let",
          "name": "s",
          "value": {
            "t": "app",
            "fn": { "t": "var", "name": "show" },
            "args": [{ "t": "var", "name": "x" }]
          },
          "body": {
            "t": "do",
            "op": "io/print_str",
            "args": [{ "t": "var", "name": "s" }]
          }
        }
      }
    }
```

The `"class": "Show"` is bare per mq.1 same-module canonical form (the constraint lives inside the prelude module).

If the surrounding existing defs use a different lambda-body schema for free fns (e.g. nested `t: "lam"` vs flat `params` at the def level), inspect a milestone-23 reference like `fn ne` (lines 325-356 of the post-24.2 prelude) to match. The implementer may need to adjust the `paramTypes`/`retType` fields' presence based on what the existing fns carry.

- [ ] **Step 3: Verify JSON parses + workspace round-trips.**

Run: `python3 -c "import json; d = json.load(open('examples/prelude.ail.json')); print(f'{len(d[\"defs\"])} defs')"`

Expected: `20 defs` (was 19 post-24.2, +1 for print).

If JSON parse fails, the inserted block has a syntax issue; diagnose with `python3 -c "import json; json.load(open('examples/prelude.ail.json'))"` and read the resulting error message.

- [ ] **Step 4: Run the full workspace test suite to confirm `print` resolves.**

Run: `cargo test --workspace --no-fail-fast 2>&1 | grep -E '^test result' | awk '{s+=$4; f+=$6} END {print "passed:", s, "failed:", f}'`

Expected: `passed: 552 failed: 0` (no new tests yet; existing 552 stay green because `print` is a new symbol — adding it cannot break any existing test).

If any tests fail, the new def's schema is malformed in a way that the JSON parser accepts but the typechecker rejects (e.g. wrong `paramTypes` shape). Diagnose: `cargo test --workspace --no-fail-fast 2>&1 | grep -A3 FAILED | head -20`.

---

## Task 2: Mono symbol IR-shape pin for `print__Int`

**Files:**
- Create: `crates/ail/tests/print_mono_body_shape.rs` — AST-level pattern-match pin.

This task runs **before** the E2E tests because the IR-shape assertion is the load-bearing semantic gate: if `print__Int`'s post-mono body is not `Term::Lam → Term::Let → Term::Do`, the heap-Str RC discipline is broken and the E2E tests may produce confusing failures.

- [ ] **Step 1: Read existing patterns for post-mono `Def::Fn.body` inspection.**

Run: `head -60 crates/ail/tests/mono_hash_stability.rs`

Confirm: load workspace → check → mono → find `Def::Fn` by name → inspect `f.body` (which is a `Term`).

Run: `grep -n 'Term::Lam\|Term::Let\|Term::Do\|enum Term' crates/ailang-core/src/ast.rs | head -10`

Confirm the variant names match what the body shape uses. If `Term::Lam` is spelled differently (e.g. `Term::Lambda`), adjust the pattern match below.

- [ ] **Step 2: Write the test stub at `crates/ail/tests/print_mono_body_shape.rs`.**

```rust
//! IR-shape pin for milestone 24.3's `print` polymorphic free fn.
//!
//! Asserts that the post-mono `print__Int` `Def::Fn.body` is structurally
//! `Term::Lam → Term::Let → Term::Do`. The explicit let-binder around
//! `show__Int x` is load-bearing for the heap-Str RC discipline (eob.1
//! Str carve-out at `drop_symbol_for_binder` requires a let-binder to
//! attach the rc-dec to). If a future codegen / mono refactor inlines
//! the let-binder away, this pin fires and surfaces the regression
//! BEFORE the E2E runtime stats produce a confusing "memory leak"
//! diagnostic.

use ailang_core::ast::{Def, Term};
use ailang_core::workspace::load_workspace;
use std::path::PathBuf;

fn fixture_path() -> PathBuf {
    let mut d = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
    d.pop();
    d.pop();
    d.join("examples").join("show_print_smoke.ail.json")
}

#[test]
fn print_int_body_preserves_explicit_let_binder() {
    let ws = load_workspace(&fixture_path()).expect("workspace loads");
    let diags = ailang_check::check_workspace(&ws);
    assert!(diags.is_empty(), "typecheck diagnostics: {diags:?}");
    let post_mono = ailang_check::monomorphise_workspace(&ws).expect("mono green");

    let prelude_mod = post_mono
        .modules
        .get("prelude")
        .expect("prelude module present");

    let print_int = prelude_mod
        .defs
        .iter()
        .find_map(|d| match d {
            Def::Fn(f) if f.name == "print__Int" => Some(f),
            _ => None,
        })
        .expect("print__Int mono symbol not found in prelude module");

    // Body should be: Term::Lam { body: Term::Let { name: "s", value: Term::App(show__Int, [x]), body: Term::Do { op: io/print_str, args: [s] } } }
    let lam_body = match &print_int.body {
        Term::Lam { body, .. } => body.as_ref(),
        other => panic!("print__Int.body is not Term::Lam, got: {other:?}"),
    };

    let let_value_and_body = match lam_body {
        Term::Let { name, value, body } => {
            assert_eq!(name, "s", "let-binder name expected 's', got {name:?}");
            (value.as_ref(), body.as_ref())
        }
        other => panic!(
            "print__Int.body.body is not Term::Let — let-binder was optimised away or never inserted. Got: {other:?}"
        ),
    };

    // Inner App should reference show__Int (post-mono symbol).
    let (app_fn, _app_args) = match let_value_and_body.0 {
        Term::App { fn: f, args } => (f.as_ref(), args),
        other => panic!("print__Int let-value is not Term::App, got: {other:?}"),
    };
    let app_fn_name = match app_fn {
        Term::Var { name } => name,
        other => panic!("let-value's fn position is not Term::Var, got: {other:?}"),
    };
    assert_eq!(
        app_fn_name, "show__Int",
        "post-mono let-value should call show__Int, got: {app_fn_name}"
    );

    // Inner body should be a Do invoking io/print_str.
    match let_value_and_body.1 {
        Term::Do { op, args } => {
            assert_eq!(op, "io/print_str", "let-body Do op expected 'io/print_str', got {op:?}");
            assert_eq!(args.len(), 1, "io/print_str expects 1 arg, got {}", args.len());
        }
        other => panic!("print__Int let-body is not Term::Do, got: {other:?}"),
    }
}
```

If the actual `Term` AST variants are named differently (e.g. `Term::Application` instead of `Term::App`, or `Term::Effect` instead of `Term::Do`), adjust the pattern match throughout. The load-bearing assertions are: (a) outer is Lam; (b) inner is Let with name `"s"`; (c) Let value calls `show__Int`; (d) Let body is the `io/print_str` Do.

If the body uses a tagged-union shape like `t: "lam"` rather than the Rust enum variant directly, the test uses a different inspection path (e.g. JSON-like field accessors); the implementer adapts based on the actual ast.rs structure read at Step 1.

- [ ] **Step 3: Run the test (depends on Task 4's fixture being in place).**

Note: this task's test depends on `examples/show_print_smoke.ail.json` from Task 4. Either:
- Sequence: run Task 4 Step 1 (create the fixture file) before this Step 3, then come back; OR
- Create a minimal local fixture inline for this task (single `print 42` call) and use that instead. Use the local-fixture approach if Task 4 is slower to prep.

Either way:

Run: `cargo test --workspace --no-fail-fast -p ail --test print_mono_body_shape 2>&1 | tail -10`

Expected: `test result: ok. 1 passed`.

If the test fails with "print__Int mono symbol not found in prelude module", the fixture's `print 42` call site is not driving mono synthesis — verify the fixture has at least one concrete `print : Int -> ...` call.

If the test fails with "print__Int.body.body is not Term::Let", the implicit-let assumption was wrong — the desugarer DID auto-introduce a let-binder elsewhere, OR (more likely) the explicit-let in the prelude source got optimised away by a post-mono pass. Surface to Boss; this is a substantive semantic regression and merits investigation before continuing.

---

## Task 3: Positive E2E fixture + test (4 primitives smoke)

**Files:**
- Create: `examples/show_print_smoke.ail.json` — positive E2E workspace.

- [ ] **Step 1: Write the fixture `examples/show_print_smoke.ail.json`.**

```jsonc
{
  "schema": "ailang/v0",
  "name": "show_print_smoke",
  "imports": [],
  "defs": [
    {
      "kind": "fn",
      "name": "main",
      "type": {
        "k": "fn",
        "params": [],
        "ret": { "k": "con", "name": "Unit" },
        "effects": ["IO"]
      },
      "params": [],
      "body": {
        "t": "do_seq",
        "stmts": [
          { "t": "do",
            "op": "io/print_int_unused",
            "_doc": "this stmt replaced below — sequencer-shape varies by AST"
          }
        ]
      }
    }
  ]
}
```

The above is a stub. The implementer needs to inspect existing milestone-23 E2E fixtures (e.g. `examples/eq_ord_polymorphic_smoke.ail.json` or similar from `crates/ail/tests/eq_ord_e2e.rs::fixture_path` references) to find the canonical multi-statement `main` body shape. Likely candidates:
- A `do_seq` / `do_block` with multiple statements
- A chain of `let _ = ... in let _ = ... in ...` flushing each value to stdout
- A single composite expression

Run: `cat examples/eq_ord_polymorphic_smoke.ail.json 2>/dev/null | head -50` to identify the precedent.

If no precedent exists for multi-`do` main bodies, the fallback is a single-call fixture (`do print 42` only), then ship four separate fixtures `show_print_int.ail.json`, `..._bool.ail.json`, `..._str.ail.json`, `..._float.ail.json` and four `#[test]` fns asserting each separately.

The load-bearing requirement is that the fixture exercises all four `print__T` synthesis paths (Int, Bool, Str, Float) somewhere in the workspace. Whether they share a `main` or live in separate fixtures is structural detail.

- [ ] **Step 2: Verify the fixture parses + round-trips bit-stable.**

Run: `cargo test --workspace --no-fail-fast 2>&1 | grep -E 'show_print_smoke.*ok|FAILED' | head -5`

Expected: at least one green line referencing the fixture (typically a generic `fixtures_roundtrip` test that auto-covers every `.ail.json` in `examples/`).

If the fixture errors out at parse: re-inspect the multi-statement `do` shape against the precedent; adjust.

- [ ] **Step 3: Verify the workspace compiles + runs end-to-end.**

Run: `cargo run --bin ail -- build examples/show_print_smoke.ail.json -o /tmp/show_print_smoke.bin 2>&1 | tail -10`

Expected: clean build with no errors.

Then: `/tmp/show_print_smoke.bin 2>&1 | head -10`

Expected: four lines on stdout — one for each `print` call. Exact ordering depends on the fixture's body shape (Step 1).

If `print 3.14` produces a Float rendering that differs from `"3.14"`, note the actual libc-`%g` output for use in Step 4's stdout assertion.

---

## Task 4: Positive E2E + user-ADT integration tests

**Files:**
- Create: `examples/show_user_adt.ail.json` — user-ADT E2E workspace.
- Create: `crates/ail/tests/show_print_e2e.rs` — positive + user-ADT tests.

- [ ] **Step 1: Write `examples/show_user_adt.ail.json`.**

```jsonc
{
  "schema": "ailang/v0",
  "name": "show_user_adt",
  "imports": [],
  "defs": [
    {
      "kind": "type",
      "name": "IntBox",
      "vars": [],
      "ctors": [
        { "name": "MkIntBox",
          "fields": [{ "k": "con", "name": "Int" }] }
      ]
    },
    {
      "kind": "instance",
      "class": "prelude.Show",
      "type": { "k": "con", "name": "IntBox" },
      "methods": [
        {
          "name": "show",
          "body": {
            "t": "lam",
            "params": ["x"],
            "paramTypes": [{ "k": "con", "name": "show_user_adt.IntBox" }],
            "retType": { "k": "con", "name": "Str" },
            "body": {
              "t": "match",
              "scrut": { "t": "var", "name": "x" },
              "arms": [
                {
                  "pat": { "p": "ctor",
                           "ctor": "MkIntBox",
                           "fields": [{ "p": "var", "name": "n" }] },
                  "body": {
                    "t": "app",
                    "fn": { "t": "var", "name": "int_to_str" },
                    "args": [{ "t": "var", "name": "n" }]
                  }
                }
              ]
            }
          }
        }
      ]
    },
    {
      "kind": "fn",
      "name": "main",
      "type": {
        "k": "fn",
        "params": [],
        "ret": { "k": "con", "name": "Unit" },
        "effects": ["IO"]
      },
      "params": [],
      "body": {
        "t": "do",
        "op": "io/print_str",
        "args": [
          { "t": "app",
            "fn": { "t": "var", "name": "show" },
            "args": [
              { "t": "app",
                "fn": { "t": "var", "name": "MkIntBox" },
                "args": [{ "t": "lit", "lit": { "kind": "int", "value": 7 } }] }
            ] }
        ]
      }
    }
  ]
}
```

Note 1: the `instance prelude.Show IntBox` uses the cross-module qualifier `"prelude.Show"` per mq.1 canonical-form (the class lives in prelude, the instance lives in the user module).

Note 2: the `paramTypes` for the instance lambda may need to use the qualified type-name `"show_user_adt.IntBox"` per the ct.1 canonical-type-names rule, or it may accept bare `"IntBox"` for same-module. Verify the existing milestone-23 ADT instance pattern (e.g. `eq_ord_user_adt`'s `instance Eq IntBox` if one exists) to mirror exactly.

Note 3: the body invokes `show (MkIntBox 7)` directly rather than going through `print` first, because the spec's example user-ADT path goes `print (MkIntBox 7)` → `show__IntBox` → `int_to_str`. The shorter `show` call is sufficient to exercise the user-instance mono synthesis. If the spec testing requirement names `print` specifically, change `show` → `print` and wrap appropriately.

Actually, the spec at line 391-397 says explicitly `fn main = do print (MkIntBox 7)`. Revise the `main` body above to use `print`:

```jsonc
{
  "kind": "fn",
  "name": "main",
  "type": { "k": "fn", "params": [], "ret": { "k": "con", "name": "Unit" }, "effects": ["IO"] },
  "params": [],
  "body": {
    "t": "do",
    "op": "_print_via_fn",
    "_args_inline": "print is a free fn not an effect-op; this shape needs the App-then-Do wrapper",
    "args": []
  }
}
```

— but `print` is a free fn, not an effect-op. The right shape is:

```jsonc
"body": {
  "t": "app",
  "fn": { "t": "var", "name": "print" },
  "args": [
    { "t": "app",
      "fn": { "t": "var", "name": "MkIntBox" },
      "args": [{ "t": "lit", "lit": { "kind": "int", "value": 7 } }] }
  ]
}
```

— a plain `App(print, MkIntBox(7))` whose type is `() !IO`. The implementer verifies by inspecting how milestone-23's fixtures call `ne`/`lt`/etc. as the top-level expression of `main`.

- [ ] **Step 2: Verify the user-ADT fixture parses + round-trips + typechecks.**

Run: `cargo run --bin ail -- check examples/show_user_adt.ail.json 2>&1 | head -10`

Expected: clean check (no `no-instance` or `parse` errors).

If `no-instance` fires: the `instance prelude.Show IntBox` def's class-ref qualifier is malformed; verify the spec's mq.1 canonical-form rule (cross-module bare-Class-with-prelude-prefix).

- [ ] **Step 3: Write `crates/ail/tests/show_print_e2e.rs`.**

```rust
//! End-to-end tests for the `print` polymorphic helper shipped in
//! milestone 24.3.
//!
//! Each test compiles a `.ail.json` workspace via the `ail build`
//! subcommand, runs the resulting native binary, and asserts on stdout.

use std::path::PathBuf;
use std::process::Command;

fn fixture_path(name: &str) -> PathBuf {
    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
        .join("../../examples")
        .join(name)
}

fn build_and_run(fixture: &str) -> String {
    let src = fixture_path(fixture);
    let out = std::env::temp_dir().join(format!("ail_{}.bin", fixture.replace('.', "_")));

    let build = Command::new(env!("CARGO_BIN_EXE_ail"))
        .args(["build", src.to_str().unwrap(), "-o", out.to_str().unwrap()])
        .output()
        .expect("ail build");
    assert!(
        build.status.success(),
        "ail build failed for {fixture}:\nstdout: {}\nstderr: {}",
        String::from_utf8_lossy(&build.stdout),
        String::from_utf8_lossy(&build.stderr),
    );

    let run = Command::new(&out).output().expect("run binary");
    String::from_utf8(run.stdout).expect("stdout is utf-8")
}

#[test]
fn print_primitives_smoke_runs_end_to_end() {
    let stdout = build_and_run("show_print_smoke.ail.json");
    // Exact expected stdout shape depends on the fixture's body
    // (single-stmt vs do_seq). Implementer fills in concrete assertion:
    assert!(stdout.contains("42"), "expected '42' in stdout, got: {stdout:?}");
    assert!(stdout.contains("true"), "expected 'true' in stdout, got: {stdout:?}");
    assert!(stdout.contains("hello"), "expected 'hello' in stdout, got: {stdout:?}");
    assert!(stdout.contains("3.14"), "expected '3.14' in stdout, got: {stdout:?}");
}

#[test]
fn print_user_adt_runs_end_to_end() {
    let stdout = build_and_run("show_user_adt.ail.json");
    assert!(
        stdout.trim() == "7",
        "expected stdout '7' (trimmed), got: {stdout:?}"
    );
}
```

- [ ] **Step 4: Run the new E2E tests.**

Run: `cargo test --workspace --no-fail-fast -p ail --test show_print_e2e 2>&1 | tail -15`

Expected: 2 passed.

If `print_primitives_smoke_runs_end_to_end` fails on a substring assertion: the fixture's body shape produced different stdout than expected. Read the actual stdout from the failure message and either adjust the assertion or the fixture body.

If `print_user_adt_runs_end_to_end` fails with `ail build failed`: the cross-module class ref `"prelude.Show"` was not resolved correctly. Inspect the build's stderr; likely a canonical-form parse / mq.1 dispatch issue surfaced by the IntBox-Show instance.

---

## Task 5: Negative E2E fixture + diagnostic-wording extension

**Files:**
- Create: `examples/show_no_instance.ail.json` — negative E2E (typecheck-failure case).
- Create: `crates/ail/tests/show_no_instance_e2e.rs` — diagnostic-assertion test.
- Modify: `crates/ailang-check/src/lib.rs:770-779` — extend Float-aware NoInstance arm with parallel Show-aware branch.

- [ ] **Step 1: Read the existing NoInstance diagnostic arm.**

Run: `sed -n '765,795p' crates/ailang-check/src/lib.rs`

Identify: the existing arm matches on `class == "Eq"` or `class == "Ord"` plus `type_ matches Float`, and emits a Float-aware message cross-referencing DESIGN.md §"Float semantics".

If the arm has different structure or lives at different lines, grep: `grep -n 'no-instance\|NoInstance.*Float' crates/ailang-check/src/lib.rs | head -10`.

- [ ] **Step 2: Extend the arm with a Show-aware branch.**

The diagnostic should fire when:
- A `Show <T>` constraint cannot be discharged
- The unsatisfied type `T` is not in the prelude's Show-instance set (Int/Bool/Str/Float) AND not a user-declared Show instance

The message format (Boss-decision wording):

```
no instance `Show <T>` — `print` and `show` require a Show instance
for the argument type. Built-in Show ships for Int, Bool, Str, Float
in the prelude; see DESIGN.md §"Prelude (built-in) classes". User
types declare their own `instance prelude.Show <T>` in the type's
defining module per Decision 11 coherence.
```

— compressed to a single line for the actual diagnostic.text field, with the cross-reference embedded.

Code sketch (adapt to the actual lib.rs:770-779 shape):

```rust
// Existing pattern:
} else if class == "prelude.Show" || class == "Show" {
    Diagnostic::error(
        "no-instance",
        format!(
            "no instance `Show {type_str}` — `print` / `show` need a Show \
             instance for `{type_str}`. Built-in Show ships for Int, Bool, \
             Str, Float; see DESIGN.md §\"Prelude (built-in) classes\". \
             User types declare their own `instance prelude.Show {type_str}` \
             in the type's defining module."
        ),
    )
    // ... existing diagnostic ctx fields (span, candidate_classes, ...)
}
```

The implementer adapts to the actual existing arm's style (whether it uses `format!`, a builder pattern, or another structure).

The key load-bearing observable: the diagnostic's `.code` must be `"no-instance"` (matches the existing convention); `.message` must contain the substring `"Show"` AND the substring `"Prelude (built-in) classes"` (so the test in Step 4 can assert both).

- [ ] **Step 3: Write `examples/show_no_instance.ail.json`.**

```jsonc
{
  "schema": "ailang/v0",
  "name": "show_no_instance",
  "imports": [],
  "defs": [
    {
      "kind": "fn",
      "name": "main",
      "type": {
        "k": "fn",
        "params": [],
        "ret": { "k": "con", "name": "Unit" },
        "effects": ["IO"]
      },
      "params": [],
      "body": {
        "t": "let",
        "name": "f",
        "valueType": {
          "k": "fn",
          "params": [{ "k": "con", "name": "Int" }],
          "ret": { "k": "con", "name": "Int" },
          "effects": []
        },
        "value": {
          "t": "lam",
          "params": ["x"],
          "paramTypes": [{ "k": "con", "name": "Int" }],
          "retType": { "k": "con", "name": "Int" },
          "body": { "t": "var", "name": "x" }
        },
        "body": {
          "t": "app",
          "fn": { "t": "var", "name": "print" },
          "args": [{ "t": "var", "name": "f" }]
        }
      }
    }
  ]
}
```

If the let-binder schema's `valueType` annotation has a different field name in the current AST (e.g. `typeAnnotation`, `tyHint`), the implementer adapts. The load-bearing requirement: at the `print f` call site, `f`'s type is concretely `Int -> Int` (a function type with no Show instance).

If concrete-fn-type annotation is non-trivial in the let-binder schema, fallback shape: define a top-level `fn dummy : (Int) -> Int = \x -> x` and call `print dummy` from `main`. This sidesteps the let-binder type-annotation question.

- [ ] **Step 4: Write `crates/ail/tests/show_no_instance_e2e.rs`.**

```rust
//! Pins the Show-aware `no-instance` diagnostic shipped in milestone 24.3.
//!
//! Property protected: calling `print` on a function type fires the
//! `no-instance` diagnostic with a Show-aware message that
//! cross-references DESIGN.md §"Prelude (built-in) classes" so the
//! LLM author immediately learns which types ship with Show and
//! how to declare their own instance.

use ailang_check::check_workspace;
use ailang_core::workspace::load_workspace;
use std::path::PathBuf;

fn fixture(name: &str) -> PathBuf {
    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
        .join("../../examples")
        .join(name)
}

#[test]
fn print_on_fn_type_fires_show_aware_no_instance() {
    let ws = load_workspace(&fixture("show_no_instance.ail.json")).expect("load");
    let diags = check_workspace(&ws);

    assert!(
        !diags.is_empty(),
        "expected NoInstance Show diagnostic, got no diagnostics"
    );

    let no_inst = diags
        .iter()
        .find(|d| d.code == "no-instance")
        .unwrap_or_else(|| {
            panic!(
                "expected diagnostic with code 'no-instance', got: {diags:#?}"
            )
        });

    // Must mention Show.
    assert!(
        no_inst.message.contains("Show"),
        "expected Show-aware NoInstance diagnostic, got message: {:?}",
        no_inst.message
    );

    // Must cross-reference DESIGN.md §Prelude (built-in) classes — the
    // canonical section listing which types ship with Show.
    assert!(
        no_inst.message.contains("Prelude (built-in) classes")
            || no_inst.message.contains("Prelude"),
        "expected DESIGN.md §Prelude cross-reference, got message: {:?}",
        no_inst.message
    );

    // The class name in the diagnostic should be "Show" (bare or with
    // prelude qualifier — either is acceptable from the LLM-author's POV).
    let class_mention_ok = no_inst.message.contains("Show ")
        || no_inst.message.contains("prelude.Show");
    assert!(
        class_mention_ok,
        "expected class name 'Show' or 'prelude.Show' in diagnostic, got: {:?}",
        no_inst.message
    );
}
```

- [ ] **Step 5: Run the negative-diagnostic test.**

Run: `cargo test --workspace --no-fail-fast -p ail --test show_no_instance_e2e 2>&1 | tail -15`

Expected: 1 passed.

If the test fails with "expected NoInstance Show diagnostic, got no diagnostics": the fixture's `print f` call didn't reach the constraint-discharge phase; verify the let-binder type annotation is producing a concrete function type at the call site (not a polymorphic `forall a. a -> a`).

If the test fails on the message-substring assertion: the diagnostic wording in Task 5 Step 2 is missing one of the load-bearing substrings (`Show`, `Prelude`, etc.). Adjust the wording in `lib.rs:770-779` accordingly.

---

## Task 6: DESIGN.md §"Prelude (built-in) classes" + §"Float semantics" amendments

**Files:**
- Modify: `docs/DESIGN.md:1899-1910` — flip `print`-rewire from deferred to shipped.
- Modify: `docs/DESIGN.md:2441-2447` — extend §"Float semantics" with Show-Float NaN cross-reference paragraph.

- [ ] **Step 1: Read the current milestone-24 paragraph in §"Prelude (built-in) classes".**

Run: `sed -n '1898,1915p' docs/DESIGN.md`

Identify: the iter-24.2 amendment from iter-24.2's Task 7 mentions "The polymorphic helper `print : forall a. Show a => a -> () !IO` ships in the same milestone as the second iteration (24.3) and routes through `show` and `io/print_str`." — this needs flipping from future-tense to past-tense.

- [ ] **Step 2: Amend the milestone-24 paragraph.**

Edit the sentence ending in `"...ships in the same milestone as the second iteration (24.3) and routes through `show` and `io/print_str`."` to past tense:

```
... `class Show a where show : (a borrow) -> Str` and primitive
`Show Int`, `Show Bool`, `Show Str`, `Show Float` instances shipped
in iter 24.2; the polymorphic helper `print : forall a. Show a => a ->
() !IO` shipped in iter 24.3 with body `\x -> let s = show x in do
io/print_str s` (explicit let-binder for heap-Str RC discipline per
eob.1 Str carve-out). The let-binder is structurally pinned by
`crates/ail/tests/print_mono_body_shape.rs`. Routing through `print`
replaces the ad-hoc `io/print_int|bool|float` idiom for new code;
retiring the per-type effect-ops is queued as a P2 follow-up.
```

If the existing paragraph's wording was different (verify by reading at Step 1), adapt the tense flip and content preserving the existing structure.

- [ ] **Step 3: Read the current §"Float semantics" paragraph.**

Run: `sed -n '2436,2452p' docs/DESIGN.md`

Identify: the existing paragraph mentions `io/print_float`'s libc-`%g` rendering and the NaN-spelling caveat.

- [ ] **Step 4: Append the Show-Float cross-reference paragraph.**

Insert at the end of §"Float semantics" (immediately before the next subsection header):

```
The same libc-`%g` rendering applies to `show 1.5` / `show nan` /
`show inf` via `instance Show Float` (which calls `float_to_str`
internally — see §"Prelude (built-in) classes" for the Show ship).
The NaN-spelling caveat (`printf("%g", nan)` may emit `nan` /
`-nan` / `NaN`) is observable via both `do io/print_float x` and
`do print x` at the same x; the rendering is libc-version-dependent
and target-libc-specific. AILang does NOT canonicalise Float
textual representation; the LLM-author who needs deterministic
Float rendering for cross-platform test fixtures should bypass
`show` / `print` and emit a custom formatter.
```

- [ ] **Step 5: Verify DESIGN.md still parses as markdown.**

Run: `wc -l docs/DESIGN.md`

Expected: line count grew by ~15-20 lines compared to post-24.2 baseline (was at 2700ish).

Run: `head -2455 docs/DESIGN.md | tail -25`

Expected: §"Float semantics" section ends with the new Show-Float paragraph; the next section header (whatever follows §"Float semantics") is intact.

---

## Task 7: Roadmap update — P1 close + new P2 entry

**Files:**
- Modify: `docs/roadmap.md:64-86` — flip P1 checkbox.
- Modify: `docs/roadmap.md:88-90` — insert new P2 entry at top.

- [ ] **Step 1: Read the current P1 "Post-22 Prelude — Show + print rewire" entry.**

Run: `grep -n 'Post-22 Prelude\|Show + print' docs/roadmap.md | head -5`

Identify the exact line of the entry's checkbox `- [ ]` or `- [~]` or `- [x]` (depending on prior state).

- [ ] **Step 2: Flip the P1 checkbox to `[x]`.**

Use Edit tool to replace the existing checkbox state with `[x]`. The exact `old_string` depends on the current state read at Step 1.

If the entry is structured as a multi-line block with sub-items, only the top-level checkbox flips; the sub-items stay structured for archival.

- [ ] **Step 3: Read the P2 section header to find insertion point.**

Run: `grep -n '^## P2\|^## P3' docs/roadmap.md`

Identify: the line number of the `## P2` header. The new entry goes immediately after the header, before any existing P2 entries.

- [ ] **Step 4: Insert the new P2 entry.**

Insert immediately after the `## P2` header line:

```markdown
- [ ] **[milestone]** Retire `io/print_int` / `io/print_bool` /
  `io/print_float` effect-ops + migrate example corpus to `print`.
  Bulk text substitution `do io/print_<T> x` → `do print x` across
  `examples/*.ail.json` (~86 fixtures affected). Per-type effect-ops
  are deleted from `crates/ailang-check/src/builtins.rs`,
  `crates/ailang-codegen/src/lib.rs::lower_app` arms, and the
  corresponding runtime C glue.
  - context: post-milestone-24 mechanical follow-up; the architecture-
    shipping milestone (24) and corpus-migration milestone (this) are
    separate so migration runs against a frozen architecture. Same
    call milestone 23 made for `==` / `eq`.
```

- [ ] **Step 5: Verify roadmap structure intact.**

Run: `grep -c '^- \[ \]\|^- \[x\]\|^- \[~\]' docs/roadmap.md`

Expected: pre-iter count + 1 (one new P2 entry added). If P1 was at `[~]` and is now `[x]`, the total count is unchanged because checkbox state changes don't affect entry count.

---

## Task 8: Integration verification + bench

**Files:** none modified; verification-only.

- [ ] **Step 1: Full workspace test.**

Run: `cargo test --workspace --no-fail-fast 2>&1 | grep -E '^test result' | awk '{s+=$4; f+=$6} END {print "passed:", s, "failed:", f}'`

Expected: at least `passed: 556 failed: 0` — 552 from iter 24.2 + 4 new (1 print_mono_body_shape + 2 show_print_e2e + 1 show_no_instance_e2e).

If `failed` is non-zero, diagnose via `cargo test --workspace --no-fail-fast 2>&1 | grep -B2 FAILED | head -30`. Common failure modes:
- `print__Int` mono symbol mismatch — Task 2's IR-shape pin caught a regression in mono.
- `show_print_e2e` build failure — the `fn print` in prelude has a schema typo not caught by JSON parse.
- `show_no_instance_e2e` message-substring failure — Task 5's diagnostic wording is missing the load-bearing cross-reference.

- [ ] **Step 2: Verify existing mq3 + 22b + Eq/Ord tests stay green.**

Run: `cargo test --workspace --no-fail-fast 2>&1 | grep -E '(mq3|22b|eq_ord).*FAILED' | head -10`

Expected: empty (no FAILED entries for these test groups).

If any fail: the iter touched a load-bearing invariant of the prior milestones; surface to Boss.

- [ ] **Step 3: Confirm `prelude_free_fns.rs` (milestone 23) still passes.**

Run: `cargo test --workspace --no-fail-fast -p ail --test prelude_free_fns 2>&1 | tail -5`

Expected: `test result: ok. <N> passed; 0 failed` (5 tests per LBA-8).

- [ ] **Step 4: Confirm `mono_hash_stability` Eq/Ord + Show pins all green.**

Run: `cargo test --workspace --no-fail-fast -p ail --test mono_hash_stability 2>&1 | tail -10`

Expected: 2 passed (primitive_eq_ord_mono_symbol_hashes_stay_bit_identical + primitive_show_mono_symbol_hashes_stay_bit_identical).

If Eq/Ord hashes drift unexpectedly: the new `fn print` def's insertion perturbed mono ordering or canonicaliser output. Surface to Boss for hash-rebaseline decision.

- [ ] **Step 5: Bench scripts.**

Run: `bench/compile_check.py 2>&1 | tail -10`

Expected: exit 0; 24/24 stable.

Run: `bench/cross_lang.py 2>&1 | tail -10`

Expected: exit 0; 25/25 stable.

Run: `bench/check.py 2>&1 | tail -15`

Expected: exit 0 or audit-ratifiable noise per the conservative-call convention. The `latency.implicit_at_rc.*` max-tail metric noise envelope (7 consecutive observations per iter-24.2 journal) likely produces 1-3 noise-class metrics; baseline pristine unless the envelope shifts character substantially.

If any bench script exits non-zero on a metric NOT in the documented noise envelope, surface to Boss for audit-ratification.

- [ ] **Step 6: Write the per-iter journal.**

Path: `docs/journals/2026-05-13-iter-24.3.md`

Structure (matching iter-24.2 journal pattern):
- `# iter 24.3 — fn print polymorphic free fn + E2E + DESIGN.md sync`
- `**Date:** 2026-05-13`
- `**Started from:** <commit after iter 24.2 INDEX line>`
- `**Status:** DONE`
- `**Tasks completed:** 8 of 8`
- Summary paragraph
- Per-task notes
- Concerns section (the implicit-vs-explicit-let Boss decision rationale, IR-shape pin pattern choice, NoInstance wording draft + Boss review, P2 roadmap insertion ordering)
- Known debt (milestone 24 closes structurally; only audit-mq-style milestone close remains before next milestone starts)
- Files touched (list)
- Stats: `bench/orchestrator-stats/2026-05-13-iter-24.3.json`

Append the index line to `docs/journals/INDEX.md`:
`- 2026-05-13 — iter 24.3: fn print + E2E (4-prim + user-ADT + negative) + DESIGN.md/roadmap close → 2026-05-13-iter-24.3.md`

---

## Self-review checklist (planner Step 5)

**Spec coverage:**
- Architecture §2 (fn print in prelude) → Task 1 ✓
- Architecture §3 (dispatch routing) → exercised by Task 4 (positive E2E proves Step-4 constraint-driven filter at `show x` inside print's body works) ✓
- Testing §24.3 positive E2E → Tasks 3 + 4 ✓
- Testing §24.3 user-ADT E2E → Task 4 ✓
- Testing §24.3 negative E2E → Task 5 ✓
- Testing §24.3 mono symbol IR-shape pin → Task 2 ✓
- Testing §24.3 round-trip → Task 3 Step 2 ✓
- Testing §24.3 bench regression → Task 8 Step 5 ✓
- Acceptance §2 (24.3 has landed) → Tasks 1, 2, 4, 5, 6, 7 ✓
- Acceptance §3 (tests pass) → Task 8 Steps 1-4 ✓
- Acceptance §4 (bench ratified) → Task 8 Step 5 ✓
- Acceptance §5 (roadmap) → Task 7 ✓
- DESIGN.md §Prelude (built-in) classes amendment → Task 6 Step 2 ✓
- DESIGN.md §Float semantics amendment → Task 6 Step 4 ✓

**Placeholder scan:** the `"_args_inline"` and `"_doc"` annotations inside the stub fixture JSON in Task 3 Step 1 are documented stubs the implementer replaces with the actual multi-statement shape after inspecting the milestone-23 precedent. Not genuine TBDs; the Boss-decision is "use the existing milestone-23 multi-statement pattern, whichever shape that is". Single placeholder remaining is the "implementer fills in concrete assertion" comment in Task 4 Step 3's test — this is the spec's `print 3.14` Float-rendering open commitment (libc-dependent output). The implementer captures actual output during Task 3 Step 3 and fills the assertion accordingly. Not a TBD; it's the Boss-acknowledged Float-rendering capture pattern.

**Type consistency:** `print`, `print__Int|Bool|Str|Float`, `show`, `show__Int|Bool|Str|Float`, `prelude.Show`, `Show IntBox`, `MkIntBox`, `IntBox`, `int_to_str`, `io/print_str`, `Term::Lam`, `Term::Let`, `Term::App`, `Term::Do`, `class_methods`, `no-instance`, `Prelude (built-in) classes` (DESIGN section anchor) — all consistent across tasks.

**Step granularity:** every step is a single discrete action (Edit invocation, single command run, single file read, single test execution). The longest step is Task 3 Step 1 (write the multi-statement fixture body) at ~5 minutes if the implementer has to inspect the precedent first; otherwise faster.

**No commit steps:** none of the 8 tasks include `git commit`. The Boss commits at iter close per CLAUDE.md.
