# nullary-app.1 — Accept `(app f)` as the canonical zero-arg call form

> **No parent spec.** Gitea issue #12 carries the resolved design
> fork verbatim; two prior fieldtest specs
> (`docs/specs/0030-fieldtest-mut-local.md` F3,
> `docs/specs/0035-fieldtest-loop-recur.md` spec_gap) are the
> LLM-natural-shape evidence. User accepted `(app f)` + Ctor-style
> serde convention on 2026-05-20.
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement`
> to run this plan.

**Goal:** Lift the surface parser's "at least one argument" rule on
`app-term` (and `tail-app-term`) so a niladic function can be called
as `(app f)`. Round-trip and downstream stages already accept empty
`args` mechanically — typechecker's arity check is `args.len() ==
params.len()` (passes for `0 == 0`), codegen's `args.iter().zip(...)`
loop is a no-op for empty args, surface printer's arg-emit loop
prints nothing for empty args. The fix is the parser guard plus one
serde-attribute edit on `Term::App.args` to mirror `Term::Ctor.args`
(read-tolerant when `args` is absent in canonical JSON).

**Doc-honesty side-fix:** the existing comment on `Term::Ctor` in
`design/contracts/0002-data-model.md:130-131` claims "`args` omitted when
empty", which is *factually false* for the current code — `Ctor.args`
has `#[serde(default)]` (read-tolerant) but NO `skip_serializing_if`,
so writes always emit `"args":[]`. Fix the comment to describe actual
behaviour. This is `design/contracts/0007-honesty-rule.md` enforcement
folded into the same iter (the discovery happened while researching
this iter's serde change, and a stale comment in the same jsonc
block we're modifying must not be left behind).

**Architecture:** Three surface-layer edits + one schema-doc edit + one
new E2E test (which is also the RED-then-GREEN pin for the parser
change). No typechecker, codegen, or runtime changes — every layer
below the parser already handles empty `args` correctly today.

**Tech Stack:** `ailang-surface` (parser), `ailang-core` (AST serde),
`design/contracts/0002-data-model.md` (schema doc), `crates/ail` (E2E
test).

**Files this plan creates or modifies:**

- Create: `examples/nullary_app_smoke.ail` — single nullary user fn
  `greet : fn() -> Unit !IO` called as `(app greet)`.
- Create: `crates/ail/tests/nullary_app_e2e.rs` — builds and runs
  the fixture; asserts `"hello\n"` stdout.
- Modify: `crates/ailang-surface/src/parse.rs:1322-1342` — drop the
  4-line empty-args guard from `parse_app_body`; the doc comment on
  `parse_app_body` changes from "callee + 1+ args + closing `)`" to
  "callee + 0+ args + closing `)`". The grammar comment near the top
  of the file (`parse.rs:48`) changes `term+` to `term*`.
- Modify: `crates/ailang-core/src/ast.rs:419-425` — annotate
  `Term::App.args` with `#[serde(default)]`, mirroring
  `Term::Ctor.args` at line 472-477. `skip_serializing_if` is NOT
  added (mirror Ctor's *actual* attrs, not the doc claim).
- Modify: `design/contracts/0002-data-model.md:112` — add an inline comment
  to the `app` jsonc shape that `args` MAY be empty (e.g. nullary
  call `(app f)`).
- Modify: `design/contracts/0002-data-model.md:130-131` — rewrite the
  misleading "args omitted when empty" comment on the `ctor` jsonc
  shape so it describes the actual write/read behaviour: write
  always emits `"args":[]`; read tolerates absent `args`.

---

## Task 1: RED — fixture + E2E test for nullary `(app f)`

**Files:**
- Create: `examples/nullary_app_smoke.ail`
- Create: `crates/ail/tests/nullary_app_e2e.rs`

The RED test compile-blocks NOTHING (it is a new file using public
CLI surface). It fails at runtime because `ail build` returns a parse
error from `parse_app_body`'s "expected at least one argument" guard
on the `(app greet)` call. Watch the failure carry that exact error
string; that confirms the RED hits the targeted symptom.

- [ ] **Step 1.1: Write the example fixture**

  Create `examples/nullary_app_smoke.ail`:

  ```lisp
  (module nullary_app_smoke
    (fn greet
      (doc "Nullary user function — no params, prints a fixed line.")
      (type (fn-type (params) (ret (con Unit)) (effects IO)))
      (params)
      (body (do io/print_str "hello\n")))

    (fn main
      (doc "Calls the nullary `greet` as `(app greet)` — the zero-arg surface form accepted under Gitea #12.")
      (type (fn-type (params) (ret (con Unit)) (effects IO)))
      (params)
      (body (app greet))))
  ```

  Confirm `greet` and `main` both have empty `(params)` and both type
  to `fn() !IO -> Unit`. The fixture exercises BOTH a nullary fn
  definition AND a nullary call to it.

- [ ] **Step 1.2: Write the RED E2E test**

  Create `crates/ail/tests/nullary_app_e2e.rs`:

  ```rust
  //! Gitea #12 RED→GREEN pin — nullary `(app f)` surface acceptance.
  //!
  //! Pre-fix, `ail build` rejects `examples/nullary_app_smoke.ail`
  //! at parse with `parse_app_body`'s "expected at least one
  //! argument" error. Post-fix (parse guard dropped), the file
  //! builds, runs, and prints `"hello\n"` from the nullary `greet`
  //! invoked as `(app greet)`.
  //!
  //! Two independent fieldtest specs flagged the same gap:
  //! `docs/specs/0030-fieldtest-mut-local.md` F3 and
  //! `docs/specs/0035-fieldtest-loop-recur.md` spec_gap. The
  //! Form-A surface refusing nullary calls had no schema backing —
  //! `Term::App.args` is `[Term...]` (zero-or-more), so the parser
  //! guard was an unbacked rule.

  use std::path::Path;
  use std::process::Command;

  fn ail_bin() -> &'static str {
      env!("CARGO_BIN_EXE_ail")
  }

  fn build_and_run(example: &str) -> String {
      let manifest_dir = env!("CARGO_MANIFEST_DIR");
      let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
      let src = workspace.join("examples").join(example);
      let tmp = std::env::temp_dir().join(format!(
          "ailang_e2e_{}_{}",
          example.replace('.', "_"),
          std::process::id()
      ));
      std::fs::create_dir_all(&tmp).unwrap();
      let out = tmp.join("bin");
      let status = Command::new(ail_bin())
          .args(["build", src.to_str().unwrap(), "-o"])
          .arg(&out)
          .status()
          .expect("ail build failed to run");
      assert!(status.success(), "ail build failed for {example}");
      let output = Command::new(&out).output().expect("execute binary");
      assert!(
          output.status.success(),
          "binary {} exited non-zero",
          out.display()
      );
      String::from_utf8(output.stdout).expect("stdout utf8")
  }

  /// A nullary user fn called as `(app greet)` builds and runs; the
  /// nullary application surface form is the canonical zero-arg
  /// shape under the resolution of Gitea #12.
  #[test]
  fn nullary_app_builds_and_runs() {
      let out = build_and_run("nullary_app_smoke.ail");
      assert_eq!(
          out, "hello\n",
          "nullary `(app greet)` must build, run, and emit `hello\\n`; \
           got `{out}` — pre-fix surface parser rejects empty-args at \
           `parse_app_body` with `expected at least one argument` (see \
           Gitea #12).",
      );
  }
  ```

- [ ] **Step 1.3: Confirm RED**

  Run: `cargo test --quiet -p ail --test nullary_app_e2e`

  Expected: FAIL. The failure must mention "expected at least one
  argument" in the `ail build` stderr (visible because `build_and_run`
  asserts `status.success()` and the build failure propagates the
  parser diagnostic). If the failure mode is anything else — wrong
  stdout instead of build failure, panic from a missing file, etc. —
  STOP and re-read; the RED is not pinning the targeted symptom.

---

## Task 2: GREEN — drop the empty-args guard in `parse_app_body`

**Files:**
- Modify: `crates/ailang-surface/src/parse.rs:48`
- Modify: `crates/ailang-surface/src/parse.rs:1315-1342`

- [ ] **Step 2.1: Update the grammar comment near the file top**

  `crates/ailang-surface/src/parse.rs:48` currently reads:

  ```
  //! app-term    ::= "(" "app" term term+ ")"
  ```

  Change `term+` to `term*`. Also update the next line (the
  `tail-app-term` grammar) to match — that line currently mirrors the
  same `term+`.

  Re-verify by re-reading lines 47-50 to ensure both productions now
  read `term*` (zero or more args).

- [ ] **Step 2.2: Update the `parse_app_body` doc comment**

  `crates/ailang-surface/src/parse.rs:1315-1316` reads:

  ```rust
  /// Body shared by [`Self::parse_app`] and [`Self::parse_tail_app`]:
  /// callee + 1+ args + closing `)`.
  ```

  Change `1+ args` to `0+ args`.

- [ ] **Step 2.3: Drop the empty-args guard**

  Delete `crates/ailang-surface/src/parse.rs:1323-1331` — the entire
  guard block:

  ```rust
          let callee = self.parse_term()?;
          // 1+ args
          if matches!(self.peek(), Some(Token { tok: Tok::RParen, .. })) {
              let pos = self.peek().map(|t| t.span.start).unwrap_or(0);
              return Err(ParseError::Production {
                  production,
                  message: "expected at least one argument".into(),
                  pos,
              });
          }
          let mut args = Vec::new();
  ```

  becomes:

  ```rust
          let callee = self.parse_term()?;
          // 0+ args — empty arg list is the nullary call shape `(app f)`,
          // resolution of Gitea #12. The schema permits empty `args` on
          // `Term::App` (data-model.md: `args: [Term...]` is zero-or-more);
          // typechecker, codegen, and surface printer already handle
          // empty `args` cleanly today.
          let mut args = Vec::new();
  ```

- [ ] **Step 2.4: Confirm Task 1's RED now flips to GREEN**

  Run: `cargo test --quiet -p ail --test nullary_app_e2e`

  Expected: PASS with the `"hello\n"` stdout assertion.

---

## Task 3: GREEN — serde `default` on `Term::App.args`

**Files:**
- Modify: `crates/ailang-core/src/ast.rs:419-425`

- [ ] **Step 3.1: Annotate `App.args` with `#[serde(default)]`**

  `crates/ailang-core/src/ast.rs:419-425` currently reads:

  ```rust
      App {
          #[serde(rename = "fn")]
          callee: Box<Term>,
          args: Vec<Term>,
          #[serde(default, skip_serializing_if = "is_false")]
          tail: bool,
      },
  ```

  Add `#[serde(default)]` to `args` so a canonical JSON document that
  omits the `args` key parses to an empty `Vec`. Do NOT add
  `skip_serializing_if = "Vec::is_empty"` — Ctor's actual write
  behaviour always emits `"args":[]`; this iter mirrors that exactly:

  ```rust
      App {
          #[serde(rename = "fn")]
          callee: Box<Term>,
          #[serde(default)]
          args: Vec<Term>,
          #[serde(default, skip_serializing_if = "is_false")]
          tail: bool,
      },
  ```

  The doc comment on `App` (lines 411-418) does not need an edit —
  the variant docstring is about semantics, not serialisation, and
  `tail`'s comment already covers the "additive variants must
  hash-stably" pattern.

- [ ] **Step 3.2: Verify the existing corpus still serialises identically**

  Run: `cargo test --workspace --quiet`

  Expected: every existing test still PASS. The new `#[serde(default)]`
  is read-tolerance-only — it changes deserialisation behaviour for
  documents missing `args`, but every existing `.ail.json` fixture
  has a non-empty `args` (verified at plan-write time:
  `grep -rn '"args":\[\]' examples/*.ail.json` returns zero matches).
  Write behaviour is unchanged. Canonical-byte hashes of every
  existing fixture are therefore unchanged.

---

## Task 4: Doc-honesty fix — `data-model.md` `ctor` and `app` comments

**Files:**
- Modify: `design/contracts/0002-data-model.md:110-131`

This task is `design/contracts/0007-honesty-rule.md` enforcement — the
prose-authoritative contract must describe present-state behaviour,
not aspirational behaviour. The current `Ctor` comment is
aspirational (the omit-when-empty rule is not implemented), and the
`App` jsonc shape benefits from an explicit note now that nullary
calls are legal.

- [ ] **Step 4.1: Add an empty-args note to the `app` shape**

  `design/contracts/0002-data-model.md:110-112` currently reads:

  ```jsonc
  // fn application; tail flag triggers musttail under codegen.
  // `tail` is omitted when false (hash-stable when omitted).
  { "t": "app",  "fn": Term, "args": [Term...], "tail": false }
  ```

  Extend the comment block above the jsonc to mention that `args` may
  be empty (nullary call). New text:

  ```jsonc
  // fn application; tail flag triggers musttail under codegen.
  // `tail` is omitted when false (hash-stable when omitted).
  // `args` may be empty: a nullary call is the surface form
  // `(app f)` (resolution of Gitea #12). Read-tolerant: a JSON
  // document omitting the `args` key deserialises to `[]`.
  { "t": "app",  "fn": Term, "args": [Term...], "tail": false }
  ```

- [ ] **Step 4.2: Rewrite the misleading `ctor` comment**

  `design/contracts/0002-data-model.md:130-131` currently reads:

  ```jsonc
  // Ctor application; `args` omitted when empty.
  { "t": "ctor", "type": "<id>", "ctor": "<id>", "args": [Term...] }
  ```

  Replace with the actual behaviour:

  ```jsonc
  // Ctor application. `args` is always emitted on write (including
  // as `"args": []` for niladic ctors); reads tolerate the key being
  // absent and treat it as `[]`. This mirrors the read/write
  // asymmetry on `Term::App.args` (see above).
  { "t": "ctor", "type": "<id>", "ctor": "<id>", "args": [Term...] }
  ```

- [ ] **Step 4.3: Verify drift-pin still green**

  Run: `cargo test --quiet -p ailang-core --test design_schema_drift`

  Expected: PASS. The drift test scans only jsonc-fenced blocks (per
  bug #10's GREEN fix); the anchor strings `"t": "app"` and
  `"t": "ctor"` remain inside the same jsonc block and are still
  found. The comment changes do NOT move any schema anchor.

---

## Task 5: Workspace-wide sanity

**Files:** none modified.

- [ ] **Step 5.1: Full workspace test**

  Run: `cargo test --workspace --quiet`

  Expected: PASS across all crates. No regressions; new
  `nullary_app_e2e` test in green state from Task 2.4.

- [ ] **Step 5.2: Drift-pin re-run for honesty-rule compliance**

  Run: `cargo test --quiet -p ailang-core --test design_index_pin --test design_schema_drift`

  Expected: PASS. Both drift tests are jsonc-fenced-only after bug
  #10's GREEN; the present iter touches doc-comments inside an
  existing jsonc block but does not move any anchor.

- [ ] **Step 5.3: Round-trip invariant**

  Run: `cargo test --quiet -p ailang-surface --test round_trip`

  Expected: PASS. The new fixture
  `examples/nullary_app_smoke.ail` has no `.ail.json` counterpart
  (none authored), so it falls under the test's "parse-only" branch.
  This is correct: the canonical form a Form-A author would produce
  for nullary `(app f)` is `{"t":"app","fn":{"t":"var","name":"f"},
  "args":[]}` — write-emit always carries `"args":[]`. A future iter
  may author the JSON counterpart explicitly if a hash-stable canonical
  fixture is wanted; not required for this iter.
