AILang/design/contracts/0002-data-model.md

# Data model

## Data model

The on-disk JSON-AST is what the toolchain hashes, typechecks, and
lowers. **This section is the canonical schema.** The Rust types in
`crates/ailang-core/src/ast.rs` are the in-memory projection of it;
when the two disagree, this section wins, and the drift test
`crates/ailang-core/tests/design_schema_drift.rs` fires. Every
additive field is declared with `skip_serializing_if` so pre-existing
fixtures keep bit-identical canonical-JSON hashes — that gating
contract is what makes growing the schema cheap.

### Module

```jsonc
{
  "schema": "ailang/v0",
  "name": "<id>",
  "imports": [{ "module": "<id>", "as": "<id>" }],
  "defs": [Def...]
}
```

### Def

`kind ∈ { "fn", "const", "type", "class", "instance" }`. All five
are real surface forms. Class and type cross-module references
(canonical-form rule, qualified `<module>.<Class>` /
`<module>.<TypeName>`) follow the scoping rule in
[memory model](0008-memory-model.md); the `class`/`instance` schema
narrative — defaults, superclasses, diagnostics — lives in
[typeclasses](0013-typeclasses.md). Exported `fn` defs interact with
[embedding ABI](0003-embedding-abi.md).

```jsonc
// fn (the unit that gets a content hash)
{ "kind": "fn",
  "name": "<id>",
  "type": Type,            // typically Type::Fn, optionally wrapped in Forall
  "params": ["<id>"...],   // names bound in body, in type.params order
  "body": Term,
  "doc": "<optional string>",
  "export": "<optional C symbol>", // omitted when absent (hash-stable when omitted); embedding-ABI surface — see prose below
  "suppress": [Suppress...] // omitted when empty
}

// const (top-level value; codegen emits as a global; body must be pure)
{ "kind": "const",
  "name": "<id>",
  "type": Type,
  "value": Term,
  "doc": "<optional string>"
}

// type (algebraic data type; parameterised)
{ "kind": "type",
  "name": "<id>",
  "vars": ["<id>"...],     // type parameters; omitted when empty (hash-stable when omitted)
  "ctors": [
    { "name": "<id>", "fields": [Type...] }  // nullary ctor: fields = []
    ...
  ],
  "doc": "<optional string>",
  "drop-iterative": true   // opt-in; omitted when false (hash-stable when omitted)
}

// class (typeclass declaration; narrative in contracts/0013-typeclasses.md)
{ "kind": "class",
  "name": "<id>",          // class name (e.g. "Show")
  "param": "<id>",          // single class parameter, kind *
  "superclass": null,       // or { "class": "<id>", "type": "<param>" } — "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module)
  "methods": [
    { "name": "<id>",
      "type": Type,         // FnSig over the class param
      "default": Term       // optional fallback body; null = abstract-required
    }
    ...
  ],
  "doc": "<optional string>"
}

// instance (typeclass instance; narrative in contracts/0013-typeclasses.md)
{ "kind": "instance",
  "class": "<id>",          // class being instantiated; canonical form (bare for same-module, "<module>.<Class>" for cross-module)
  "type": Type,             // concrete type expression (never the class param)
  "methods": [
    { "name": "<id>", "body": Term }
    ...
  ],
  "doc": "<optional string>"
}
```

**`Suppress`** (entry in `FnDef.suppress`):

```jsonc
{ "code": "<diagnostic-code>",   // e.g. "over-strict-mode"
  "because": "<author reason>"   // must be non-empty;
                                 // empty/whitespace fires `empty-suppress-reason` (Error)
}
```

### Term (expression)

```jsonc
{ "t": "lit", "lit": Literal }
{ "t": "var", "name": "<id>" }

// 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 }

{ "t": "let",  "name": "<id>", "value": Term, "body": Term }

// Local recursive let. Always fn-shaped. The desugar pass
// lifts most `letrec` to a synthetic top-level fn; `lift_letrecs`
// finishes the job after typecheck for the residue that captures
// let-bound names. Post-codegen, no `letrec` survives.
{ "t": "letrec",
  "name": "<id>", "type": Type, "params": ["<id>"...],
  "body": Term, "in": Term }

{ "t": "if",   "cond": Term, "then": Term, "else": Term }

// Effect-op invocation. `op` is "<eff>/<op>" (e.g. "io/print_str").
// `tail` triggers musttail (omitted when false).
{ "t": "do",   "op": "<eff>/<op>", "args": [Term...], "tail": false }

// 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...] }

{ "t": "match", "scrutinee": Term, "arms": [Arm...] }

// Anonymous fn value; free vars captured from enclosing scope.
{ "t": "lam",
  "params": ["<id>"...],
  "param-types": [Type...],
  "ret-type": Type,
  "effects": ["<id>"...],
  "body": Term }

// Sequencing. Semantically `let _ = lhs in rhs`; lhs must be Unit.
{ "t": "seq",   "lhs": Term, "rhs": Term }

// Explicit RC clone. Codegen lowers as
// `call void @ailang_rc_inc(ptr %v)` before returning %v under `--alloc=rc`.
{ "t": "clone", "value": Term }

// Explicit reuse-as hint. `body` must be allocating
// (typically `ctor` or `lam`); `source` must be a bare `var`. Codegen
// lowers as in-place rewrite under `--alloc=rc`.
{ "t": "reuse-as", "source": Term, "body": Term }

// loop: strict iteration block. `binders` declares
// one or more loop parameters (name, type, init), evaluated in
// order on loop entry; `body` is in scope of all binders. The
// loop's value is `body`'s value on the iteration that exits via a
// non-`recur` branch. Strictly additive (no `skip_serializing_if`;
// pre-existing fixtures hash bit-identically — none carry the tag).
// No totality claim — an infinite loop is legal. See
// `docs/specs/0034-loop-recur.md`.
{ "t": "loop",
  "binders": [ { "name": "<id>", "type": Type, "init": Term }, ... ],
  "body": Term }

// recur: re-enter the lexically innermost enclosing
// `loop`, rebinding its binders positionally to `args`. Transfers
// control (no fall-through); valid only in tail position of its
// enclosing loop (enforced at typecheck, `recur-not-in-tail-position`).
{ "t": "recur",
  "args": [ Term, ... ] }
```

In the MVP, `do` is only a direct call to a built-in effect op (no
handler); the effect system is described in [effects](../models/0002-effects.md).
A `lam` term constructs an anonymous function value; free
variables of its body are captured from the enclosing scope.

Loop binders are alloca-resident: typecheck binds them in the
ordinary local scope plus a positional `loop_stack`, and codegen
lowers them as entry-block allocas. Capturing a `loop` binder into a
lambda body is rejected at typecheck via
`CheckError::LoopBinderCapturedByLambda`. See
`docs/specs/0034-loop-recur.md`.

**`Literal`**:

```jsonc
{ "kind": "int",  "value": <i64> }
{ "kind": "bool", "value": <bool> }
{ "kind": "str",  "value": "<utf-8>" }
{ "kind": "unit" }
{ "kind": "float", "bits": "<16-lowercase-hex>" }
```

**`Pattern`** (the `pat` field of an `Arm`; discriminator `p`):

```jsonc
{ "p": "wild" }                                   // _
{ "p": "var",  "name": "<id>" }                   // x — binds the value
{ "p": "lit",  "lit": Literal }
{ "p": "ctor", "ctor": "<id>", "fields": [Pattern...] } // fields omitted when empty
```

Patterns are linear: each pattern variable may appear at most once.

### Type

The `Type::Con.name` canonical-form rule (bare for same-module /
primitives, qualified `<module>.<TypeName>` for cross-module) lives
in [memory model](0008-memory-model.md); `Type::Fn`'s parameter-mode
metadata is defined and gated there as well.

```jsonc
// Type-constructor application. `args` omitted when empty
// (hash-stable when omitted, for non-parameterised cases like Int, Bool, ...).
{ "k": "con", "name": "<id>", "args": [Type...] }   // "name": canonical form (bare for same-module / primitives, "<module>.<TypeName>" for cross-module)

// Function type. paramModes/retMode are metadata on Type::Fn —
// they are NOT separate Type variants, so every existing match-arm
// in the typechecker (unify, occurs, apply) keeps working.
// `paramModes` omitted when every entry is "implicit"; `retMode`
// omitted when "implicit" (hash-stable when omitted). Full mode
// contract lives in contracts/0008-memory-model.md.
{ "k": "fn",
  "params":     [Type...],
  "paramModes": [ParamMode...],
  "ret":        Type,
  "retMode":    ParamMode,
  "effects":    ["<id>"...] }

{ "k": "var", "name": "<id>" }

// Top-level polymorphism only. `constraints` carries class
// constraints (narrative in contracts/0013-typeclasses.md); omitted when
// empty (hash-stable when omitted).
{ "k": "forall",
  "vars": ["<id>"...],
  "constraints": [{ "class": "<id>", "type": "<id>" }, ...],   // "class": canonical form (bare for same-module, "<module>.<Class>" for cross-module)
  "body": Type }
```

**`ParamMode`** (full contract in
[memory model](0008-memory-model.md)):

```
"implicit"   — unannotated / back-compat. Treated as `own` by the typechecker.
"own"        — (own T) — caller transfers ownership; callee consumes.
"borrow"     — (borrow T) — caller retains ownership; callee may not consume.
```

`implicit ≡ own` semantically; the distinction exists so existing
unannotated fixtures continue to serialize without the mode wrapper and keep their
canonical-JSON hash. The full mode contract (codegen consequences,
the over-strict-mode lint, the `Suppress` mechanism) lives in
[memory model](0008-memory-model.md); the four language-design
preconditions that make RC sound live in
[language constraints](0015-language-constraints.md).

Ratified by: `crates/ailang-core/tests/design_schema_drift.rs`.