# AILang authoring reference

## 1. Header

AILang is a small data-as-source functional language designed for an
LLM author. This document is the complete authoring reference. Every
well-formed program is a module. Read the whole thing once before you
write code; the order of sections is the order of dependencies.

## 2. Modules and the on-disk form

A module is the top-level container. It has a name (matching the file
stem), an optional list of imports, and a flat list of definitions.
Every program you write is exactly one module. Cross-module imports
are out of scope for the tasks in this experiment — every task is a
single self-contained module with an empty `imports` array.

A definition is the unit that gets a content hash. There are five
kinds of definition: function, constant, data type, class, and
instance. Their order within the module is preserved on disk but is
not load-bearing semantically — forward references are legal.

The schema of the on-disk form is fixed. The toolchain rejects any
module that does not conform.


The AIL surface is parenthesised tagged-head Lisp-style notation.
Every form begins with `(` followed by a tag identifier; positional
arguments follow; the form closes with `)`. There are no infix
operators, no operator precedence, no implicit conversions.

A module starts with `(module NAME ...)` and contains import forms
followed by definition forms. Whitespace and `;`-prefixed line
comments are insignificant; indentation is for readability only.

Identifier characters allowed: letters, digits, `_`, `-`, `+`, `*`,
`/`, `<`, `>`, `=`, `!`, `?`, `%`. An identifier may not start with a
digit. String literals are double-quoted with `\"`, `\\`, `\n`, `\t`
escapes; integer literals are signed decimal; the `(unit)` keyword
denotes the unit value.

The AIL form is the printed dual of the JSON form: `ail parse
file.ail` produces the same canonical bytes as the JSON form would,
and `ail render file.ail.json` produces the AIL text. Round-trip
through this pair is gated by an integration test.

```ail
(module fn_returns_int
  (fn answer
    (doc "The simplest possible fn — no params, returns a fixed Int.")
    (type (fn-type (params) (ret (own (con Int)))))
    (params)
    (body 42)))
```

## 3. Types

A type is one of four shapes. Every type position in a function
signature must be filled in; there is no implicit type inference at
the binding form. Constructor types may carry type arguments.
Functions are first-class — function types appear as parameter and
return types.

A type constructor names a base type or a user-defined ADT. The
prelude provides `Int`, `Bool`, `Str`, `Float`, `Unit`. User ADTs are
introduced with the `data` form (see section 6); after declaration
their name is usable wherever a type constructor is expected.

A type variable is a placeholder introduced by `forall` at the top
of a function signature. Within the signature the variable behaves
as an opaque type; the toolchain instantiates it at each call site.

A function type names parameter types, parameter modes (see section
4), a return type, a return mode, and an effect row (see section 8).

A `forall` is universal quantification: it lists the type variables
bound and the body type they appear in. Forall is only valid at the
top of a function or constant signature.


Type::Con: `Int`, `Bool`, `Str`, `Float`, `Unit`, or a user type
name. For parameterised types: `(con List Int)` reads "List of Int";
nested: `(con Map (con Str) (con Int))`.

Type::Var: a bare identifier alone in type position (no wrapper),
e.g. `a` inside a `(forall (a) ...)`. Type variables are
syntactically distinguished from type constructors by being declared
in the enclosing `forall`.

Type::Fn: `(fn-type (params T1 T2) (ret RET) (effects E1 E2))`. The
parameter and effect lists may be empty. Modes wrap the relevant
type: `(own T)` or `(borrow T)` — see section 4.

Type::Forall: `(forall (a b) BODY-TYPE)` quantifies over `a` and
`b`. Class constraints attach as `(forall (a) (constraints (Eq a))
BODY-TYPE)`.

```ail
(module forall_polymorphic
  (fn id
    (doc "The polymorphic identity function. Exercises Type::Forall and Type::Var.")
    (type (forall (vars a) (fn-type (params (own a)) (ret (own a)))))
    (params x)
    (body x)))
```

## 4. Mode annotations

Every function parameter carries a mode, and so does the return
position. There are exactly two modes. `own` means the caller
transfers ownership and the callee consumes the value; `borrow`
means the caller retains ownership and the callee may inspect but
not consume. `borrow` is legal only over a boxed type (an algebraic
data type): unboxed value types (`Int`, `Bool`, `Float`, `Unit`)
are always `own`, and borrowing one is a `borrow-over-value` error.

The return position is always `own` — a function returns a fresh
owned value; borrow-returns are not permitted.

Modes are mandatory: every parameter and the return position carries
exactly one mode. There is no default or omitted mode.


Modes are surface wrappers around the type in parameter and return
position. Inside a `(fn-type ...)`:

- `(params (own T1) (borrow T2))` — `T1` is owned, `T2` is borrowed.
- `(ret (own T))` — the return is always owned.

Every parameter and the return carries a mode wrapper; there is no
bare-type (mode-less) form. `borrow` is legal only over a boxed
type: `(borrow (con Int))` is a `borrow-over-value` error — write
`(own (con Int))`. The mode is part of the fn's signature and
contributes to its content hash.

```ail
(module param_modes_all
  (data List
    (ctor Nil)
    (ctor Cons (con Int) (con List)))
  (fn f_own_value
    (doc "(own Int) — unboxed value types (Int/Bool/Float/Unit) are always own; borrow over them is a borrow-over-value error.")
    (type (fn-type (params (own (con Int))) (ret (own (con Int)))))
    (params x)
    (body x))
  (fn f_borrow_boxed
    (doc "(borrow List) — boxed types may be borrowed; the callee reads without consuming.")
    (type (fn-type (params (borrow (con List))) (ret (own (con Int)))))
    (params xs)
    (body (match xs
      (case (pat-ctor Nil) 0)
      (case (pat-ctor Cons h t) (app + 1 (app f_borrow_boxed t)))))))
```

## 5. Functions

A function definition binds a name to a body of code. The signature
declares the parameter types, the return type, and the effect set
(if any). The body is an expression — there is no statement form.
Functions are first-class values: they may be passed to other
functions, returned, stored in data structures.

Inside a function body, four term forms appear most often: a
variable reference (the name of a parameter, a local binding, or a
top-level definition); a function application (a callee plus
positional arguments); a lambda (an anonymous function with its
own signature); a literal value (see section 9).

Lambdas carry their own parameter list, parameter types, return
type, and effect set. Lambdas may close over names from the
enclosing lexical scope; the closure is captured by value (the
captured values are reference-counted under `--alloc=rc`).

A `let` binds a name to the value of one expression for the
duration of another expression. A `letrec` is the recursive variant:
the bound name is visible inside its own body. Both are
expressions, not statements — they have a value.


Function definition: `(fn NAME (doc STRING)? (type TYPE) (params
NAME*) (body TERM))`. The `doc` clause is optional but recommended.

Term forms relevant here:

- Variable reference: a bare identifier, e.g. `x` or `+`.
- Application: `(app CALLEE ARG*)` — head is `app`, then the callee
  term, then positional arguments.
- Lambda: `(lam (params NAME*) (paramTypes TYPE*) (ret TYPE)
  (effects EFFECT*) BODY)`.
- Let: `(let (NAME VALUE) BODY)` — binds `NAME` to `VALUE` in
  `BODY`.
- Letrec: `(letrec NAME (type FN-TYPE) (params NAME*) BODY IN-BODY)`
  — `NAME` is recursively visible inside `BODY`.

Inside any term position, an integer literal is a bare decimal
number, a string is a double-quoted string, a bool is `true` or
`false`, unit is `(unit)`.

```ail
(module fn_calls_prelude
  (fn add
    (doc "Add two Ints via the polymorphic prelude `+`. Also exercises TermLet by binding the sum to a local name before returning it, and TermClone by re-using the let-bound value.")
    (type (fn-type (params (own (con Int)) (own (con Int))) (ret (own (con Int)))))
    (params x y)
    (body (let s (app + x y) (clone s)))))
```

```ail
(module fn_with_lambda
  (fn make_adder
    (doc "Curried adder: takes an Int and returns an Int -> Int closure. Exercises TermLam and Type::Fn appearing as a return type.")
    (type (fn-type (params (own (con Int))) (ret (own (fn-type (params (own (con Int))) (ret (own (con Int))))))))
    (params x)
    (body (lam (params (typed y (con Int))) (ret (con Int)) (body (app + x y))))))
```

## 6. Algebraic data types

A data type declaration introduces a new named type with one or
more constructors. Each constructor takes zero or more positional
argument types and produces a value of the declared type. ADTs may
be parameterised by type variables; the variables are listed before
the constructors and may appear in the constructor argument types.

A constructor is invoked at the term level by naming the type and
the constructor plus the positional arguments. The runtime
representation under `--alloc=rc` is a tagged heap cell with one
slot per constructor field; the tag identifies which constructor
was used.

Constructor names live in their own namespace, separate from
function names. The same identifier may be a function and a
constructor without conflict, though for readability the convention
is constructors are capitalised and functions are lower-case.


Type definition: `(data NAME (vars TYVAR*)? (doc STRING)? (ctor
CTOR-NAME ARG-TYPE*)*)`. The `vars` clause is optional; nullary
constructors have an empty argument list.

Example: `(data List (ctor Nil) (ctor Cons Int (con List)))` is a
monomorphic Int list with a Nil and a Cons constructor.

Constructor invocation: `(term-ctor TYPE-NAME CTOR-NAME ARG*)`.
The Form-A keyword is `term-ctor`, not `ctor` — the bare `(ctor
...)` is reserved for the inside of `(data ...)` definitions and
the parser rejects it in term position. `TYPE-NAME` precedes the
constructor name so the parser does not need a separate registry
to resolve overlapping constructor names across ADTs. Example:
`(term-ctor List Cons 1 (term-ctor List Nil))` builds a single-
element list.

```ail
(module data_simple
  (data Box (vars a)
    (ctor MkBox a))
  (fn wrap_one
    (type (fn-type (params (own (con Box (con Int)))) (ret (own (con Box (con Int))))))
    (params src)
    (body (match src
      (case (pat-ctor MkBox v) (reuse-as src (term-ctor Box MkBox 1)))))))
```

## 7. Pattern matching

Pattern matching is the way to inspect an ADT value. A `match`
form takes a scrutinee expression plus one or more arms; each arm
pairs a pattern with a body. Arms are tried top-to-bottom; the
first matching arm's body is evaluated. The toolchain checks
exhaustiveness: every constructor of the scrutinee's type must be
reachable through some arm, otherwise the typechecker errors.

There are four pattern shapes. A wildcard matches anything and
binds nothing. A variable pattern matches anything and binds the
matched value to the named identifier for use inside the arm body.
A literal pattern matches only when the scrutinee is bit-equal to
the literal value; literal patterns work on Int, Bool, Str, and
Unit (Float patterns are rejected at typecheck — see section 9).
A constructor pattern matches when the scrutinee is built by the
named constructor and recursively matches the constructor's
fields.


Match expression: `(match SCRUT (case PAT BODY) (case PAT BODY)
...)`. Arms are introduced by `(case ...)`; the order matters (top-
to-bottom).

Pattern shapes:
- Wildcard: `_` — bare underscore is the wildcard pattern.
- Variable: a bare identifier alone, e.g. `h`, binds the scrutinee.
- Literal: a literal value alone, e.g. `0`, `true`, `"abc"`,
  `(unit)`, matches bit-equal scrutinees.
- Constructor: `(pat-ctor CTOR-NAME SUBPAT*)` — the Form-A
  pattern keyword is `pat-ctor` (not `ctor`). No `type` slot
  here, because the constructor name is resolved against the
  scrutinee's type.

Pattern variables introduced by a `(case PAT BODY)` are in scope
inside `BODY` and bind there only. They do not leak into sibling
arms or into code outside the `match`.

```ail
(module data_with_match
  (data List
    (doc "Monomorphic singly-linked Int list — boxed, recursive.")
    (ctor Nil)
    (ctor Cons (con Int) (con List)))
  (fn head_or_zero
    (doc "Return the head of xs, or 0 if empty. Exercises TermMatch with two arms, PatternCtor (both nullary Nil and binary Cons), and PatternWild on the tail field.")
    (type (fn-type (params (borrow (con List))) (ret (own (con Int)))))
    (params xs)
    (body (match xs
      (case (pat-ctor Nil) 0)
      (case (pat-ctor Cons h _) h))))
  (fn count_via_letrec
    (doc "Walk xs counting nodes using a local recursive let. Exercises TermLetRec and TermVar; the recursive `go` is bound locally.")
    (type (fn-type (params (own (con List))) (ret (own (con Int)))))
    (params xs)
    (body (let-rec go (params ys) (type (fn-type (params (own (con List))) (ret (own (con Int))))) (body (match ys
      (case (pat-ctor Nil) 0)
      (case (pat-ctor Cons _ t) (app + 1 (app go t))))) (in (app go xs))))))
```

```ail
(module match_literal_pattern
  (fn classify
    (doc "Classify an Int via literal patterns plus a wildcard fallback. Exercises PatternLit and PatternWild.")
    (type (fn-type (params (own (con Int))) (ret (own (con Int)))))
    (params n)
    (body (match n
      (case (pat-lit 0) 100)
      (case _ 200))))
  (fn sign_if
    (doc "Return -1 / 0 / 1 for the sign of n using nested TermIf. Exercises TermIf in both then/else positions plus TermApp on the polymorphic comparison.")
    (type (fn-type (params (own (con Int))) (ret (own (con Int)))))
    (params n)
    (body (if (app lt n 0) -1 (if (app eq n 0) 0 1)))))
```

## 8. Effects

Every function type carries an effect row — a set of effect labels
that the function may raise. The two currently wired effects are
`IO` (required to call effect operations like `io/print_int`) and
`Diverge` (used for functions that may not terminate). An empty
effect row marks the function as pure.

Effect operations are reached through the `do` term, not through a
regular function application. The `do` form names the operation
(e.g. `io/print_int`) and lists the operation's arguments; the
typechecker resolves the operation against the prelude's effect-op
table, checks the arguments, and accumulates the operation's
effect label into the enclosing function's effect row.

A `seq` term sequences two effectful expressions: the left-hand
side is evaluated for its effects and its result discarded, then
the right-hand side is evaluated and its value is the value of the
whole expression. `seq` is the canonical way to perform multiple
IO operations one after the other inside a function body.


Effect row on a fn type: `(effects E1 E2 ...)` inside the
`(fn-type ...)`. Empty row: omit the `(effects)` clause or write
`(effects)`. Common effectful row: `(effects IO)`. Order is sorted
in canonical output.

Effect operation: `(do OP-NAME ARG*)` — the leading `do` head is
the parser's signal that the next identifier is an effect op, not a
function. Example: `(do io/print_int 42)` prints the integer 42
and raises the `IO` effect.

Sequencing: `(seq LHS RHS)`. The value of `(seq X Y)` is `Y`'s
value; `X`'s value is discarded. Chains of sequencing are written
nested: `(seq A (seq B C))` runs `A`, then `B`, then yields `C`'s
value.

```ail
(module fn_with_do_seq
  (fn main
    (doc "Print two Ints in sequence. Exercises TermDo, TermSeq, and the IO effect on a fn type. The trailing unit return is implicit in the last Do (op returns Unit).")
    (type (fn-type (params) (ret (own (con Unit))) (effects IO)))
    (params)
    (body (seq (seq (do io/print_str (app int_to_str 1)) (do io/print_str "\n")) (seq (do io/print_str (app int_to_str 2)) (do io/print_str "\n"))))))
```

## 9. Literals

There are five literal shapes. An integer literal is a signed
64-bit decimal. A boolean literal is `true` or `false`. A string
literal is a UTF-8 sequence in double quotes; AILang restricts the
authored byte set to ASCII printable (Decision 6 Constraint 3) — non-
ASCII bytes are an authoring error. A unit literal is the value of
type Unit, written `(unit)` in AIL and `{"kind": "unit"}` in JSON.

A Float literal is an IEEE-754 binary64 value. When you author
Floats, write the decimal form (`3.14`) — the AIL parser converts
to the 64-bit bit pattern and emits the canonical 16-character
lowercase hex string into the JSON. You never author the hex
encoding by hand; the toolchain owns that representation. NaN and
±Inf are representable through the prelude constants `nan`, `inf`,
and `neg_inf` (see section 11).

Float pattern matching is rejected at typecheck. IEEE-`==` makes
Float patterns semantically dubious (NaN never matches; equality
is bit-exact, not approximate), so the language hard-rejects them
to push authors toward comparison-based dispatch.


Literal surface forms:

- Int: bare signed decimal, e.g. `42`, `-7`, `0`.
- Bool: `true` or `false` as bare keywords.
- Str: double-quoted with `\"`, `\\`, `\n`, `\t` escapes, e.g.
  `"hello"`. Bytes outside ASCII printable are rejected by the
  authoring rule (Decision 6 Constraint 3).
- Unit: the keyword `(unit)`.
- Float: bare decimal with at least one digit on each side of the
  point, e.g. `3.14`, `0.5`, `-2.0`. The parser converts to the
  bit-pattern hex string when emitting JSON; you never author the
  hex form.

There is no separate syntax for exponent-form floats in the v1
authoring surface; for non-finite values use the prelude constants
`nan`, `inf`, `neg_inf`.

```ail
(module floats
  (const pi
    (doc "Pi as a Float constant. Exercises Def::Const and Literal::Float — the bit pattern below is f64::to_bits(3.14).")
    (type (con Float))
    (body 3.14)))
```

```ail
(module bool_str
  (fn is_true
    (doc "Trivial Bool literal. Exercises Literal::Bool.")
    (type (fn-type (params) (ret (own (con Bool)))))
    (params)
    (body true))
  (fn greeting
    (doc "Trivial Str literal. Exercises Literal::Str.")
    (type (fn-type (params) (ret (own (con Str)))))
    (params)
    (body "hi"))
  (fn unit_value
    (doc "Trivial Unit literal — needed so Literal::Unit appears at least once in the corpus.")
    (type (fn-type (params) (ret (own (con Unit)))))
    (params)
    (body (lit-unit))))
```

## 10. Typeclasses

A typeclass is a named bundle of operations that may be
implemented for multiple types. The class declaration lists the
methods; each instance declaration provides the bodies for a
specific type. The compiler resolves method calls at the call site
and monomorphises the implementation: there is no runtime dispatch.

A class is parameterised by a single type variable (multi-parameter
classes are not supported in v1). Methods are declared with their
full type signature; the class parameter appears as a type variable
inside the method type. Methods may carry an optional default body
that instances inherit unless they explicitly override.

Class constraints attach to polymorphic function types: a function
`forall a. (Eq a) => (a, a) -> Bool` requires its callers to
provide an `Eq` instance for `a` at the call site. The constraint
appears inside the `forall` quantifier, in the `constraints` slot.

The prelude provides four built-in classes: `Eq` (equality), `Ord`
(ordering with `<`, `<=`, `>`, `>=`), `Num` (arithmetic — `+`, `-`,
`*`, `/`, `neg`), and `Bounded`. Their instances for Int, Bool,
Str, Float, and Unit are built in; you do not declare them.


Class declaration: `(class NAME (param TYVAR) (method NAME (type
METHOD-TYPE) (default BODY)?)*)`. The `default` clause is
optional; methods without a default must be supplied by every
instance.

Instance declaration: `(instance NAME (type CONCRETE-TYPE) (method
NAME BODY)*)`. The method bodies must match the class's method
signatures with the class parameter substituted to the concrete
type.

Constraints inside a forall: `(forall (a) (constraints (Eq a) (Ord
a)) BODY-TYPE)`. Each constraint is `(CLASS-NAME TYPE)` — usually
the type is a single bound type variable but a concrete type is
also legal (typically a typecheck error unless an instance exists).

```ail
(module class_def
  (class MyShow
    (param a)
    (doc "A toy single-method class. The instance in instance_def.ail.json provides a body for `show` at type Int.")
    (method show
      (type (fn-type (params (own a)) (ret (own (con Str))))))))
```

```ail
(module instance_def
  (class MyShow
    (param a)
    (doc "Re-declared here so this fixture is self-contained; class_def.ail.json declares the same class verbatim.")
    (method show
      (type (fn-type (params (own a)) (ret (own (con Str)))))))
  (instance
    (class MyShow)
    (type (con Int))
    (doc "Trivial MyShow Int — show always returns the fixed string \"int\".")
    (method show
      (body "int"))))
```

## 11. The prelude

The prelude provides a fixed set of operators, conversion functions,
and effect operations that are in scope without any import. The
operators are polymorphic over a small set of types and resolve at
the call site by the resolved argument type.

| Name | Type | Notes |
|------|------|------|
| `+` `-` `*` `/` | `forall a. (a, a) -> a` | Int and Float; codegen filters at use site |
| `%` | `(Int, Int) -> Int` | Int only |
| `==` `!=` `<` `<=` `>` `>=` | `forall a. (a, a) -> Bool` | Int, Bool, Str, Float, Unit |
| `not` | `(Bool) -> Bool` | Boolean negation |
| `neg` | `forall a. (a) -> a` | Int and Float negation |
| `int_to_float` | `(Int) -> Float` | Widening; codegen uses `sitofp` |
| `float_to_int_truncate` | `(Float) -> Int` | Saturating truncation to zero |
| `float_to_str` | `(Float) -> Str` | Formatted decimal |
| `is_nan` | `(Float) -> Bool` | True only for NaN bit patterns |
| `nan` `inf` `neg_inf` | `Float` | Bare-value constants |
| `__unreachable__` | `forall a. a` | Polymorphic bottom; LLVM `unreachable` |

Effect operations (reached through `do`, not `app`):

| Op | Signature | Effect |
|----|----------|-------|
| `io/print_int` | `(Int) -> Unit` | `IO` |
| `io/print_bool` | `(Bool) -> Unit` | `IO` |
| `io/print_str` | `(Str) -> Unit` | `IO` |
| `io/print_float` | `(Float) -> Unit` | `IO` |

`int_to_str` is intentionally NOT in the prelude — it is type-
installed in a future milestone but codegen-deferred pending the
heap-Str ABI work. Do not call it.

## 12. Content addressing

AILang has content-addressed identity: every top-level definition
has a 16-character hash derived from its canonical bytes. The
toolchain computes hashes on `ail parse` and `ail check`; the
author writes no hash literal in either form, and the JSON form
has no `hash` field. This is symmetric across forms and removes
hash as a form-distinguishing factor. You author code; the
toolchain owns the identity.

## 13. Out of scope, and closing directive

This reference does NOT teach: cross-module imports (every task is
a single self-contained module with an empty `imports` array),
refinement types (reserved in the schema but pass-through in this
version), point-free style (cut by Decision 6), operator
overloading (cut), syntactic sugar of any kind (cut). The four
tasks you will be given do not need any of these features.

When asked to write a module, return the complete module text and
nothing else. No markdown fences. No prose explanation. No
preamble. The harness parses your response verbatim; any wrapping
text is treated as a parse error.
