sx/library/modules/std/meta.sx

// Comptime type metaprogramming — `declare` / `define` (construct a NEW nominal
// type from data), plus `type_info` / `field_type` (reflect a type → data) and
// the data model they reflect INTO and construct FROM.
//
// This is a SEPARATE on-demand module rather than part of the prelude: its data
// types would otherwise intern into every module's type table and shift every
// `.ir` snapshot. Import it explicitly:  #import "modules/std/meta.sx";
//
// `declare` is ordinary sx (over the `abi(.compiler)` primitive `declare_type`);
// `define` / `type_info` / `field_type` are comptime-only compiler builtins —
// reaching one at runtime is a hard error (the type must be minted / reflected
// at compile time).

// One variant of a constructed enum: a name plus an optional payload type.
// `payload = void` means a tagless variant (e.g. `closed`).
EnumVariant :: struct {
    name: string;
    payload: Type;
}

// The shape of an enum/tagged-union being reflected or constructed. The type's
// NAME is supplied to `declare(name)`, not here — `declare` needs it at compile
// time to register the forward type so the body can reference itself (`*Name`).
EnumInfo :: struct {
    variants: []EnumVariant;
}

// One field of a constructed struct: a name plus its type.
StructField :: struct {
    name: string;
    type: Type;
}

// The shape of a struct being reflected or constructed. As with `EnumInfo`, the
// type's NAME travels in `declare(name)`, not here.
StructInfo :: struct {
    fields: []StructField;
}

// The element types of a tuple being reflected or constructed. Tuples are
// POSITIONAL (no field names), so this is just an ordered list of types.
TupleInfo :: struct {
    elements: []Type;
}

// The reflected/constructed type shape. A tagged union over the kinds of type
// that can be minted — `` .`enum ``, `` .`struct `` and `` .`tuple `` all ship.
// The variants use the backtick raw-identifier escape so they read as the
// keywords (`` .`enum(...) `` / `` .`struct(...) `` / `` .`tuple(...) ``) rather
// than mangled `enum_` / `struct_` / `tuple_`.
TypeInfo :: enum {
    `enum: EnumInfo;
    `struct: StructInfo;
    `tuple: TupleInfo;
}

// ── The low-level compiler-API type-construction primitive ───────────────────
//
// `declare_type` is an `abi(.compiler)` function — the compiler's primitive for
// minting a forward nominal slot, serviced by `comptime_vm.callCompilerFn`. It
// runs at LOWERING time (when a `-> Type` builder's result is first referenced),
// where the compiler still resolves references to the new type. The DSL's
// `declare` below is ordinary sx written over it. (Declared here, not imported
// from `modules/compiler.sx`, so `meta.sx` stays self-contained and doesn't
// intern `compiler.sx`'s `List(string)` types into every importer's table.)
declare_type :: (name: string) -> Type abi(.compiler);

// ── The metatype DSL: `declare` (sx), `define`/`type_info`/`field_type` (builtins)
//
//   declare(name)        — mint a NEW empty (undefined) nominal type NAMED
//                          `name`, returned as a `Type` handle. The compiler
//                          registers the forward type at compile time (it scans
//                          for the literal `declare("Name")` spelling), so the
//                          body of `define` can reference it BY NAME — that's how
//                          self-reference works (`payload = *List` resolves to the
//                          forward `List`). Using the type before its `define` is
//                          a loud error; a pointer to it is fine. A trivial alias
//                          for `declare_type` (both mint the same forward slot).
//   define(handle, info) — fill a declared handle's body from a `TypeInfo`, and
//                          RETURN the handle so the one-shot form chains:
//                              List :: define(declare("List"), .enum(.{ variants = .[
//                                  EnumVariant.{ name = "cons", payload = *List },
//                                  EnumVariant.{ name = "nil",  payload = void } ] }));
//                          Still a compiler builtin: decoding the `TypeInfo`
//                          tagged-union VALUE needs comptime-VM tagged-union
//                          matching (`enum_tag` on a tagged union), which the VM
//                          doesn't model yet — so `define` can't be plain sx until
//                          that lands (it would `match` the union).
//   type_info($T)        — reflect a type INTO a `TypeInfo` value (the inverse of
//                          `define`). Still a compiler builtin: building the
//                          tagged-union value byte-compatibly is a compiler job.
//   field_type($T, idx)  — the i-th field / variant-payload / element type. A
//                          compiler builtin that folds at LOWER time, so it
//                          composes inside `type_eq` / `type_name` / any type-arg
//                          slot — a property `type_field_type` (a runtime value)
//                          can't provide.
define     :: (handle: Type, info: TypeInfo) -> Type #builtin;
type_info  :: ($T: Type) -> TypeInfo #builtin;
field_type :: ($T: Type, idx: i64) -> Type #builtin;

// `declare(name)` is plain sx over the `declare_type` primitive: both mint (or
// find) the same forward nominal slot — `declare` adds nothing but a name the
// compiler scans for to forward-register self-references (`*Name`).
declare :: (name: string) -> Type {
    return declare_type(name);
}

// --- Type constructors built in sx library code (no compiler machinery) ---
//
// The channel result types, expressed as type-fns over declare/define. They
// demonstrate that a programmatically-built enum carries a full enum through
// codegen: `RecvResult(i64)` constructs and matches like any hand-written enum,
// and is one nominal type across sites (the type-fn identity path). The channel
// library (N3) consumes these once it lands.

// The GENERAL enum constructor: mint a nominal enum NAMED `name` from a variant
// list passed as a VALUE (a `[]EnumVariant`), rather than a hardcoded literal.
// Because `variants` is an ordinary comptime value, a caller can ASSEMBLE it in
// a local (conditionally, in a loop, from type args) before minting — see
// `examples/0620`. `define` decodes the slice via `decodeVariantElements`. The
// channel constructors above are the special-cased shapes; `make_enum` is the
// open-ended one every other constructor could be written over.
//
// Call it from a non-generic `() -> Type` builder (whose whole body is
// comptime-evaluated, so locals are in scope) or inline with a literal arg
// (`E :: make_enum("E", .[ … ])`). A *generic* type-fn comptime-evaluates only
// its return EXPRESSION, so build the list inline in the return there, not in a
// preceding local.
make_enum :: (name: string, variants: []EnumVariant) -> Type {
    return define(declare(name), .enum(.{ variants = variants }));
}

// A blocking recv: a value, or the channel was closed (drained).
RecvResult :: ($T: Type) -> Type {
    return define(declare("RecvResult"), .enum(.{ variants = .[
        EnumVariant.{ name = "value",  payload = T    },
        EnumVariant.{ name = "closed", payload = void },
    ] }));
}

// A non-blocking try-recv: a value, currently empty, or closed — three states
// a bool can't express.
TryResult :: ($T: Type) -> Type {
    return define(declare("TryResult"), .enum(.{ variants = .[
        EnumVariant.{ name = "value",  payload = T    },
        EnumVariant.{ name = "empty",  payload = void },
        EnumVariant.{ name = "closed", payload = void },
    ] }));
}