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";
//
// All four are comptime-only 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;
}

// The reflected/constructed type shape. A tagged union over the kinds of type
// that can be minted. Only `` .`enum `` ships today; struct/tuple land later.
// The variant uses the backtick raw-identifier escape so it reads as the
// keyword `enum` (`` .`enum(...) ``) rather than a mangled `enum_`.
TypeInfo :: enum {
    `enum: EnumInfo;
}

// The compiler's ONLY type-construction primitives (comptime-only #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, 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.
//   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 } ] }));
declare    :: (name: string) -> Type #builtin;
define     :: (handle: Type, info: TypeInfo) -> Type #builtin;
type_info  :: ($T: Type) -> TypeInfo #builtin;
field_type :: ($T: Type, idx: i64) -> Type #builtin;

// --- 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 },
    ] }));
}