// Phase 6 — the canonical heterogeneous `map`, end to end. A pack-fn whose
// return type `$R` is inferred from the mapper's closure return:
//   - `mapper: Closure(..sources.T) -> $R` types the lambda's params from the
//     projected pack element types, and its body (`a + b`) drives `$R`.
//   - `$R` is inferred at the call site from the lowered mapper's closure ret,
//     bound into the mono (`-> VL($R)` ⇒ `VL(i64)`, `Combined($R, ..)` ⇒
//     `Combined(i64, ..)`), and folded into the mangle.
//   - `(..sources)` materializes the pack into the `(..VL(Ts))` field (per-element
//     erase) and `mapper(..sources.get)` projects+spreads; `xx c` erases the
//     generic-struct instance to `VL(i64)` via the generic impl's monomorphized
//     thunk.

#import "modules/std.sx";

VL :: protocol(T: Type) { get :: () -> T; }
IntCell :: struct { v: i64; }
impl VL(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }

Combined :: struct($R: Type, ..$Ts: []Type) {
    sources: (..VL(Ts));
    value:   $R;
}
impl VL($R) for Combined($R, ..$Ts) { get :: (self: *Combined) -> $R => self.value; }

map :: (mapper: Closure(..sources.T) -> $R, ..sources: VL) -> VL($R) {
    c : Combined($R, ..sources.T) = ---;
    c.sources = (..sources);
    c.value = mapper(..sources.get);
    return xx c;
}

main :: () -> i32 {
    r := map((a, b) => a + b, IntCell.{ v = 40 }, IntCell.{ v = 2 });
    print("{}\n", r.get());     // 42
    0
}