// Feature 1 — TYPE-position pack projection `xs.T`. The per-element protocol
// type-arg `T` projects into a Pack of types, usable in type/signature
// positions: a tuple type `(..xs.T)` and a closure signature
// `Closure(..xs.T) -> R`. (`T` of each element comes from its
// `impl Box(T) for <elem>`.)

#import "modules/std.sx";

Box :: protocol(T: Type) {
    get :: () -> T;
}

IntCell :: struct { v: i64; }
StrCell :: struct { s: string; }
Dbl :: struct { n: i64; }
impl Box(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
impl Box(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
impl Box(i64) for Dbl { get :: (self: *Dbl) -> i64 => self.n * 2; }

// Tuple type `(..xs.T)` — heterogeneous (i64, string), matched by the
// value-projection `(..xs.get)`.
snap :: (..xs: Box) -> void {
    t : (..xs.T) = (..xs.get);
    print("0={} 1={}\n", t.0, t.1);
}

// Closure signature `Closure(..xs.T) -> i64` — here `Closure(i64, i64) -> i64`.
// The closure literal's params are contextually typed from the projection.
fold :: (..xs: Box) -> i64 {
    cb : Closure(..xs.T) -> i64 = (a, b) => a + b;
    return cb(xs[0].get(), xs[1].get());
}

main :: () -> i32 {
    snap(IntCell.{ v = 42 }, StrCell.{ s = "hi" });   // (i64, string)
    print("fold={}\n", fold(IntCell.{ v = 10 }, Dbl.{ n = 5 }));   // 10 + 10 = 20
    0
}