Callbacks

# Callbacks (design/ffi.md, 2026-09-21): a capture-free, C-shaped Zag function handed to C.
extern.C fn qsort ['base': ptr.mut[int32], 'n': uint64, 'size': uint64,
                 'cmp': fn[ptr[int32], ptr[int32]] -> int32];

fn ascending ['a': ptr[int32], 'b': ptr[int32]] -> int32 {
    no_check return ptr.read['a'] minus ptr.read['b'];        # C hands pointers to two elements
}

fn descending ['a': ptr[int32], 'b': ptr[int32]] -> int32 {
    no_check return ptr.read['b'] minus ptr.read['a'];
}

MAIN {
    let 'nums' := [5, 1, 4, 2, 3];
    no_check qsort[lend.mut 'nums', 5, size.of[int32], ascending];
    std::print.useful['nums', newln];
    no_check qsort[lend.mut 'nums', 5, size.of[int32], descending];
    std::print.useful['nums', newln];
}