hlist-to-tuple.clay

import printer.*;

/// true iff type is HList
[Something when Type?(Something)]
define HList?(#Something): Bool private overload;


[Head, Tail when HList?(Tail)]
record HCons[Head, Tail](
    head: Head,
    tail: Tail,
);

record HNil();


default HList?(type) = false;

[Head, Tail]
overload HList?(#HCons[Head, Tail]) = true;

overload HList?(#HNil) = true;


[HList when HList?(HList)]
define unpackHList(hlist: HList) private overload;

[Head, Tail]
overload unpackHList(hlist: HCons[Head, Tail]) = hlist.head, ..unpackHList(hlist.tail);

overload unpackHList(hlist: HNil) = ;


hlistToTuple(hlist) = [..unpackHList(hlist)];


define packHList(..values) private overload;

overload packHList() = HNil();

[Head, ..Tail]
overload packHList(head: Head, ..tail: Tail) =
    HCons(head, packHList(..tail));


tupleToHList(tuple) = packHList(..unpackTuple(tuple));


main() {
    println(hlistToTuple(HNil()));
    println(hlistToTuple(HCons(1, HNil())));
    println(hlistToTuple(HCons(1, HCons(true, HNil()))));
    println(hlistToTuple(HCons(1, HCons(true, HCons('a', HNil())))));
    // prints:
    // []
    // [1]
    // [1, true]
    // [1, true, a]

    println(tupleToHList([]));
    println(tupleToHList([1]));
    println(tupleToHList([1, true]));
    println(tupleToHList([1, true, 'a']));
    // prints:
    // HNil()
    // HCons[Int32, HNil](1, HNil())
    // HCons[Int32, HCons[Bool, HNil]](1, HCons[Bool, HNil](true, HNil()))
    // HCons[Int32, HCons[Bool, HCons[Char, HNil]]](...)
}