Late and Rec

[mattbierner]

Bennu currently defines two parser late and rec that would be useful as a more general solution for Akh monads

Late resolves the content of function only when a computation is actually run.

var a;

// `a` is not defined yet but we don't care since it is resolved when `c` is run
var c = M.late(\-> a).map(+, 10);

a = M.of 3;

run c; // 13

Rec allows references to itself and is defined using late

rec = \def -> {
    var value = def <| late\->value;
    return value;
};

var c = rec \ self -> M.get.chain(\x -> x > 10 : M.of x : M.modify(+, 1).concat(self))