sentinel value for recursive types

[davidchambers]

Haskell:

data List a = Nil | Cons a (List a)

JavaScript:

const List = Type({Nil: [], Cons: [a, undefined]});

There's also a hack which looks better but relies on confusing JavaScript behaviour and doesn't work with const.

If we're to use a sentinel value (such as undefined), why not use one which is more descriptive? We could support one of the following:

const List = Type({Nil: [], Cons: [a, Type.Self]});

const List = Type({Nil: [], Cons: [a, Type.Recur]});

const List = Type({Nil: [], Cons: [a, Type.Recursive]});

The value of Type.Self (or whatever we were to call it) could simply be undefined, but I'd prefer something like {'@@functional/recursive-type-reference': true}.

What do you think, @JAForbes? Do you agree that one of these would be better than undefined? If so, what would you like the property to be named?

[JAForbes]

@davidchambers I think a sentinel makes for a cleaner/clearer API. Whichever name you prefer is fine with me. I also agree an explicit value for the recursive type reference is a better choice.

Pending your decision on #6 of course, I say 👍