instances for Lazy

[matthewleon]

There should be instances for these classes for the Lazy data type from purescript-lazy.

[matthewleon]

Realizing this is far easier said than done due to the kinds involved.

[hdgarrood]

How so? Lazy has kind Type -> Type, and Foldable and Traversable both expect something of kind Type -> Type, no?

[matthewleon]

Well, you can't do, say, Foldable a => Foldable (Lazy a), which is the most natural way of expressing an instance that comes to mind. You'd need some kind of higher-order Lazy that does something like (Type -> Type) -> (Type -> Type).

Good chance I'm missing something obvious here, though. Let me know.

[hdgarrood]

Oh, I thought you were imagining an instance which would give you e.g. toList = pure <<< force.

[matthewleon]

Interesting. There is that. I'll give it a shot.

[matthewleon]

I'm not really seeing that instance... How it would work, or what its benefit would be.

[matthewleon]

On the other hand... Why not?

[hdgarrood]

Yeah, I haven't really used the data type Lazy very much so I can't comment on its benefit. I think it can be defined as follows though:

instance Foldable Lazy where
  foldr f z x = f (force x) z
  foldl f z x = f z (force x)
  foldMap f = f (force x)

[LiamGoodacre]

I'm thinking that these instances might be better off in purescript-lazy. Thoughts?

[matthewleon]

I'm thinking that these instances might be better off in purescript-lazy. Thoughts?

That makes sense to me.