PureScript by example (custom Foldable)

Question

Currently I'm reading PureScript by example book and have question about one of exercises:

Chapter 6, exercise 5 at the end of the chapter.

Write a Foldable instance for NonEmpty. Hint: resuse the foldable instance for arrays.
data NonEmpty a = NonEmpty a (Array a)

My initial thought was to implement it like this:

instance foldableNonEmpty :: Foldable (Array (a)) => Foldable (NonEmpty a) where
  foldr f b (NonEmpty a ar) = NonEmpty a (foldr f b ar)
  foldl f b (NonEmpty a ar) = NonEmpty a (foldl f b ar)
  foldMap m (NonEmpty a ar) = NonEmpty a (foldMap m ar)

But this doesn't work and currently I can't figure out why (compiler error can't help me)

Could not match kind
  Type -> Type
with kind
  Type

while checking the kind of Foldable (Array a) => Foldable (NonEmpty a)

in value declaration foldableNonEmpty

Can someone point out in which direction should I dig to find an answer?

Take a look at how Foldable is defined for Array. Do you see any differences from your definition? — Fyodor Soikin
Thank you for this suggestion, previous it was not helpful for me, but with explanation from wuct I found it usefull — Alex

wuct wuct · Accepted Answer · 2018-02-22T16:27:38

The error message is saying that Foldable expected to receive a type constructor (Type -> Type), but you are giving it a concrete type (Type). For example: Maybe and Array are type constructors; Maybe a and Array a are concrete types. In your case, NonEmpty a is the concrete type. I believe now you get the idea how to fix it.

Moreover, your implementation is also wrong. The type of foldr is

foldr :: forall a b. (a -> b -> b) -> b -> f a -> b

You should return a value with type b, but you are returning a value with f a, which is NonEmpty a in your case.

I left my answer in this link in case you still can not figure it out.

PureScript by example (custom Foldable)

1 Answers