Also, if I understand it correctly, higher kinds a...

# arrow

[Bob Glamm]

Also, if I understand it correctly, higher kinds are just type constructors. But don't ask me how Either l r :: * -> * -> * is implemented via Kind<F, A> in Arrow

[simon.vergauwen]

It's just Kind<Kind<F, A>, B or Kind2<F, A, B>.

[simon.vergauwen]

You can see Kind as the way to apply a type argument to a type constructor.

If we say that List is a type constructor that takes 1 type argument (*-> *) then it can be made into a type using a single type application Kind<ForList, A> (= List<A>).

For a map, which takes 2 type arguments we need to apply it twice. Which means we can also "curry" it into a type which only needs 1 type argument, for example we could already apply the type argument of the keys. Kind<ForMap, String> and now this type fits all maps that have Map<String,?>.
Which is the same shape as List (* -> *), and you can use it in the same way with typeclasses such as Functor or Traverse.

[simon.vergauwen]

And by telling the compiler that Kind<F, A> is the same as F<A>, we can eliminate fix from the code. We can do so with compiler plugins.