Why is the class/constructor required as a parameter for `traverse` (or sequence)?

A Fantasy-land compliant version of traverse might go something like this:

S.traverse (Array) (S.words) (S.Just ('foo bar baz'))

But why are we asking for the first (Array) parameter? If the words function must return an Array, could we not access the constructor of the Array as such:

const foo  = [1,2,3]
const foo2 = foo.constructor.of( 4,5,6 )

So why do we need Array at all? Here S.words returns an Array Monad contained within the Maybe. We can then derive the Array constructor as mentioned above to sequence an Array of Maybes.

[evilsoft]

Pardon the silly pattern, but this is one way that I have implemented traverse on a Maybe type: https://github.com/evilsoft/crocks/blob/master/src/core/Maybe.js#L210

The left side of that either function is for the Nothing case, so it composes a Nothing constructor before the af or Applicative function resulting in the Applicative m => m Maybe a

So in short: the traverse is “run”, but the “lifing” function is not. That af needs to come from somewhere, and that is what the TypeRep or a -> m a in the case of crocks (we accept both the Applicative TypeRep or an Apply returning function, which is why this pattern looks so silly.)

EDIT: notice that af is based on the f that is passed in and it only appears in the left or Nothing case.

EDIT AGAIN: Oh looks like the comment I was replying to was deleted. So sorry about that!

[babakness]

Ah, right ok, you get Nothing not [ Nothing ]