use `TypeRep f` in traverse instead of `a -> f a`

As continuation of #206 I think we should change the signature of traverse

-traverse :: Applicative f, Traversable t => t a ~> (a -> f b, c -> f c) -> f (t b)
+traverse :: Applicative f, Traversable t => t a ~> (TypeRep f, a -> f b) -> f (t b)

sanctuary-type-classes TypeReps are first arguments and i think it’s fine, but if someone things that TypeRep f should be last argument let’s discuss that too.

This would be a breaking change so we might hold it for some time.

[joneshf]

so it puzzles me why that extra argument is ever needed as the desired of can be already accessed via v.constructor.of.

It is incredibly instructive to implement traverse for a data type like Maybe a. Here’s a link to a partial implementation: https://glot.io/snippets/elw3v9lfkc You should only have to implement the two traverse functions. There’s a couple of assertions at the bottom that should all pass.

For more insight, try to understand why the traverse1 function cannot be implemented for Nothing. If you think traverse1 can be implemented for Nothing, please provide an implementation.

[SimonRichardson]

It doesn’t work like that for haskell because of type inference.