Add FUN 'One Profunctor instance

[sjoerdvisscher]

I'm trying to add support for generating linear traversals to my one-liner package, and noticed this instance is missing. I could use the LinearArrow instance instead, but having this instance makes things a bit cleaner.

[aspiwack]

Mmm… That's a good point. We must have used the LinearArrow newtype when FUN was not a thing. I'm kind of wondering if we should remove the LinearArrow newtype altogether, since FUN 'One does the trick, and that would save us some coerce.

Does it make some code less readable? We will have to experiment.

[b-mehta]

Mmm… That's a good point. We must have used the LinearArrow newtype when FUN was not a thing.

Can confirm this is what happened.

[sjoerdvisscher]

In this case it is even possible to merge the (->) and FUN 'One instances into one FUN m instance, but that made it too magical imho.

[aspiwack]

@b-mehta
thanks for the feedback. I'm thinking we can just kill LinearArrow then.

@sjoerdvisscher
I think the FUN m instance may not be super robust to changes in the future. So maybe let's avoid it for now, indeed.

[sjoerdvisscher]

I removed the wrapper.

Note that it is now possible to combine over and overU:

over :: Optic_ (FUN m) s t a b -> (a %m-> b) -> s %m-> t

But I guess we should wait for this until we can combine traverseOf and traverseOfU too.

[aspiwack]

Thanks. Feel free to merge.