Add input list transformation functions

[gereeter]

This change adds combinators to change the list that is being folded over. For example, a Fold that multiplies the sum of the even numbers and the sum of the odd numbers in a list could be written like this:

weird :: Fold Int Int
weird = (*) <$> filtered even sum <*> filtered odd sum

Showing it off:

> fold weird [1, 2, 3]
8
> fold weird [1, 2]
2

Without these functions, there was no way to easily do this in one traversal.

[Gabriella439]

Sorry for the delay. I'm busy preparing the pipes-4.0.0 release which will be done really soon. I have been doing a lot of thinking about this and I will have more to say on this topic in about a week.

[mitchellwrosen]

Any reason why the functions all have -ed suffixes? It looks like it would be more idiomatic to just call them 'map', 'filter', etc. as foldl was designed to be imported qualified.

Anyways, I came here to make a similar pull request, and, because @Gabriel439 has been thinking hard about this, I'm sure I'm overlooking something =)

[gereeter]

I added the -ed suffixes because without them, it sounds like I am working on the output of the fold, not the input. The other (possibly better) options would be using premap, prefilter, etc. or using mapInput, filterInput, etc.

Also, I'm glad to see that this is being revived.

[mitchellwrosen]

Hm, it looks like mapped has already been in the repo since 1.0.2, as premap

[Gabriella439]

Yeah, premap implements the proposed mapped. Also, I recently added the pretraverse function in foldl-1.0.6:

http://hackage.haskell.org/package/foldl-1.0.6/docs/Control-Foldl.html#v:pretraverse

This function supersedes some of the proposed features here, but not all of them, since it lets you apply a traversal upstream of the fold to pick which targets you want to process. For example, you can now implement the proposed filtered by using pretraverse and the following filtered traversal:

http://hackage.haskell.org/package/lens-4.4.0.1/docs/Control-Lens-Fold.html#v:filtered

I'm still leaving this ticket open because there are several features proposed which have not been implemented yet. I'm not adding them just yet because I'm trying to see if I can find fold them into a more general pattern.