spanWith, takeWhileWith

[devvythelopper]

It would be nice to have a variant of span and a variant of takeWhile that do not just apply a predicate, but also fold over a value.

The practical application I have in mind is to span or takeWhile but only up to a maximum number of characters. This of course could be implemented if there was a spanMax and takeWhileMax but a generalized spanWith and takeWhileWith that also do a fold (in this case over the number of characters already selected by the predicate) seems to be the better choice to me.

[lehmacdj]

takeWhileMax n predicate = takeWhile predicate . take n and should have fairly similar performance characteristics to the dedicated version due to fusion. So at least for your practical application you should be good to go!

The more general version maybe still is necessary if you want to do something more complicated though; I don't see a way to cobble it together out of existing library functions in a good way.

[phadej]

@lehmacdj there is no implicit fusion (#348)

[lehmacdj]

Good to know, sad to hear though, but the discussion in that linked issue makes sense

[Lysxia]

This is implemented in PR #437 (which was originally motivated by issue #178, which is the same issue as this).