Pipes.Prelude: smaller type constraints on fold, length, maximum, minimum, ...

[fmap]

...null, sum, product, toList, and toListM.

[Gabriella439]

Okay, so I finally had a chance to look at this. So the narrow types are there to prevent accidentally discarding unused values. pipes generally tries to make it hard to do the wrong thing by accident, even if that means making things slightly more difficult to use.

However, there is a solution that gives the best of both worlds, which is to provide alternative fold functions that take any return value and return it alongside the fold's result.

Here is an example:

fold' :: Monad m => (x -> a -> x) -> x -> (x -> b) -> Producer a m r -> m (b, r)

That is designed to work with the foldl library, so that you can write things like:

-- `impurely` and `head` come from `foldl`

impurely fold' head :: Producer a m r -> m (Maybe a, r)

That will work for any return value. All the other folds (like maximum and minimum) could be similarly generalized, although chances are that those generalized versions would go in pipes-extras.

I've also added void to the exports of the Pipes module to make it easier to explicitly discard unused return values.