Proposal: add unzips for Data.Sequence

[David Feuer]

The semantics that seem most natural from an implementation standpoint are

unzip xs = xs `seq` (fmap fst xs, fmap snd xs)

Is there some reason that extra drop of strictness will be troublesome?

It is true that all functors give an unzip, but it seems valuable to
offer a custom version
anyway for situations where the universal implementation could lead to
a space leak.
For example, suppose I have a function

f :: A -> (B, C)

where B is very large and C is not. Suppose I then

let (bs, cs) = unzipWith f xs

immediately fold bs up into a small summary value, and never use bs
again. With the
universal implementation of unzip, all the B values will be kept alive
by cs. With the
hand-written implementation, assuming the GC hack works out [*], we
should be able
to free the Bs promptly. Of course, nothing is free, so we'll build up
some structure for
cs even if we never use it. I think it makes sense to offer users the choice.

[*] http://homepages.inf.ed.ac.uk/wadler/papers/leak/

On Fri, Jan 12, 2018 at 2:07 AM, Tony Morris <tmorris at tmorris.net> wrote:
> This idea has been around a while:
>
> https://hackage.haskell.org/package/category-extras-0.52.1/docs/Control-Functor-Zip.html
>
> Here is an article:
>
> http://comonad.com/reader/2008/zipping-and-unzipping-functors/
>
> Note that all functors give unzip:
>
> \x -> (fmap fst x, fmap snd x)
>
>
> On Fri, Jan 12, 2018 at 1:28 PM, David Feuer <david.feuer at gmail.com> wrote:
>>
>> Paolo G. Giarrusso (Blaisorblade) would like to add an unzip function to
>> Data.Sequence. I agree. I propose adding
>>
>>   unzip :: Seq (a,b) -> (Seq a, Seq b)
>>
>>   unzipWith :: (x -> (a, b)) -> Seq x -> (Seq a, Seq b)
>>
>> Does anyone object?
>>
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>