[Haskell-cafe] Why does `mzip /= liftM2 (,)`

[Dmitry Olshansky]

Instances for MonadZip not always defined as "liftM2 (,)".

Compare:

> liftM2 (,) (Alt [1..2]) (Alt ("foo"))
Alt {getAlt = [(1,'f'),(1,'o'),(1,'o'),(2,'f'),(2,'o'),(2,'o')]}

> mzip (Alt [1..2]) (Alt ("foo"))
Alt {getAlt = [(1,'f'),(2,'o')]}


2017-04-20 23:07 GMT+03:00 Mateusz Kowalczyk <fuuzetsu at fuuzetsu.co.uk>:

> On 04/20/2017 08:19 PM, MarLinn wrote:
> >
> >> The question is why does
> >>
> >> ```
> >> mzip /= liftM2 (,)
> >>
> >> ```
> >
> > I don't have any evidence, but my gut feeling is that indeed mzip ==
> > liftM2 (,) == liftA2 (,), but that MonadZip just predates many advances,
> > and that it's not used often enough to warrant changing. Especially
> > because changing stuff in base comes with huge costs and long debates.
>
> Normally I would agree but in case of adding instances, there are no
> real costs or long debates. Indeed to add a lawful instance you could
> just send a diff to GHC and see it in next version. In my experience
> there are only arguments where there are multiple possible "reasonable"
> instances &c. Hence my question in this case.
>
> Even if it was legacy, IO was around in 2011 and there seems to be no
> reason why it wouldn't have been one of the default instances.
>
> > Today I would expect something similar to look somewhat more like this:
> >
> > ```
> > class Applicative f => Unzippative f where
> >      unzipF :: f (a,b) => (f a, f b)
> > ```
> >
> > I personally can't remember a single time that function would have come
> > in handy, so I'm happy with Applicative. And I'm projecting that
> > experience onto others and drawing the conclusion of "Meh.".
> >
>
> I think we have a bunch across projects.
>
> > But if you find out more after
> >> investigating for next 30 minutes
> >
> > I'd be interested to hear. ;)
>
> Me too! Maybe on weekend if there are no good replies.
>
> > Cheers,
> > MarLinn
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>
>
> --
> Mateusz K.
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