Proposal: Add Semigroup/Monoid instance to Data.Functor.Compose

[Daniel Cartwright]

Yeah, the instance being defined with Ap is certainly a good way to go. We
can use DerivingVia and Data.Monoid.Ap.


On Fri, Nov 30, 2018, 4:55 PM David Feuer <david.feuer at gmail.com wrote:

> The alternate one can be written much more simply with
> FlexibleContexts instead of QuantifiedConstraints:
>
> instance Semigroup (f (g a)) => Semigroup (Compose f g a) where
>   Compose x <> Compose y = Compose (x <> y)
> instance Monoid (f (g a)) => Monoid (Compose f g a) where
>   mempty = Compose mempty
>
> That strikes me as the most natural way to do it. The lifting
> instances are more naturally done using this oft-proposed type:
>
> newtype Ap f a = Ap (f a)
> instance (Applicative f, Semigroup a) => Semigroup (Ap f a) where
>   Ap x <> Ap y = Ap (liftA2 (<>) x y)
> instance (Applicative f, Monoid a) => Monoid (Ap f a) where
>   mempty = pure mempty
>
> Now Ap (Compose f g) a will do the lifting you want.
>
> On Fri, Nov 30, 2018 at 4:13 PM Daniel Cartwright <chessai1996 at gmail.com>
> wrote:
> >
> > Data.Functor.Compose:
> >
> > newtype Compose f g a = Compose { getCompose :: f (g a) }
> >
> > there's clear-cut, more traditional instances if `f` and `g` are
> Applicative:
> >
> > instance (Applicative f, Applicative g, Semigroup a) => Semigroup
> (Compose f g a) where
> >   (<>) = liftA2 (<>)
> >
> > instance (Applicative f, Applicative g, Monoid a) => Monoid (Compose f g
> a) where
> >   mempty = pure mempty
> >
> > There's an alternative with `QuantifiedConstraints`, but it's arguable
> that this is desirable:
> >
> > instance (forall x. Semigroup x => Semigroup (f x), forall x. Semigroup
> x => Semigroup (g x), Semigroup a) => Semigroup (Compose f g a) where
> >   Compose x <> Compose y = Compose (x <> y)
> >
> > Both to seem to fit the commonplace spirit of lifting monoids up through
> applicative contexts.
> > _______________________________________________
> > Libraries mailing list
> > Libraries at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20181130/8d096344/attachment.html>