[Haskell-cafe] Semigroup and Monoid instances for ReaderT

[Louis Pan]

I'm glad you agree that it will be worthwhile. Thank you for cutting out
the cruft from my code.

I've added a request to http://hub.darcs.net/ross/transformers/issue/38

On 22 March 2017 at 08:09, David Feuer <david.feuer at gmail.com> wrote:

> Yes, this is valid, and we should probably do it.
>
> mempty = pure mempty
> mappend = liftA2 mappend
>
> will make a valid Monoid for any Applicative instance, and
>
> x <> y = (<>) <$> x <.> y
>
> will make a valid Semigroup for any Apply instance. Proving this seems
> a bit gross. It's probably best to go via a well-known alternative
> formulation of Applicative:
>
> class Functor f => Apply' f where
>   pair :: f a -> f b -> f (a, b)
> instance Apply' f => Applicative' f where
>   pure' :: a -> f a
>
> This formulation lets you reassociate by switching between  (a, (b,c))
> and  ((a,b),c)  rather than shifting fmaps around.
>
> On Tue, Mar 21, 2017 at 3:53 AM, Louis Pan <louis at pan.me> wrote:
> > Hi all,
> >
> > In my Glazier GUI library, I had to use newtype wrappers to create
> Semigroup
> > and Monoid instances for ReaderT.
> >
> > Is there a reason why ReaderT doesn't have an instance of Semigroup and
> > Monoid?
> >
> > The reader ((->) a) is a Monoid and a Semigroup.
> > https://hackage.haskell.org/package/base-4.9.1.0/docs/src/
> GHC.Base.html#line-268
> > https://hackage.haskell.org/package/base-4.9.1.0/docs/src/
> Data.Semigroup.html#line-150
> >
> > Could the following be added to the transformers package? Or is it not
> > lawful?
> >
> > instance (Applicative m, Semigroup a) => Semigroup (ReaderT r m a) where
> >     f <> g = (<>) <$> f <*> g
> >     {-# INLINABLE (<>) #-}
> >
> > instance (Applicative m, Monoid a) => Monoid (ReaderT r m a) where
> >     mempty = pure mempty
> >     {-# INLINABLE mempty #-}
> >
> >     f `mappend` g = mappend <$> f <*> g
> >     {-# INLINABLE mappend #-}
> >
> > Does it make sense to extend the monoid instance to all the other
> > transformers? Eg.
> >
> > instance (Monad m, Semigroup a) => Semigroup (StateT s m a) where
> >     f <> g = (<>) <$> f <*> g
> >     {-# INLINABLE (<>) #-}
> >
> > instance (Monad m, Monoid a) => Monoid (StateT s m a) where
> >     mempty = pure mempty
> >     {-# INLINABLE mempty #-}
> >
> >     f `mappend` g = mappend <$> f <*> g
> >     {-# INLINABLE mappend #-}
> >
> > instance (Monad m, Monoid w, Semigroup a) => Semigroup (WriterT w m a)
> where
> >     f <> g = (<>) <$> f <*> g
> >     {-# INLINABLE (<>) #-}
> >
> > instance (Monad m, Monoid w, Monoid a) => Monoid (WriterT w m a) where
> >     mempty = pure mempty
> >     {-# INLINABLE mempty #-}
> >
> >     f `mappend` g = mappend <$> f <*> g
> >     {-# INLINABLE mappend #-}
> >
> > and also for MaybeT, IdentityT, ExceptT, etc
> >
> >
> > Regards,
> >
> > Louis
> >
> > _______________________________________________
> > Haskell-Cafe mailing list
> > To (un)subscribe, modify options or view archives go to:
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> > Only members subscribed via the mailman list are allowed to post.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20170322/5d3890bb/attachment.html>