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 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 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

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