On Tue, Mar 21, 2017 at 3:53 AM, Louis Pan <louis@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?

There are at least three reasonable instances of Semigroup and Monoid for ReaderT.

instance (Applicative m, Monoid a) => Monoid (ReaderT r m a) where

mempty = pure mempty

mappend a b = mappend <$> a <*> b

instance (Alternative m) => Monoid (ReaderT r m a) where

mempty = empty

mappend = (<|>)

instance (Monoid (m a)) => Monoid (ReaderT r m a) where

mempty = ReaderT $ mempty

mappend a b = ReaderT $ mappend a b

In the absence of a principled reason to prefer one over the others and a general consensus, I think it’s better not to choose.

Dave Menendez <dave@zednenem.com>
<http://www.eyrie.org/~zednenem/>