Re: [Haskell-cafe] Fwd: Semigroup and Monoid instances for ReaderT

Ah, good point. I didn't think that there would be other instances. I'll close the darcs request. On Thu, 23 Mar 2017 at 6:58 am, David Menendez wrote:

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?

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 http://www.eyrie.org/~zednenem/

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