[Haskell-cafe] Fwd: Semigroup and Monoid instances for ReaderT
dave at zednenem.com
Wed Mar 22 19:55:06 UTC 2017
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
There are at least three reasonable instances of Semigroup and Monoid for
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 at zednenem.com>
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