concrete functor and monad transformers
#86Semigroup / Monoid instances for monad transformers
I would like to contribute Semigroup / Monoid instances of the form:
instance Semigroup a => Semigroup (F a) where
(<>) = liftA2 (<>)
instance Monoid a => Monoid (F a) where
mempty = pure mempty… where F is each monad transformer. Or equivalently:
deriving (Semigroup, Monoid) via (Ap F a)I motivate why these instances are useful and why the existence of Ap alone does not suffice in this post:
https://www.haskellforall.com/2022/03/applicatives-should-usually-implement.html
An alternative variation on this proposal that I'd also be fine with would be to omit the Semigroup / Monoid instance for monad transformers that have a non-lifted Alternative instance (such as MaybeT)
I just want to add my support for this proposal. I find it very convenient being able to use
foldMapwith transformers, but I currently can't do that without defining orphan instances.I'd like to propose that this issue be extended to also cover the
Applicativetransformers (BackwardsandLift).
