instance Semigroup/Monoid (f (g a)) => Semigroup/Monoid (Compose f g a)
Motivation
I am defining a higher-kinded data structure representing configuration:
data Config f = Config
{ _port :: f Int
, _db :: f FilePath
}
instance (forall a . Semigroup (f a)) => Semigroup (Config f) where
Config p1 db1 <> Config p2 db2 = Config (p1 <> p2) (db1 <> db2)
instance (forall a . Monoid (f a)) => Monoid (Config f) where
mempty = Config mempty mempty
I want to represent partial configs using the Maybe (Last a))
Monoid. To make that fit, I have to use Data.Functor.Compose.Compose
:
type FullConfig = Config Identity
type PartialConfig = Config (Compose Maybe Last)
However, Compose f g a
has no Monoid
instance that uses the underlying f (g a)
. At this point I am sad, and have to either write an orphan, or my own newtype Last a = Last (Maybe a)
which is the same as Data.Maybe.Last
(which was meant to be deprecated in 8.8, according to base-4.13.0.0
haddocks?).
Proposal
Add the following instances (possibly GeneralizedNewtypeDeriving will give them to us):
instance Semigroup (f (g a)) => Semigroup (Compose f g a)
instance Monoid (f (g a)) => Monoid (Compose f g a)
Related Issues
#15028 (closed) - Deprecate and remove Data.Semigroup.Option; Data.Monoid.{First,Last}
#16636 - Eq1/Show1 etc for Data.Monoid.{First,Last}