On Tue, Jun 30, 2009 at 6:45 PM, Bryan O'Sullivan
I've thought for a while that it would be very nice indeed if the Monoid class had a more concise operator for infix appending than "a `mappend` b". I wonder if other people are of a similar opinion, and if so, whether this is worth submitting a libraries@ proposal over.
We could use (Control.Category..) as an operator, but this would require an additional wrapping layer if we wish to use the existing Monoid instances:
import Prelude hiding (id, (.)) import Control.Category import Data.Monoid
-- Category wrapper for existing Monoid instances newtype MonoidC m a b = MonoidC {unwrapMC :: m} deriving (Show)
instance Monoid m => Category (MonoidC m) where id = MonoidC mempty MonoidC m . MonoidC n = MonoidC $ m `mappend` n
Furthermore, writing Category instances for monoids require dummy type parameters:
-- Example instance newtype SumC m a b = SumC {getSumC :: m} deriving (Show, Eq)
instance Num a => Category (SumC a) where id = SumC (fromIntegral 0) SumC x . SumC y = SumC $ x + y
Another disadvantage of this approach is that we cannot have a default monoid instance for lists (kind mismatch).