On Wed, Nov 18, 2009 at 7:43 AM, Ben Millwood <haskell@benmachine.co.uk> wrote:
It looks quite neat to use the Maybe monoid here:

> import Data.Monoid
> searchList p = foldr (\x -> if p x then mappend (Just [x]) else id) Nothing

but it seems that the Maybe Monoid instance keeps this strict. I
fiddled with this a bit, and came up with the following:

> instance (Monoid m) => Monoid (Maybe m) where
>  mempty = Nothing -- as usual
>  mappend (Just x) y = Just $ mappend x (fromMaybe mempty y)
>  mappend Nothing y = y

The existing Monoid instance for 'Maybe a' lifts what is logically a Semigroup into a Monoid by extending the domain of the operation with a unit (Nothing). Alas, This is annoyingly not the same behavior as the MonadPlus behavior for Maybe, unlike all of the other cases where MonadPlus and Monoid happen to exist in a manner in which they coincide, and since there is no Semigroup class, it lies and claims that it transforms Monoid m => Monoid (Maybe m).

Your version uses mempty from the underlying monoid, so it would break any code that relied on the existing 'lifted Semigroup' interpretation of the Maybe Monoid, which safely operate lift Semigroups-that-claim-to-be-Monoids where mempty = undefined

-Edward Kmett