Using the Endo newtype can avoid such ambiguities:
http://darcs.haskell.org/packages/base/Data/Monoid.hs
newtype Endo a = Endo { appEndo :: a -> a }
instance Monoid (Endo a) where
mempty = Endo id
Endo f `mappend` Endo g = Endo (f . g)
Endo allows you to explicitly select the monoid behavior of the
endomorphism String -> String instead of using String -> String as an
exponent. It seems 6.4.2 -> 6.6 made a change from a default Monoid
instance for (a -> a) to the more general Monoid instance for (a ->
b).
On 4/10/07, Stefan O'Rear
On Tue, Apr 10, 2007 at 02:33:41PM +0100, Chris Kuklewicz wrote:
Well, since ((.) :: ShowS -> ShowS -> ShowS) is a Monoid, you can use Writer to create the result:
Not portably.
stefan@stefans:~$ ghc-6.4.2 -e '( ("foo"++) `Data.Monoid.mappend` ("bar"++) ) "END"' "foobarEND" stefan@stefans:~$ ghc-6.6 -e '( ("foo"++) `Data.Monoid.mappend` ("bar"++) ) "END"' "fooENDbarEND"
-- 6.6 sources instance Monoid b => Monoid (a -> b) where mempty _ = mempty mappend f g x = f x `mappend` g x
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