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 <[EMAIL PROTECTED]> wrote:
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.
[EMAIL PROTECTED]:~$ ghc-6.4.2 -e '( ("foo"++) `Data.Monoid.mappend` ("bar"++) )
"END"'
"foobarEND"
[EMAIL PROTECTED]:~$ 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
Stefan
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