Data.Monoid

[Iavor Diatchki]

Hello,
I am not sure what the process is, but this seems like a good idea to
me.  I would only suggest that instead of 'runEndo' we call the
selector 'appEndo'. We could even use some sort of infix application
operator if anyone could suggest one.
-Iavor

On 9/13/05, Ross Paterson <ross at soi.city.ac.uk> wrote:
> (regurgitating
> http://www.haskell.org/pipermail/libraries/2005-July/004057.html)
> 
> I propose to replace the instance
> 
>         instance Monoid (a -> a) where
>                 mempty  = id
>                 mappend = (.)
> 
> with
> 
>         newtype Endo a = Endo { runEndo :: a -> a }
> 
>         instance Monoid (Endo a) where
>                 mempty = Endo id
>                 Endo f `mappend` Endo g = Endo (f . g)
> 
>         instance Monoid b => Monoid (a -> b) where
>                 mempty _ = mempty
>                 mappend f g x = f x `mappend` g x
> 
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