[Haskell-cafe] Re: The danger of Monad ((->) r)

[Tomasz Zielonka]

On Wed, May 16, 2007 at 09:28:31AM +0100, Jules Bean wrote:
> Tomasz Zielonka wrote:
> >You mean using the (Monoid b) => Monoid (a -> b) instance ?
> >I can see that IO () makes a perfect Monoid, but there doesn't seem to
> >be a standard instance for that.
> 
> Indeed, all Monads are Monoids (that is, if m :: * -> * is a Monad, then 
> m a :: * is a Monoid, for any fixed type a) by using >>.

Are you sure that (IO Int) is a monoid with mappend = (>>)? How do you
define mempty, so it is an identity for mappend?

It would help if type a was a Monoid, then:

    mempty = return mempty
    mappend mx my = do
        x <- mx
        y <- my
        return (x `mappend` y)

It's easier if a = ().

Regards
Tomek