Newbie qustion about monads

[Juanma Barranquero]

On Thu, 02 Oct 2003 14:27:29 +0200
"Marcin 'Qrczak' Kowalczyk" <qrczak at knm.org.pl> wrote:

> Accumulating state is fine. These definitions don't accumulate state:
> 'return' should yield a "neutral" state, and the above ">>=" ignores the
> state of the lhs.

You're right.

> data Accum s a = Ac [s] a
> 
> instance Monad (Accum s) where
>    return x      = Ac [] x
>    Ac s1 x >>= f = let Ac s2 y = f x in Ac (s1++s2) y
> 
> output :: a -> Accum a ()
> output x = Ac [x] ()

Nice. Thanks!

                                                                Juanma