how do I abstract this pattern ?

[HP Wei]

Thanks to all who has replied in such a short time.

>From your reponses, I looked at the definition of `mplus`

   Nothing `mplus` ys  = ys
   xs      `mplus` _ = xs

It catches the essence of what I am trying to do!

Later, if pn grows in number, I can scale up by using
some-kind-of-zero with some higher order foldx-like functions.

Thanks a lot,
--hp



On Wed, 21 May 2008, HP Wei wrote:

> 
> Suppose p1, p2, p3 are 3 predicates
> that take an input -- say, a String.
> They return either (True, result)
>             or     False.
> 
> I want to get an effect like in this expression:
> 
> case p1 s of 
>    (True, r) -> r
>    False -> case p2 s of 
>                (True, r) -> r
>                False -> case p3 s of
>                            (True, r) -> r
>                            False -> none
> 
> Is this a job for Maybe monad ?
> How do I do that ?
> 
> Thanks
> HP
>