Re: how do I abstract this pattern ?

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