Re: how do I abstract this pattern ?

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 ?

I think this is a job for the Either monad: data Either a b = Left a | Right b case p1 s of (Left (True, r)) -> r (Right False) -> case p2 s of (Left (True, r)) -> r (Right False) -> case p3 s of (Left (True, r)) -> r (Right False) -> none So you wrap up your (Bool, result) type using Left, and your Bool type using Right. Regards, Chris.

Thanks HP _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users