On further thought, better to use the Maybe afterall. You can use Nothing to signify your False and Just to signify your result. case p1 s of Just result -> result Nothing -> case p2 s of ... That's probably more intuitive to what you were intending. Chris. On Wed, 21 May 2008, C.M.Brown wrote:
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
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