On Wed, Nov 12, 2008 at 10:20 AM, Derek Elkins
In addition to what others have said, I recommend using functions like isTypeB :: T -> Maybe Double isTypeB (B d) = Just d isTypeB _ = Nothing
You can recover the Bool version of isTypeB just by post-composing with isJust, but this version avoids needing partial functions and often is more what you want (i.e. it's a first-class "pattern"). Combining it with catMaybes is also a common pattern.
Although if you are using Maybe as a monad, these two pieces of code are equivalent: test1 t1 t2 = do d1 <- isTypeB t1 d2 <- isTypeB t2 return (d1 + d2) test2 t1 t2 = do B d1 <- return t1 B d2 <- return t2 return (d1 + d2) This is because pattern-matching in do-notation implicitly calls "fail" when the pattern match fails. -- ryan