On 4/22/11 3:26 PM, Henning Thielemann wrote:
On Fri, 22 Apr 2011, Christopher Done wrote:
Use of Fantom's save invoke and Maybe are more or less the same.
-- Hard way email = if userList /= Nothing then let user = findUser "bob" (fromJust userList) in if user /= Nothing then getEmail (fromJust user) else Nothing else Nothing
In idiomatic Haskell you would write
case userList of Nothing -> Nothing Just plainUserList = let user = findUser "bob" plainUserList ...
Well, in *idiomatic* Haskell you'd write: plainUserList <- userList user <- findUser "bob" plainUserList getEmail user Or even just: getEmail =<< findUser "bob" =<< userList (As others have mentioned a few times.) But the greatest thing about Maybe is that you don't *have* to write code in monadic style. Because Maybe makes explicit the null-pointer shenanigans in other languages, you can simply unwrap the Maybe and pass around the raw value instead of letting Nothing permeate your whole program. Spending all your time in the Maybe monad is just as bad as spending all your time in the IO monad. Purity begets clarity! Bob Harper has a few things to say[1] about using equality tests instead of case analysis, and I agree with him. [1] http://existentialtype.wordpress.com/2011/03/15/boolean-blindness/ -- Live well, ~wren