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
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