Am Montag, 11. April 2005 15:59 schrieb Christoph Bauer:
Ok, my second haskell program ;-):
module Init where
import Maybe
left :: a -> Maybe [a] -> Maybe [a] left x None = (Just [])
^^^^^^^^ Nothing, as below :-)
left x (Just l) = (Just (x:l))
init :: [a] -> [a] init xs = fromJust . foldr left Nothing xs
Sure, there is a better solution...
I don't think so. As far as I see, it's impossible to do it with just init xs = foldr fun val xs, (unless we use some dirty trick) because we must have fun x (init ys) = x:init ys for any nonempty list ys and init [] = val forces val to be 'error "init of empty List"' or something of the sort and this has to be evaluated when we reach the end of the list, for we would need fun _ (error "blah") = [] for nonempty lists. Dirty trick: unsafePerformIO import System.IO.Unsafe fun :: a -> [a] -> [a] fun x xs = unsafePerformIO ((xs `seq` (return (x:xs))) `catch` (\ _ -> return [])) init3 :: [a] -> [a] init3 = foldr fun (unsafePerformIO (ioError (userError "init of []"))) *Init> init3 [] *** Exception: user error (init of []) *Init> init3 [1] [] *Init> init3 [1 .. 10] [1,2,3,4,5,6,7,8,9] Though this works, it is utterly horrible and despicable. DON'T DO THAT!!!!!
Best Regards, Christoph Bauer
Cheers, Daniel