Fellow Haskellers, I wanted to experiment a bit with lists and sequences (as in Data.List and Data.Sequence), and I got stuck. I wanted to dynamically select processing function depending on cmdline argument:
main = do args <- getArgs let reversor = case args of ["sequence"] -> reverseList ["list"] -> reverseSeq _ -> error "bad args" input <- getContents let output = reversor $ lines $ input mapM_ putStrLn output
In my oppinion reversor would have type
reversor :: (Foldable f) => [a] -> f b
but I couldn't get this to work. I've tried typeclass approach:
class (Foldable f) => Reversor f where reverse' :: [a] -> f a
instance Reversor ([]) where reverse' = Data.List.reverse
instance Reversor ViewR where reverse' = viewr . foldr (<|) empty
reverseList = reverse' :: (???) reverseSeq = reverse' :: (???)
but now in order to differentiate between "reverse'" functions I'd have to provide different type annotations, and then "reversor" won't typecheck... Similar problem surfaced with this try:
data Proc = SP | LP reverseList = reverse' LP reverseSeq = reverse' SP
reverse' :: (Foldable f) => Proc -> [a] -> f a reverse' LP = Data.List.reverse reverse' SP = viewr . foldr (<|) empty
So now I'm looking for some suggestions how should I approach the problem... Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: kokr@jabberpl.org Mobile IRL: +353851383329, Mobile PL: +48783303040 "Simplicity is the ultimate sophistication" -- Leonardo da Vinci