Krzysztof Kościuszkiewicz wrote:
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
No, this is the wrong type. To find the correct type, if you look at the type of the input argument in your code it will be the result of (lines), so from ghci: Prelude> :t lines lines :: String -> [String] Prelude> Therefore (reverseor) has type [String] -> ??? Now for the output type, you are using (output) as an input to (mapM_ putStrLn). (mapM_) takes a list and uses its argument to do something to each element of the list. So, since the input to (putStrLn) is (String), the input to (mapM_ putStrLn) is ([String]). Therefore reversor :: [String] -> [String] So reverseList is just Data.List.reverse as you've got it (though presumably you meant to write ["list"] -> reverseList and not reverseSeq). For using Data.Sequence to implement reversor, all you need to do is first convert [String] to Seq String, reverse the sequence, then convert back from Seq String to [String]. Hope this helps, Brian.
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,