On Fri, Sep 28, 2007 at 04:38:35PM +0100, Brian Hulley wrote:
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.
True. I forgot to mention imports in my code:
import Prelude hiding (foldr, foldr1, reverse, mapM_) import System.Environment import Data.List hiding (foldr, foldr1) import Data.Foldable import Data.Traversable import Data.Sequence
So the type of mapM_ used in the code is (Foldable t, Monad m) => (a -> m b) -> t a -> m () I'd like to keep the generic Foldable t there when "m" is specialized to IO. I thought this would allow type of "reversor" to be specialized to (Foldable f) => [String] -> f String
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].
Yes, probably that's how it works under the hood, but the reason I mentioned Foldable is that I'd like to avoid [a] -> something -> [a], but keep the type of output value from "reversor" abstract... For no particular reason, just playing with this idea :) 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