[Haskell-cafe] "Wrong kind" when attempting to build a monad for a circular list of functions

[Roel van Dijk]

Looks good! A few tips:

>  funcList :: [Int -> Int]
>  funcList = [\_ -> 1, \_ -> 2, \_ -> 3]
funcList = [const 1, const 2, const 3]

>
>  iterateCircularFL :: [a -> b] -> (a -> b, [a -> b])
>  iterateCircularFL (x:xs) = (x, concat [xs, [x]])
{- If you use cycle in main then you do not need this function at all. -}

>  applyCircularFL :: a -> [a -> b] -> (b, [a -> b])
>  applyCircularFL arg fList =
>   let
>     (currentFunc, iteratedList) = iterateCircularFL fList
>   in (currentFunc arg, iteratedList)
{- If the list of functions is infinite then we do not have to worry
about exhausting it, although an empty list will still cause a pattern
match failure. -}
applyCircularFL :: a -> [a -> b] -> (b, [a -> b])
applyCircularFL arg (f:fs) = (f arg, fs)

>testTraversal i l
> |  i == 0 = putStr "Done."
> |  i > 0 = do {
>               putStr "Execution ";
>               putStr (show i);
>               putStr " returned ";
>               putStr (show val);
>               putStr ".\n";
>               testTraversal (i - 1) newList
>            }
>              where (val, newList) = applyCircularFL i l

{- Transform funcList into an infinite list to simplify things -}
main = testTraversal 5 $ cycle funcList

I hope these tips are usefull :-)

Roel