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

Felipe Lessa felipe.lessa at gmail.com
Thu Feb 28 05:44:48 EST 2008

On Thu, Feb 28, 2008 at 4:28 AM, Aaron Altman <aaltman at pdx.edu> wrote:
>  runActionAndIterate :: [a -> a] -> a -> (a, [a -> a])
>  runActionAndIterate (currentAction:actionList) actionInput =
>  (currentAction actionInput, concat [actionList, [currentAction]])
>  shiftActionList :: [a -> a] -> [a -> a]
>  shiftActionList (currentHead:rest) = concat [rest, [currentHead]]

As a side note, it's not good to recreate the list (using 'concat')
for every item as it is an O(n) operation. Bas van Dijk's 'always'
(also called 'forever'[1]) is an option, but you can also create a
circular list using the simple function 'cycle'[2] and your functions
above would become

runActionAndIterate (currentAction:actionList) actionInput =
  (currentAction actionInput, actionList)

shiftActionList = tail

[1] http://haskell.org/ghc/docs/latest/html/libraries/base/Control-Monad.html#v%3Aforever
[2] http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v%3Acycle


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