[Haskell-cafe] List operation question

[Bryan Donlan]

Eric Olander wrote:
> Hi,
>    I'm still somewhat new to Haskell, so I'm wondering if there are 
> better ways I could implement the following functions, especially shiftl:
> 
>  >> moves the first element to the end of the list
>     shiftr :: [a] -> [a]
>     shiftr [] = []
>     shiftr (x:y) = y ++ [x]
>    
>  >> moves the last element to the head of the list
>     shiftl :: [a] -> [a]
>     shiftl [] = []
>     shiftl x = [last x] ++ init x

If you use Data.Sequence (new in 6.6, I think), these can be O(1):

import qualified Data.Sequence as Seq
import Data.Sequence ( (<|), (|>), (:<), (:>) )

shiftr seq = go (Seq.viewl seq)
   where
     go (EmptyL) = Seq.empty
     go (e :< remain) = remain |> e

shiftl seq = go (Seq.viewr seq)
   where
     go (EmptyR) = Seq.empty
     go (remain :> e) = e <| remain

Decomposing by elements like this is a bit unwieldy, but using the 
functions in Data.Traversable and Data.Foldable it shouldn't be too bad, 
depending on what you're doing.