swap08.hs

import Control.Monad.State

type Ary a = [a]
type Temp a = a

type Vars a = (Ary a, Temp a)

toTemp :: Int -> State (Vars a) ()
toTemp i = modify $ \(xs, tmp) -> (xs, xs!!i)

copyElem :: Int -> Int -> State (Vars a) ()
copyElem i j = modify $ \(xs, tmp) ->
               (take i xs ++ xs!!j : drop (i+1) xs, tmp)

fromTemp :: Int -> State (Vars a) ()
fromTemp j = modify $ \(xs, tmp) ->
             (take j xs ++ tmp : drop (j+1) xs, tmp)

swapS :: Int -> Int -> State (Vars a) ()
swapS i j = toTemp i >> copyElem i j >> fromTemp j

swap i j xs = fst $ execState (swapS i j) (xs, 0)