myswap.hs

import qualified Data.Vector.Mutable as MV
import qualified Data.Vector as V
import Control.Monad.Primitive
import Control.Monad

myswap :: PrimMonad m => MV.MVector (PrimState m) a -> Int -> Int -> m ()
myswap v i j = do
  a <- MV.read v i
  b <- MV.read v j
  MV.write v i b
  MV.write v j a

main :: IO ()
main = do
  array <- V.thaw $ V.fromList [100,200,300]
  myswap array 0 1
  frozed <- V.freeze array
  let list = (V.toList frozed)
  print list