Move indices around STArray

Main.purs

module Main where

import Prelude

import Data.Foldable (fold)
import Effect (Effect)
import TryPureScript (h1, h2, p, text, list, indent, link, render, code)
import Control.Monad.ST as ST
import Data.Array.ST as Array.ST

moveFromTo :: forall a. Int -> Int -> Array a -> Array a
moveFromTo sourceIndex targetIndex xs = ST.run do
  xs' <- Array.ST.thaw xs
  x <- Array.ST.splice sourceIndex 1 [] xs'
  _ <- Array.ST.splice targetIndex 0 x xs'
  Array.ST.freeze xs'

data MoveOperation = Up | Down

move :: forall a. MoveOperation -> Int -> Array a -> Array a
move Up n = moveFromTo n (n - 1)
move Down n = moveFromTo n (n + 1)

main :: Effect Unit
main = render do
  let xs = [1,2,3,4]
  fold
    [ p $ text ("Original: " <> show xs)
    , p $ text ("Move 3→1: " <> show (moveFromTo 3 1 xs))
    , p $ text ("Move up from 2: " <> show (move Up 2 xs))
    , p $ text ("Move down from 2: " <> show (move Down 2 xs))
    ]