Implementation of swipe gestures in Elm

Swipe.elm

import Graphics.Element exposing (show, Element)
import Touch exposing (Touch)
import Signal
import Set


type alias Swipe = Touch


touches': Signal (List Touch, List Touch)
touches' = 
  Signal.foldp 
    (\touches (a, _) -> (touches, a)) 
    ([], []) 
    Touch.touches


getSwipes: (List Touch, List Touch) -> List Swipe
getSwipes (newTouches, oldTouches) = 
  let getIds touches = 
        List.map 
          (\touch -> touch.id)
          touches
        |> Set.fromList
          
      swipeIds =         
        Set.diff 
          (getIds oldTouches) 
          (getIds newTouches)
        |> Set.toList 
  in            
    List.filter 
      (\touch -> List.member touch.id swipeIds) 
      oldTouches


swipes: Signal (List Swipe)
swipes = Signal.map getSwipes touches'


main: Signal Element
main = Signal.map show swipes