This Elm program compiles to a Web page that plays a movie of image files, reversing direction when it reaches the last or first image

SlideShow.elm

import Time (every)
import Window (dimensions)

firstImageIndex = 0
lastImageIndex = 1000
delta = lift inSeconds (fps 30)

modCount signal period = lift (\a -> a `mod` period) (count signal)

data PlayDirection = Fwd | Back

yoyoIndex direction fwdIndex reverseIndex = if (direction == Fwd) then fwdIndex else reverseIndex

yoyo signal period =
  let fwdIndices = modCount signal period
      reverseIndices = lift (\a -> period - a) fwdIndices
      turnarounds = keepIf (\a -> a == 0) false fwdIndices
      directions = (lift (\t -> if (t `mod` 2 == 0) then Fwd else Back) (count turnarounds))
  in
    lift3 yoyoIndex directions fwdIndices reverseIndices

showSlide (w,h) imageIndex =
  let filename = "morph/" ++ show imageIndex ++ ".svg"
      pic = image 800 800 filename
      textElement = text (Text.color white (toText  $ show imageIndex))
  in
    color black . container w h middle $ flow down [ container w (heightOf pic) middle pic, container w 100 midTop textElement ]

showSlides first last interval = lift2 showSlide dimensions (lift (\a -> a + first) (yoyo (every interval) (last - first)))

main = showSlides firstImageIndex lastImageIndex delta