Displays 10 boxes, one of them randomly switching off every cycle (200ms).

DiscoBoxen.elm

module DiscoBoxen where

import Array exposing (fromList, get)
import Color exposing (Color, black, white, green, red, blue, yellow, brown, purple, orange)
import Graphics.Element exposing (Element, spacer, color, flow, right, show)
import List exposing (foldr, (::))
import Random exposing (generate, initialSeed)
import Time exposing (every, millisecond, second)

drawSquare : Int -> Int -> Color -> Element
drawSquare x y c =
  spacer x y |> color c

drawSquares : Int -> List Element
drawSquares r =
  foldr
    (\x acc -> (::) 
      (if r == x then
        drawSquare 10 10 white
      else
        drawSquare 10 10 black)
    acc)
    []
    [1..10]

getColor n =
  if n % 2 == 0 then
    black
  else
    white

getRandomBoxNum n =
  let doGenerate seed =
        generate (Random.int 1 10) (initialSeed (round seed)) |> fst
  in
    Signal.map doGenerate (every (n * millisecond))

{-getRandomBoxNum seed =
  generate (Random.int 1 10) (initialSeed (round seed)) |> fst
main = 
  Signal.map (flow right) (Signal.map drawSquares (Signal.map getRandomBoxNum (every (200 * millisecond))))
-}
main = 
  Signal.map (flow right) (Signal.map drawSquares (getRandomBoxNum 200))

Just a stylistic comment - since you set things up in the let for the expression in the in, I would personally do:

getRandomBoxNum n =
  let doGenerate seed =
    generate (Random.int 1 10) (initialSeed (round seed)) |> fst
  in 
    Signal.map doGenerate (every (n * millisecond))

@ccapndave ty, updated with your comment 😄