Demonstrates scanl to paste into http://www.elm-lang.org/try

RectangleOffsets.elm

import Color exposing (..)
import Graphics.Collage exposing (..)
import Graphics.Element
import Debug exposing (..)


type alias Rect = { width: Float, color: Color }


toForm : (Rect, Float) -> Form
toForm (r, offset) =
    filled r.color (rect r.width 100)
    |> alpha 0.8
    |> moveX (r.width / 2)
    |> moveX offset
  

main =
  let
    rects = [ Rect 20 yellow, Rect 100 lightGreen, Rect 20 lightBlue ]
    widths = List.map (\r -> r.width) rects
    offsets = List.scanl (+) 0 widths
    rectsWithOffsets = List.map2 (,) rects offsets   
  in
    collage 400 400 (List.map toForm rectsWithOffsets)