understanding graphics checkboxes in Elm

ElmCheckbox.elm

import Graphics.Collage exposing (..)
import Graphics.Element exposing (..)
import Graphics.Input exposing (..)
import Signal exposing (..)
import Color exposing (..)

checkRed : Signal.Mailbox Bool
checkRed = Signal.mailbox False
checkBlue : Signal.Mailbox Bool
checkBlue = Signal.mailbox False
checkGreen : Signal.Mailbox Bool
checkGreen = Signal.mailbox False

box: Bool -> Address Bool -> Element
box checked address =
  container 40 40 middle (checkbox (Signal.message address) checked)
  
boxes: Bool -> Bool -> Bool -> Element
boxes red blue green =
  flow down
  [
    flow right 
    [
      box red checkRed.address
    , box blue checkBlue.address
    , box green checkGreen.address
    ]
  , collage 200 100
    [
      colorBox red Color.red (-60.0, 0.0)
    , colorBox blue Color.blue (0.0, 0.0)
    , colorBox green Color.green (60.0, 0.0)
    ]
  ]
  
colorBox: Bool -> Color -> (Float, Float) -> Form
colorBox fill color position =
  -- yeah, this can be written better. easier to read this way.
  if fill
  then (circle 20 |> filled color |> move position )
  else (circle 20 |> traced (solid color) |> move position)

main : Signal Element
main = boxes <~ checkRed.signal ~ checkBlue.signal ~ checkGreen.signal