Is it possible to iterate over union type in Elm?

Question

I have a type union of colors that I want to render to the user. Is it possible to iterate over all type union values?

type Color = Red | Blue | Green | Black

colorToStirng color = 
    case color of
        Red -> "red"
        Blue -> "blue"
        Green -> "green"
        Black -> "black"

colorList = 
    ul
        []
        List.map colorListItem Color  -- <- this is the missing puzzle

colorListItem color = 
    li [class "color-" ++ (colorToString color) ] [ text (colorToString color) ]

Answer 1

The problem with declaring a function like:

type Foo 
  = Bar 
  | Baz

enumFoo = 
  [ Bar 
  , Baz ]

is that you will probably forget to add new enums to it. To solve this, I've been playing with this (hacky, but less hacky than the idea above) idea:

enumFoo : List Foo
enumFoo =
  let
    ignored thing =
      case thing of
          Bar ->  ()
          Baz -> ()
          -- add new instances to the list below!
  in [ Bar, Baz ]

This way you at least get an error for the function and hopefully don't forget to add it to the list.