Bugger. I suspect, comparing the elm wiki page with the ruby one that the packages are currently limited to elm/core and elm-explorations/test. If so, that's pretty limiting. Without it, I suppose one could manually copypaste the whole Regex module in to the preloaded section, but that seems clumsy. Summary: I don't know.

This could also be solved arguably more neatly via a Regex e.g.

import Regex

stripComments : String -> List Char -> String
stripComments input markers =
    let
        markersString =
            String.concat <|
                List.map (\marker -> "\\" ++ String.fromChar marker) markers
        regex =
            Maybe.withDefault Regex.never <|
                Regex.fromString ("[ \t]*[" ++ markersString ++ "].*")
    in
    Regex.replace regex (\_ -> "") input

One other thought is that the markers input could be a List String rather than a List Char. I don't know what Elm best practice is here. I suppose there is some benefit to forcing the list to be single characters rather than allow a multi-character through.

To get this working I needed a shuffle which isn't natively provided by the Random library. I've pulled in the relevant functions from elm-community/random-extra and owanturist/elm-union-find packages. Bit annoying though.

That's a fair point. I'm still learning the Elm test framework so I'll have a think about how to go about this.

Elm translation for review.