Alphanumeric.elm

allAlphanumeric : String -> Bool
allAlphanumeric str =
    List.map isAlphanumeric (String.toList str)
        |> List.foldr (||) False


isAlphanumeric : Char -> Bool
isAlphanumeric char =
    String.contains (String.fromChar char) alphanumeric


alphanumeric : String
alphanumeric =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvxwyz0123456789"

You could also achieve this pretty succinctly using regex and partial application, like this. :)