PasswordValidation.elm

validatePassword : String -> Result String ValidPassword
validatePassword password =
    validate password passwordConditions


passwordConditions : List (String -> Bool, String)
passwordConditions =
    [ (isBlank, "Password can't be empty")
    , (isTooShort, "Password too short")
    ]


validate : String -> List (String -> Bool, String) -> Result String ValidPassword
validate password conditions =
    conditions
        |> List.map (checkCondition password)
        |> combineErrors password


checkCondition : String -> (String -> Bool, String) -> Maybe String
checkCondition password (condition, error) =
    if condition password then
        Just error
    else
        Nothing


combineErrors : String -> List (Maybe String) -> Result String ValidPassword
combineErrors password errors =
    case errors of
        Just error :: rest ->
            Err error

        Nothing :: rest ->
            combineErrors password rest

        [] ->
            Ok (ValidPassword (Password password))