An add operation that appears to be a semilatice

semilatice.elm

addLists : (a -> a -> Order) -> List a -> List a -> List a
addLists compareFn lst1 lst2 =
    case ( lst1, lst2 ) of
        ( _, [] ) ->
            lst1

        ( [], _ ) ->
            lst2

        ( h1 :: t1, h2 :: t2 ) ->
            case compareFn h1 h2 of
                EQ ->
                    h1 :: addLists compareFn t1 t2

                GT ->
                    h1 :: addLists compareFn t1 lst2

                LT ->
                    h2 :: addLists compareFn lst1 t2