(() => {
    // ROMAN INTEGER STRINGS ----------------------------------------------------

    // roman :: Int -> String
    const roman = n =>
        concat(snd(mapAccumL((balance, [k, v]) => {
            const [q, r] = quotRem(balance, v);
            return [r, q > 0 ? k.repeat(q) : ''];
        }, n, [
            ['M', 1000],
            ['CM', 900],
            ['D', 500],
            ['CD', 400],
            ['C', 100],
            ['XC', 90],
            ['L', 50],
            ['XL', 40],
            ['X', 10],
            ['IX', 9],
            ['V', 5],
            ['IV', 4],
            ['I', 1]
        ])));

    // GENERIC FUNCTIONS -------------------------------------------------------

    // concat :: [[a]] -> [a] | [String] -> String
    const concat = xs =>
        xs.length > 0 ? (() => {
            const unit = typeof xs[0] === 'string' ? '' : [];
            return unit.concat.apply(unit, xs);
        })() : [];

    // map :: (a -> b) -> [a] -> [b]
    const map = (f, xs) => xs.map(f);

    // 'The mapAccumL function behaves like a combination of map and foldl;
    // it applies a function to each element of a list, passing an accumulating
    // parameter from left to right, and returning a final value of this
    // accumulator together with the new list.' (See Hoogle)

    // mapAccumL :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y])
    const mapAccumL = (f, acc, xs) =>
        xs.reduce((a, x) => {
            const pair = f(a[0], x);
            return [pair[0], a[1].concat([pair[1]])];
        }, [acc, []]);

    // quotRem :: Integral a => a -> a -> (a, a)
    const quotRem = (m, n) => [Math.floor(m / n), m % n];

    // show :: a -> String
    const show = (...x) =>
        JSON.stringify.apply(
            null, x.length > 1 ? [x[0], null, x[1]] : x
        );

    // snd :: (a, b) -> b
    const snd = tpl => Array.isArray(tpl) ? tpl[1] : undefined;

    // TEST -------------------------------------------------------------------
    return show(
        map(roman, [2016, 1990, 2008, 2000, 1666])
    );
})();