Trying to apply ES6 generators to create monad comprehensions. Unfortunately it looks like this will not work with monads that contain multiple values, such as arrays.

array_monad.js

/*
 * The implementation
 */

function du(comprehension) {
    return function(/* args */) {
        var gen = comprehension.apply(null, arguments)
        return next();

        function next(v) {
            var res = gen.next(v)
            return (!res.done && res.value) ? flatMap(res.value, tryNext) : res.value
        }

        function tryNext(v) {
            try {
                return next(v)
            }
            catch (err) {
                return gen.throw(err)
            }
        }
    }
}

function flatMap(m, f) {
    if (typeof m.then === 'function') {
        return m.then(f)
    }
    else if (typeof m.map === 'function' && typeof m.length === 'number') {
        return flatten(m.map(f))
    }
    else {
        throw new TypeError("No implementation for this argument!")
    }
}

function flatten(xs) {
    if (!xs || !xs.reduce) {
        return xs
    }
    return xs.reduce(function(flat, x) {
        return flat.concat(x)
    }, [])
}


/*
 * An array monad in action
 */

var guests = [
    { name: "Sophia",    vegetarian: true,   peanutAllergy: false },
    { name: "Madison",   vegetarion: false,  peanutAllergy: false },
    { name: "Liam",      vegetarion: true,   peanutAllergy: true  },
    { name: "Noah",      vegetarian: true,   peanutAllergy: false },
    { name: "Elizabeth", vegetarian: false,  peanutAllergy: true  }
]

var entrees = [
    { name: "Meatballs",    vegetarian: false, peanut: false },
    { name: "Pad Thai",     vegetarion: true,  peanut: true  },
    { name: "BLT",          vegetarion: false, peanut: false },
    { name: "Caesar Salad", vegetarian: true,  peanut: false },
    { name: "Game Hen",     vegetarian: false, peanut: true  }
]

var acceptablePairings = du(function*() {
    var guest  = yield guests
    var entree = yield entrees
    if (guest.peanutAllergy && entree.peanut) {
        return []
    }
    else if (guest.vegetarian && !entree.vegetarian) {
        return []
    }
    else {
        return [[guest.name, entree.name]]
    }
})();

// Upon calling `gen.next()` with the second entree, an error is reported:
//
// > Error: Generator has already finished