applicatives.js

function flatMap(f, xs) {
  return xs.reduce(function(ys, x){ return ys.concat(f(x)) }, [])
}

function ap(fs, ys) {
  return flatMap(function(f){ return ys.map(f) }, fs)
}

function inverses(a){ 
  return [a, -a] 
}

function flatten(xs){ 
  return flatMap(function(a){ return a }, xs) 
}

var xs = flatMap(inverses, [1, 2, 3, 4, 5, 6, 7, 8, 9])
var fs = xs.map(function(a){
                  return function(b) {
                    return [a + b, a - b] }})

flatten(ap(fs, xs))
// => Array(648): [2, 0, 0, 2, 3, -1, -1, 3, 4, ...]