stream-zip.js

// zip : (Stream a, Stream b) => Stream [a,b]
Stream.prototype.zip = function(s2) { 

  return  this.isEmpty || this.isAbort  ? this 
        : s2.isEmpty || s2.isAbort      ? s2 
        : this.isCons && s2.isCons      ? 
            Stream.Cons([this.head, s2.head], this.tail.zip(s2.tail)) 
        : this.isCons && s2.isFuture    ? 
            Stream.Future(s2.promise.then(s => this.zip(s), Stream.Abort))
        : // this.isFuture && (s2.isCons || s2.isFuture)
            Stream.Future(this.promise.then(s => s.zip(s2), Stream.Abort))
}