stream-reduce.js

// reduce : ( Stream a, ((b, a) -> a), b  ) -> Promise b
Stream.prototype.reduce = function(f, seed) { 
  return  this.isEmpty  ? Promise.resolve(seed) 
        : this.isAbort  ? Promise.reject(this.error) 
        : this.isCons   ? this.tail.reduce( f, f(seed, this.head) ) 
        : /* isFuture */  this.promise.then( s => s.reduce(f, seed), Promise.reject )  
}

Stream.seq([1,2,3,4,5], 0, 1000)
  .reduce((x, y) => x + y, 0)
  .then (v => console.log('sum =', v) )