stream-filter-async.js

// filter : (Stream a, a -> aBoolean | Promise aBoolean) => Stream a
Stream.prototype.filter = function(p) {
  let tailF, futP;

  return  this.isEmpty || this.isAbort ? this 
        : this.isCons ?
            ( tailF = this.tail.filter(p),
              futP = Promise.resolve( p(this.head)  )
                            .then( ok => ok ? Stream.Cons(this.head, tailF) : tailF, Stream.Abort )
              Stream.Future(futP))
        : Stream.Future(
            this.promise.then( s => s.filter(p), Stream.Abort ))
}