stream-filter.js

// aBoolean : Object (JavaScript view of truth)
// filter : (Stream a, a -> aBoolean) => Stream a
Stream.prototype.filter = function(pred) {

  return  this.isEmpty || this.isAbort ? this
        : this.isCons ?
            (pred(this.head) ?
                Stream.Cons(this.head, this.tail.filter(pred)) 
              : this.tail.filter(pred))
        : Stream.Future(
            this.promise.then( 
              s => s.filter(pred), 
              Stream.Abort))
}

//filters a stream of keypress events to yield only Ctrl keys
Stream.fromDomEvent(document, 'keydown').filter( e => e.keyCode === 17 )