stream-span.js

// span : (Stream a, a -> aBoolean) -> [Stream a, Stream a]
Stream.prototype.span = function(p) { 
  let s1, s2, futs;
 
  return  this.isEmpty || this.isAbort ? [this, this]
        : this.isCons ? 
           (p(this.head) ?
              ([s1, s2] = this.tail.span(p), [Stream.Cons(this.head, s1), s2])
            : [Stream.Empty, this])
        : (futs = this.promise.then(s => s.span(p), Stream.Abort),
           [
            Stream.Future( futs.then( sp => sp[0] ) ),
            Stream.Future( futs.then( sp => sp[1] ) )
           ])
}

// example : span a 1s separated sequence form 1 to 10
var [s1, s2] = Stream.range(1, 10, 0, 1000).span( x => x < 3)
s1.log('< 3')
s2.log('>= 3')