stream-groupBy.js

// groupBy : (Stream a, (a, a) -> aBoolean) => Stream (Stream a)
Stream.prototype.groupBy = function(p) {
  var s1, s2;

  return  this.isEmpty || this.isAbort ? this 
        : this.isCons ?
          ( [s1, s2] = this.tail.span( x => p(this.head, x) ), 
            Stream.Cons(
              Stream.Cons(this.head, s1),
              s2.groupBy(p))) 
        : Stream.Future( this.promise.then( s => s.groupBy(p), Stream.Abort ) );
};