stream-debounce.js

// undefined equivalent for streams    
const undef = {}

// debounce : (Stream a, () => Promise) => Stream a
Stream.prototype.debounce = function(event, last=undef) {

  return  this.isEmpty || this.isAbort ? 
            (last !== undef ? Stream.Cons(last, this) : this) 
        : this.isCons ? 
            this.tail.debounce(event, this.head) 
        : ( last === undef ? 
              Stream.Future(this.promise.then( s => s.debounce(event), Stream.Abort )) 
            : Stream.Future(
                Promise.race([
                  event().then( 
                    _ => () => Stream.Cons(last, this.debounce(event, undef)), 
                    Stream.Abort),
                  this.promise.then( s => () => s.debounce(event, last), Stream.Abort  )
                ]).then( lazy => lazy())))
}