filter.go

func (l *Lexer) Tokens() rxgo.Observable {
  return l.observable.
    Filter(func(i interface{}) bool {
      var str = i.(string)
      return !isWhitespace(str)
    })
    // next operator
}

func isWhitespace(ch string) bool {
  return ch == " " || ch == "\t" || ch == "\n" || ch == "\r"
}