Why REPL is a good thing

Here's a great article on why REPL (read-evaluate-print loop) is a good thing. He mentions using IronPython for .Net, but you can get the same functionality in java with Jess, Groovy, jRuby, and of course, jPython. I love the mentions of Smalltalk. REPL allows you to try out designs and code out quickly. Smalltalk compiles code when you save and you never notice it. One thing that people don't realize is that you are inside a running system that you can change. Any change you make to the system is live, it's not operating on a dead instance of the system. It's the same functionality that gets blank stares at "Ruby On Rails" demos. Sad thing is we've had this for over 20+ years in Smalltalk and Lisp. I'm just glad our industry is finally coming around.