the programmer language,

Jun 18, 2011
Rexpl - an interactive bytecode console for Rubinius

A bit of a background

Since the beginning of the year I've been gradually becoming more and more interested in language design. Last December I implemented Brainfuck, the most trivial Turing-complete programming language, in pure Ruby.

In January I discovered parslet, a really smart piece of software written by Kaspar Schiess). Having tried a different approach at generating PEG parsers before, I found parslet quite smarter and friendlier to use, so I decided that the best way of giving it a try would be rewriting my Brainfuck implementation with it.

Since I liked it very much, in February I started an attempt to implement the Scheme programming language in Ruby, using what I already knew about parslet and trying to figure out the rest (not without the help of this awesome book and a refreshing trip to Paris). Although it's far from finished, it turned out to be an enlightening experience and it just brought my interest on language design further.


A few months earlier I had become aware of Rubinius, which up until that point was nothing more than a strange word that came up whenever I typed rvm list known in the terminal. It turned out to be an amazing project to implement Ruby... in Ruby itself! Taking advantage of modern research about language design, they are working hard to make it both faster and more user-friendly than the most widely used implementation, MRI.

That was last October, when I attended to Ruby And Rails Conf 2010, a really awesome Ruby conference held in Amsterdam, and there were not only one, but two talks about Rubinius. The first one was given by Dirkjan Bussink about the Rubinius VM itself. The other was given by Christopher Bertels, and he talked about Fancy, his own programming language, which was bootstrapped on the Rubinius VM.

I found those two talks really inspiring, and in May I rewrote (again) my Brainfuck implementation targeting the Rubinius VM. It was a lot of fun! I also freaked out a bit when I realized that every programming language, well, everything in computing, could be represented by a stack and a memory heap. Man! My educational background is in Political Science, where everything is sooo high level :) So that was kind of shocking to me.

Pen and paper

While tinkering with the Rubinius VM, I found it useful to have pen and paper around whenever I was coding. Given a set of VM instructions, I'd go instruction after instruction, drawing what the stack should look like at each step. I'd freak out at net stack underflow errors, try to follow along the instructions that were being executed, keeping track of the stack size and spotting that extra (or missing) pop. As a last resort, I'd also go crying in the #rubinius IRC channel on Freenode :)

Once, in that very channel, I asked if there was a way to print the stack after a particular instruction. Since there wasn't, I started reading the Rubinius source code and came up with a short script that seemed to do exactly that — except that it didn't work at all, so I abandoned the idea, drown in sadness.

Then Jeremy Tregunna, the guy behind Metis (an implementation of IO targeting the Rubinius VM), tweeted that he tried to do the same, and that he wanted to bring it further, like an IRB for bytecode. I thought this was even better than what I had in mind, so I started hacking on it and came up with rexpl, a REPL for Rubinius bytecode.

Installing and playing with rexpl

Rexpl is built to be a fun tool to use when learning how to use Rubinius bytecode instructions, for example when bootstraping a new language targeting the Rubinius VM for the first time.

Its main feature is stack introspection, which means you can inspect what the stack looks like after each step of your instruction set.

After you have installed Rubinius (you can find how to do so in their website), open a terminal and type:

$ gem install rexpl
$ rexpl

Now you should see a welcome banner and an IRB-like prompt, and you're good to go! Just start typing some VM instructions and see what happens.

There are three extra commands to take advantage of the stack introspection:

  • list lists the instruction set of the current program.
  • reset empties the instruction set and starts a new program.
  • draw prints a visual representation of the stack after each instruction of your program.

Here's a screenshot of what it looks like:

rexpl in action


The first version of Rexpl was just released yesterday, so it should contain a number of unforeseen use cases and unexpected behaviors.

If you have never played with the Rubinius VM, now it's time to start! You can tinker around, make yourself comfortble and who knows, maybe you're the author of the next mainstream programming language!

And if you already are a language master and dream in bytecode, you can surely find a lot of edge cases where rexpl would need more work — which I will be surely glad to do.

Make sure to check out the Github repo and create a bunch of issues! I love them :)