JRuby bills itself as the pragmatic Ruby, the go-to implementation when you need to fit into the Java universe or support a ton of platforms. Who knew it was also a tool for having fun exploring the realms of computer science?

1. JRUBY, NOT JUST FOR HARD-
HEADED PRAGMATISTS ANYMORE
IAN DEES • @UNDEES
WHISKEY-
CONF 2011
✹ ALSO
JR UBY
Hi, I’m Ian. Hope everyone’s having a great WhiskyConf. I hear we’ll have some JRuby material at
this conference as well. ;-)
http://www.flickr.com/photos/etnobofin/3786071796

2. Write a Compiler
Most JRuby talks are pragmatic: either a case study of how JRuby helped a project, or a specific
library that developers can use. This talk will be more theoretical. We’ll use JRuby as a vehicle to
explore an area of computer science; namely, writing a compiler.

3. The Three Paths
Abstraction
compilers
Time
Depending on your path through computer science, you may have encountered the topic already.

4. Abstraction
compilers
xor eax, eax
e-
Time
If, like me, you grew up on the hardware path, a compiler is the next logical step beyond learning
programming languages.

5. logic
λx.x grammars
Abstraction
compilers
xor eax, eax
e-
Time
If you took the software path and learned the theory of computation first, then compilers are a
practical application of those ideas.

6. logic
λx.x grammars
Abstraction
compilers
xor eax, eax
e-
Time
If you took the self-made path, you may or may not have encountered compilers by this point on
your journey. Either way, I hope the tools we talk about today pique your interest.

7. The Plan
✓Stay up late drinking whisky
✓Wear the T-shirt of the band that’s playing
✓Choose the wrong topic
✓Go at a pace that’s awkward for everyone
✓Code live on stage
The plan for this talk is to show up tired, wear a JRubyConf t-shirt (this is like showing up at a
concert wearing the t-shirt of the band that’s playing), choose a topic and pace different from all
the other talks, and introduce an element of uncertainty by coding on stage. (If you’re reading this,
I’ll show you GitHub commits instead.)

8. This might be a train wreck, but train wrecks can be entertaining as long as they’re toy trains.
http://www.flickr.com/photos/cianginty/3148870954

9. The Tools
• Parslet
http://kschiess.github.com/parslet
• BiteScript
https://github.com/headius/bitescript
• Gritty
https://github.com/undees/gritty
• Graph
http://rubygems.org/gems/graph
• JRuby!
The first two tools are the ones we’ll use to write the compiler. The next two are the ones I used to
draw the pictures on the subsequent slides.

10. The Language
The language we’re going to be writing a compiler for is an extremely simple one.

11. Thnad
(thank you, Dr. Seuss!)
Most of the real letters of the alphabet are already used for programming languages (C, D, J, K, R,
etc.). So we’ll use the fictional letter “Thnad,” which comes to us from Dr. Seuss.

12. function factorial(n) {
if (eq(n, 1)) {
1
} else {
times(n, factorial(minus(n, 1)))
}
}
print(factorial(4))
Here’s the Thnad program we’d eventually like to compile. It has integers, function calls,
conditionals, and function definitions—and that’s about it.

13. 1. Integers
One big step is to write a compiler top to bottom that parses an integer and outputs a working
Java .class file.

14. 42
First, we’re going to write code that parses the text “42” into a Ruby hash representing the tree on
the right.

15. Following Along
• Test → test_parser.rb
• Code → parser.rb
Slides like this one will contain a link to the tests, code, or full examples for the steps we’ve just
been over.

16. Next, we’re going to transform that Ruby hash (which isn’t as useful by itself) into a custom Ruby
class that will eventually be able to output Java bytecode.

17. • Test → test_transform.rb
• Code → transform.rb
• Full Example

18. 2. Function Calls
Now, let’s add the ability to compile function calls.

19. foo
We need to be able to parse identifiers; we’ll use these for function and parameter names.

20. As we did with integers, we want to transform the first hash representation into a more specific
Ruby object.

21. (42, foo)
Next, we’ll parse an argument list, which is a set of parentheses containing integers and parameter
names.

22. We’ll need code to transform that into an array of the Ruby objects we’ve already defined.

23. baz(42, foo)
Now, we’re finally ready to parse a function call...

24. ...and transform it into a Ruby object that will be able to emit bytecode.

25. • Full Example

26. 3. Conditionals
The next big piece of our compiler is the conditional expression.

27. if (0) {
42
} else {
667
}
Here’s the kind of conditional statement we’d like to be able to parse. The “else” clause is
mandatory (I hope Zed Shaw will approve).

28. Paradoxically, the more complicated the expression we parse, the more our tree of custom Ruby
classes resembles the Ruby hash we started with.

29. • Full Example

30. 4. Function Definitions
The last piece we need for our factorial Thnad program is the ability to compile function definitions.

31. function foo(x) {
5
}
A function definition is the keyword “function” followed by a list of parameters in parentheses, then
a computation between braces.

32. And here’s the resulting tree of Ruby objects.

33. • Full Example

34. Marc-André Cournoyer
Marc-André Cournoyer’s book on creating your own programming language was helpful during this
process as a way of “checking my answers” (just like checking your answers to today’s crossword
puzzle by looking in tomorrow’s paper).

35. Code
https://github.com/undees/thnad
The full compiler, complete with bytecode generation, is available here.

36. For more about JRuby (which made this whole exercise possible in half an hour), see our book at
http://pragprog.com/titles/jruby.