JRuby, Not Just For Hard-Headed Pragmatists Anymore

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

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

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

Abstraction compilers xor eax, eax e- TimeIf, like me, you grew up on the hardware path, a compiler is the next logical step beyond learningprogramming languages.

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

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

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 stageThe plan for this talk is to show up tired, wear a JRubyConf t-shirt (this is like showing up at aconcert wearing the t-shirt of the band that’s playing), choose a topic and pace different from allthe other talks, and introduce an element of uncertainty by coding on stage. (If you’re reading this,I’ll show you GitHub commits instead.)

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

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 todraw the pictures on the subsequent slides.

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

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.

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.

1. IntegersOne big step is to write a compiler top to bottom that parses an integer and outputs a workingJava .class ﬁle.

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

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

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

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

2. Function CallsNow, let’s add the ability to compile function calls.

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

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

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

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

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

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

• Full Example

3. ConditionalsThe next big piece of our compiler is the conditional expression.

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

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

• Full Example

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

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

And here’s the resulting tree of Ruby objects.

• Full Example

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

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

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

JRuby, Not Just For Hard-Headed Pragmatists Anymore

by Ian Dees

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