Symbiotic Languages: Transpiling into JavaScriptΒΆ

Authors:Jeremy Ashkenas
Time:9:30 am - 10:00 am

“I like to think of this as the place where the back wood medicine folk of programming languages get to come out of the woodwork and talk about what they’re doing.”

Only a few people here since the first emerging languages camp, which was in a cramped room in the first floor of OSCON. What made it great was discussion, people asking questions. So do that.

Here today to talk about Symbiotic Language: languages that “compile” to the source code of an existing language. These are interesting because writing VMs is really, really hard, and we’ve developed some strong VMs that might be 15-20 years old, but they’re still useful. HotSpot/Java, V8/Dart. Using an existing VM means you get to pick some of the properties of the host language you think are useful, and cherry pick those, and leave behind the ones you don’t want.

There’s a spectrum of options between compiling and *trans*piling. Transpiling means just translating some language to the source (or occassionaly the byte code) of the target, host language. Retains a lot of the semantics of the host language, since you’re working primarily with keywords and tokens. Once you start to change the semantics (i.e., by implementing “special functions” in your language that the new language calls into to implement). CoffeeScript tries not to add these “features”, because they want to stay close to the JavaScript semantics.

Charlie Nutter, who works on JRuby, has explored this: he implemented 99.5% of the Ruby semantics on Java. He’s also tried the other approach: implementing Java semantics with Ruby code.

CoffeeScript maintains a wiki of different languages that can be compiled into JavaScript. It’s a long list!

Jeremy’s experience is primarily around CoffeeScript: “It’s Just JavaScript”. JavaScript has proven remarkably versatile and robust for something designed in about ten days. CoffeeScript transpiles to that, and has come a long way since it was first developed during the first Emerging Langauges Camp [shows graph of StackOverflow vs GitHub from RedMonk; CoffeeScript has grown to something like #11 in two years].

So the choice is what you want to preserve vs deviate. One thing they can’t do is negative array indices: you’d need to inject some function into the transpiled source code to support it, since you don’t have nearly enough information at compile time.

One thing they can do is “everything is an expression”. Lots of existing Javascript is expressions, but there are also control flow objects that aren’t in plain JavaScript. CoffeeScript enables this by function wrapping: shows examples of using a complex if, a try/catch, and a loop as an expression.

The other semantic change CoffeeScript makes is the addition of classes, which CoffeeScript transpiles into the appropriate prototype declaration. You can also do interesting things like executable class bodies, which allows you to do interesting things like change the body of a class based on some value. [shows example of a Pirate class that speaks in English if century > 1700, otherwise Spanish]

The politics of programming languages are such that even if you build something interesting, there are significant barriers to adoption: “I have to use the JVM”, “It doesn’t work on the web”, etc. By transpiling you give people a shim to start playing with your new language. CoffeeScript is a really interesting case study of this: it’s come very far in two years without corporate support or backers.

It used to be that you learned a new language to program on a new platform. With symbiotic languages, we’re building the same sort of systems we were already building, but hopefully doing so in a more expressive, clean, powerful way. This means our code has two audiences: other programmers, and other programmers in the target language [I think I got this right] – and by extension, the target platform compiler.


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