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8 March 32011 / Robin Wellner

How I implemented Déjà Vu in two days

I’ve been thinking a lot about Déjà Vu (the programming language I invented) lately.

A while ago, I’ve written a post about it. I couldn’t quite get it to work. After much distress and failed attempts to fix things, I let it rest.

Once in a while I revisited it. For example, when I (re)discovered Parrot, it seemed like a good idea to implement it that way. The problem was that you pretty much need to do each part in one of Perl, some variant of Perl, or Parrot assembly. Perl is horribly opaque and I never learned it, which was problematic. I’m a high-level thinker, so assembly works against me. But the worst of all is that writing grammars and parsers and whatnot is just plain hard. No, scratch that: the absolute worst is the combination of those things. Learning a language you’ll probably never like is much harder if your equivalent to a “hello world” is a top-down grammar parser.

So I let that rest as well.

But I kept thinking about Déjà Vu. Not on how to implement it — I’m a high-level thinker, remember? No, I was thinking about concepts, about constructs, about connotations…

All ideas were kept in a file called “idea”. The language grew and matured, even though it had no implementation. I added new kinds of statements, full lexical scoping and new datatypes, and improved the function definition syntax.

This spring break, I decided to try it one more time last Sunday. Following the success of implementing a tree parser for Lith, (by the way, the answer to the question posed in that post, “Will I ever implement more?”, is “No, I will not.”) I decided to start simple: read the file line by line, perform a series of simple text transformations to deal with indentation, strings, comments, words and statements (it had to be in that order — I did spend some time thinking about implementation). After the transformations, produce a tree. I’d see what would happen next if I finished it.

I finished it within a day. I was shocked. Sure, Déjà Vu is not a large language — I kept the amount of syntax and the number of statements purposefully small — but I had so much problems with it before, and now it worked! In a day! It had everything. It had full lexical scoping! It took Lua 8 years to get that, and this implementation of Déjà Vu had it within a day! All I had to do now is find some way to deal with that parsed tree.

Let’s see. I had a tree filled with conveniently semantically typed nodes. Why not just traverse it, instead of struggling to convert it to byte code? (And probably back again at that.) So I started on that. When I went to bed that day, the first bits where in place for interpreting trees and what’s more: it looked like it was going to work.

The following day, Monday (it is past midnight as I’m writing this post), I implemented the interpreter. It was unbelievably easy. After figuring out some minor problems, I was even able to implement the For statement without much trouble, and I had expected that to hold up the project for a while.

It was the second day, and the language worked. It just worked. I had hastily added an interactive interpreter, modeled after Python’s. That worked too. I have written a neat standard library, implemented in Python. My idea file is over 400 lines, and I had implemented everything I proposed in it, save for those that were superseded by other ideas.

It’s all up on GitHub.

Tomorrow, I will write the much-needed documentation.

One more thing: the implementation as a whole defines 40 classes to parse, interpret, and represent Déjà Vu source. 26 of which are subclasses of the Node class alone. I think that’s the highest number of classes I’ve ever used for a single project.

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