How to Write a Simple Interpreter in JavaScript | Hacker News Reader

How to Write a Simple Interpreter in JavaScript

codeproject.com

84 pointsjim_lawless15 comments

Nice article - covers pretty much everything in a reasonable amount of detail.

As for the author, it should be noted that he's a 17 year old who has been programming for 2 years. Good job - we need more people like that!

I'm a bit disappointed, that the author didn't built a turing-complete interpreter. Frankly I've seen several of such articles which tell you how to build this kind of deterministic evaluator, but they all seem to stop before loops and branching.

And then there are articles about building compilers which do support loops etc, but the trick is that these constructs are implemented by some other already existing machine.

So I feel like there's some gap in my education. Pointers, anybody?

I'm not sure I'd be so quick to say it's not turing complete--the language includes building functions (not first-class functions though), but it might be possible that they can be used to emulate conditionals and branching.

Anyway, I'm pretty sure it wouldn't be that hard to extend the language to add some looping and branching constructs to the language, there's nothing really special there to add, it's just another simple type of construct to evaluate.

In my understanding, loops are implemented at the lowest level with jump instructions, but I've never really done much low-lever programming so maybe someone else can explain that better.

There is no conditional, and without a conditional you can't get a conditional.

However modifying it to support conditionals is easy. Just add a built-in function if(condition, expr1, expr2). Now you can build if's. Since the language already properly supports recursive function calls, now it is possible to implement loops through recursion, and you're off and running.

If you have function application, then you don't truly need a primitive conditional. You can represent Boolean values and conditional functions with the Church encoding [1]. This is how conditionals are represented in, for instance, the lambda calculus.

[1] http://en.wikipedia.org/wiki/Church_encoding#Church_booleans

Good point. But you need to have first class functions for that. This toy language does not. And converting from "this native number equals that native number" to your encoding of "true" is going to be an interesting trick.

There is a slight snag that arguments to a function are eagerly evaluated. A built-in if function would be more useful as a special form.

> There is no conditional, and without a conditional you can't get a conditional.

A two element array of function pointers (or code addresses) and an operator that turns true/false into 0/1 is enough to do conditionals.

All loops and branching logic has to ultimately come down to machine language instructions that your computer chip interprets as conditional, if this do that.

To get an example of what can be done at the lowest level, see http://zsmith.co/intel.html for the instructions that a Pentium chip accepts. In particular look at http://zsmith.co/intel_j.html and http://zsmith.co/intel_l.html#loop.

A classic, "Let's Build a Compiler," by Jack Crenshaw:

http://compilers.iecc.com/crenshaw/

edit: BTW, https://www.coursera.org/course/compilers just started...

This is interesting, but I feel as though the author shouldn't be instructing one on writing a parser by hand, because this is highly impractical and there are parser generators out there that target JavaScript. One's time is better spent studying the abstract math behind how parsing works. Context free grammars are complex, and it's far too easy to do something wrong.

That being said, writing an interpreter is fun and anyone with even a mild interest in parsing or compilers should do it.

This article made me want to try making a compiler or interpreter when I get a block of free time. I actually stopped reading because I want to try to figure out as much as I can on my own.

It's a very fun thing to do! Especially if you construct your own virtual machine--function calls aren't as simple as they appear. Also fun is targeting the Java Virtual Machine using a bytecode library such as ASM.

I personally enjoy using ANTLR for parsing, so that might give you a place to start.

If you want to see an interesting way to parse using Javascript check out Douglas Crockfords Top Down Operator Precedence paper here:

http://javascript.crockford.com/tdop/tdop.html

I think the parsing method in this article was borrowed from Crockford's TDOP article, except it parses a simpler language than Javascript.

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