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The Beginnings of an Interpreter

2018-05-16

One of the projects I’ve always wanted to work on has been developing my own programming language. From a practical perspective at this point it serves of little value besides the academic exercise of how it’s actually accomplished, but for me that is more then enough. I suspect that in exploring different parsing and execution techniques I’ll discover useful algorithms for use in other areas of development.

To start off I’ve decided after some reading to build a simple interpreter, using the algorithms and techniques that I’ve found in the book “Writing an Interpreter in Go” and some other sources on the internet. I’ve also decided that the first step is to of course construct one that I can throw away, and so, to start, I’m basically going to build a very simple interpreter to explore how some techniques can be accomplished. After this I’ll do some further research and analysis and build something else better - hopefully.

To start off with, I’m going to build the follow features into the first take:

Even this is a fairly significant undertaking, however with the sources available on the internet and the “Writing an Interpreter in Go” book I’ve started construction in C#. Currently I’ve gotten the beginnings of the scanner and parser working, and I’m able to lex and parse basic math expressions with the correct operator associativity. The parsing algorithm that I’m using is called Pratt Parsing, or, Top-Down Operator Precedence Parsing, which is an elegant recursive algorithm that builds up the abstract syntax tree.

Before tacking on too much more to the parsing I’m going to code some of the evaluation part in, which will basically give me a calculator. Then I plan on expanding the lexer and parser to further support some of the language constructs such as functions and classes so that I can develop and work with the techniques and algorithms necessary to accomplish this and complete the evaluation part. The evaluation algorithm that I plan on using for this first phase will be to perform tree-walking, which again is a relatively simple way to interpret the abstract syntax tree generated from the parsing.

Upon completion of this “simple” interpreter I plan on doing a fair bit more research. There is a lot to learn with regards to building a high-performance interpreter, or compiler, and before diving into the second one, I’d like to have completed further research into varying techniques and their benefits or pitfalls.

Writing an interpreter is certainly an interesting project, and more algorithmic than most side-projects that I’ve undertaken before. While it’s unlikely that whatever the language I ultimately put together would find practical use, I think constructing something like this provides a good educational project and is fun at the same time.

As a follow up to this I plan an in-depth article on Pratt Parsing to go over just how the abstract syntax tree is built, and how it actually handles the operator associativity. I basically understand how it works, but a post on it to help others will help it sink deeper into my brain as well.

If you’re interested in writing an interpreter or finding out more about Pratt Parsing, the following resources will probably be of assistance:

If you’re interested in following the development of the first interpreter I’m working on, you can find the code here. It’s a slightly modified version of the monkey interpreter built in the book “Writing an Interpreter in Go” by Thorsten Ball.