Run on

This blog post tells the short story of how I wrote a simple interpreter for a toy programming language!

I am excited to tell you that I just released the alpha version of my “Pydont's” book, a book that compiles all my “Pydon't” articles. You can get the book at leanpub:

A computer screen with some code in it, everything in a dark setting

Python is the programming language I am more comfortable with, by far; at an early stage of my learning I heard that Python was built on top of C. Whatever that meant. Then I started thinking of it in this way: programming languages are used to build programs. Someone used C to create another program, that can be used to create even more programs! That seemed reasonable.

At some point in time I got interested in compilers and interpreters and all those weird things, and I came across this wonderful blog post. I immediately started following the "Let's Build a Simple Interpreter" series to write my own Pascal interpreter with Python, which is the aim of that series.

At that time there were only 5 or 6 posts, and I got to the point where I had done everything he told us to do and couldn't wait for the other posts, as I noticed the frequency with which the author posted in that series was rather low. At that point, my project's aim diverged of that of the blog and I tried using what I had learned to create a very basic programming language I called Roj.

The end product of that is a programming language with some common constructs, such as the while loop and the if-else statement. The code and some snippets written in Roj can be downloaded from this GitHub repo. In there you can also find my attempt at formalising Roj's grammar with BNF notation .

To run one of the snippets, run the file, which starts a REPL. You can type Roj programs in one line or you can type execute filename where filename is the path to the file you want to execute:

A screenshot of a basic REPL session in the Roj interpreter

Please bear in mind that the REPL does not save variables and whatnot from the previous lines. If you want to type a Roj program directly into the REPL, it will have to be a one-liner.

You can also run the code online; just click the button on top of the article, hit the green Run button at the website and try pasting this Roj code in the console:

readint n; out "The square of your number is"; out n*n;

Let me know, down in the comments, what you think!

If you liked this article and would like to support the mathspp project, then you may want to buy me a slice of pizza 🍕.

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