Crafting Interpreters

I’ve spent a fair amount of time over the last three months following through the second part of Robert Nystrom’s fantastic Crafting Interpreters free online book, consisting of a complete step-by-step guide to how to “craft” a complete and fully-functional interpreter language virtual machine.

I made an attempt to start two years ago on the tree-walk version, but didn’t have the motivation at the time to complete it (I think the fact I knew the bytecode VM version would be better and more useful/interesting subconsciously made me not bother as much given the bytecode VM version’s chapters were still being written), but I realised just before Christmas that the book had been complete for a few months now, so made a bit of an effort to find time to complete it by writing my version based off the book, as I had a use-case for scripting language embedding in some of my apps that I thought having my own language/VM for could be quite useful/interesting.

I’ve enjoyed the process of following along, learning and writing my own version immensely, and I now have a version - called “Mint” - which is heavily based off the book, but with some subtle language syntax changes and written in C++ instead of C (not sure yet whether that was the best idea in the end, although I hope having a namespace might be useful for embedding in other codebases), and has several additional features over the stock book version of ‘Lox’, for example:

• Built-in dictionary/map variable type support for key/value lookups and storage
• Support for compound assignment operators (+=, *=, etc)
• Array/list slicing / assignment support
• break and continue loop-control syntax support
• Support for multi-line comments (C/C++-style) and both “#” and “//” single-line comments
• Ternary ? operator syntax support.

Example code listings, demonstrating fizzbuzz:

// fizzbuzz

for (var i = 1; i <= 100; i += 1)
{
	if (i % 15 == 0)
		print "fizzbuzz";
	else if (i % 3 == 0)
		print "fizz";
	else if (i % 5 == 0)
		print "buzz";
	else
		print i;
}

Calculating the approximate value of PI:

# calculate PI with the Leibniz algorithm...

def calculatePI(iterations)
{
	var x = 1.0;
	var pi = 1.0;
	
	for (var i = 2; i < iterations + 2; i += 1)
	{
		x *= -1.0;
		pi += x / (2.0 * i - 1.0);
	}
	
	pi *= 4.0;
	return pi;
}

var piVal = calculatePI(40000000);
print "PI: " + toStr(piVal);

producing:

PI: 3.14159266192262

and a fully-functional Mint Class, which can convert Roman numeral strings to numbers and vice versa, downloadable here

I’m going to continue developing it in the background, in order to attempt to integrate it into several of my applications, and also try and optimise its performance a bit and see how it compares to other interpreter languages.