Why Go Rocks for Building a Lua Interpreter

Summary

I recently needed to build a custom Lua interpreter in Go. The exact reasons aren’t important for this blog post, but neither the reference implementation — which I will be referring to as “C Lua” throughout this article — nor the other open source Go Lua intepreters I could find were a good fit for my needs. Building a Lua interpreter ended up being a rather enjoyable months-long side quest. I’ve had a number of folks ask me to write about the experience since these sorts of projects usually highlight interesting aspects of both the implementation language and the interpreted language. So here we are! What is Lua? First, let’s talk a little bit about the Lua language from the perspective of an implementer. Lua is a dynamically typed language, so any variable can hold any value. Values in Lua can be one of a handful of types: nil, the singleton null value. Booleans: either true or false. Numbers, which are generally 64-bit IEEE 754 floating-point numbers. Lua 5.2 introduced transparent representation of 64-bit signed integers, so if a numeric literal does not contain a fractional part nor an exponent and fits in a 64-bit signed integer (like -345 or 0x100000000), then it will be stored as an integer. Strings, which are immutable sequences of bytes. By convention, strings are UTF-8 encoded. Userdata, which are implementation-defined values. Tables, which are maps of values to other values. Lua raises an error if nil or NaN (the IEEE 754 sentinel value for undefined numeric results) are used as keys. Furthermore, table entries cannot use nil as a value: assigning a nil value to an entry removes it from the table. Functions. Functions in Lua can take any number of arguments and can return any number of values. They may be closures, which means they have references to variables from their containing function. Lua calls such references “upvalues”. The reference manual describes the syntax of the Lua language in depth, but most of these details don’t matter at a high leve...