# 07 Sep 2016

In this series of posts, I’ll try to explain how one can write an interpreter in Go and for Go. If, like me, you lack a bit in terms of interpreters know-how, you should be in for a treat.

## Introduction

Go is starting to get traction in the science and data science communities. And, why not? Go is fast to compile and run, is statically typed and thus presents a nice “edit/compile/run” development cycle. Moreover, a program written in Go is easily deployable and cross-compilable on a variety of machines and operating systems.

Go is also starting to have the foundation libraries for scientific work:

And the data science community is bootstrapping itself around the gopherds community (slack channel: #data-science).

For data science, a central tool and workflow is the Jupyter and its notebook. The Jupyter notebook provides a nice “REPL”-based workflow and the ability to share algorithms, plots and results. The REPL (Read-Eval-Print-Loop) allows people to engage fast exploratory work of someone’s data, quickly iterating over various algorithms or different ways to interpret data. For this kind of work, an interactive interpreter is paramount.

But Go is compiled and even if the compilation is lightning fast, a true interpreter is needed to integrate well with a REPL-based workflow.

The go-interpreter project (also available on Slack: #go-interpreter) is starting to work on that: implement a Go interpreter, in Go and for Go. The first step is to design a bit this beast: here.

Before going there, let’s do a little detour: writing a (toy) interpreter in Go for Python. Why? you ask… Well, there is a very nice article in the AOSA series: A Python interpreter written in Python. I will use it as a guide to gain a bit of knowledge in writing interpreters.

## PyGo: A (toy) Python interpreter

In the following, I’ll show how one can write a toy Python interpreter in Go. But first, let me define exactly what pygo will do. pygo won’t lex, parse nor compile Python code.

No. pygo will take directly the already compiled bytecode, produced with a python3 program, and then interpret the bytecode instructions:

shell> python3 -m compileall -l my-file.py
shell> pygo ./__pycache__/my-file.cpython-35.pyc


pygo will be a simple bytecode interpreter, with a main loop fetching bytecode instructions and then executing them. In pseudo Go code:

func run(instructions []instruction) {
for _, instruction := range instructions {
switch inst := instruction.(type) {
// perform a+b
case opPRINT:
// print values
// ...
}
}
}


pygo will export a few types to implement such an interpreter:

• a virtual machine pygo.VM that will hold the call stack of frames and manage the execution of instructions inside the context of these frames,
• a pygo.Frame type to hold informations about the stack (globals, locals, functions’ code, …),
• a pygo.Block type to handle the control flow (if, else, return, continue, etc…),
• a pygo.Instruction type to model opcodes (ADD, LOAD_FAST, PRINT, …) and their arguments (if any).

Ok. That’s enough for today. Stay tuned…

In the meantime, I recommend reading the AOSA article.