Table of Contents

1. Effigy
   1. How to play with it
      1. Currently Supported Types of Values
      2. Language Features
      3. Very useful things missing
   2. Host Language
   3. Resources
      1. On Parsing & Parsing Expression Grammars
      2. On the Python Compiler & Bytecode Format

Effigy

This is an experiment on building a small language compiler on top of a home brewed parsing expression grammar implementation.

The language implemented in this project, effigy, currently compiles down to a subset of the Python 3.7 bytecode format. More specifically, the Effigy compiler produces .pyc files.

Effigy's runtime is the Python 3.7 Virtual Machine. The difference is just how the bytecode gets generated. Most idioms like declaring literals, calling functions, assigning variables etc have the exact same semantics as in regular Python code.

Effigy differs from Python on the use of functions for control flow a little more often and the absence of classes (might be added later).

More to come!

How to play with it

Effigy is currently a teeny little JavaScript program. You can install it with npm i efgc.

Here's what's available and some of what's not:

Currently Supported Types of Values

• integers
• strings
• lists
• functions

Language Features

• [X] Arithmetic Operators
• [X] Logic Operators
• [X] Comparison Operators
• [-] Flow Control (if/else/while/for)
• [-] Exceptions
• [ ] Imports

Very useful things missing

• [ ] Slice notation
• [ ] Variadic arguments
• [ ] Named/Default parameters
• [-] floating points

Host Language

Although the first target of the little compiler is a subset of Python, JavaScript was chosen as the host language for a few reasons:

1. I didn't want to do it in Python because it'd be very tempting to use one of its modules for parsing, scope analysis or code generation. I wanted to implement all the pieces of the compiler to be able to reason how far I could leverage the PEG to do those tasks.

2. Python and JavaScript have very similar semantics for closures but present slight differences in how side-effect (assignment) of values declared in enclosed scopes work. Java Script separates assignment from declaration, Python provides the nonlocal keyword.

   I wanted something right in the middle for Effigy: Assignment is coupled to declaring a variable, but provides the keyword let to mark names to be saved as closures so assignments in deeper scopes will know its not a new value.

3. It doesn't really matter. The goal is to rewrite Effigy with Effigy.

Resources

On Parsing & Parsing Expression Grammars

• Parsing Expression Grammars: A Recognition-Based Syntactic Foundation
• Parsing Expression Grammars for Structured Data
• PEG-based transformer provides front-, middle and back-end stages in a simple compiler
• Modular Semantic Actions

On the Python Compiler & Bytecode Format

• https://devguide.python.org/compiler
• https://github.com/python/cpython/tree/master/Python
• https://codewords.recurse.com/issues/seven/dragon-taming-with-tailbiter-a-bytecode-compiler