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:

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- integers
- strings
- lists
- functions

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

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- [ ] 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

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- 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

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