Plox

Plox is an interpreter for the Lox programming language described in the book Crafting Interpreters by Robert Nystrom.

Features

The program is a relatively direct translation of the tree-walk interpreter developed in the first part of the book from Java to Python, with a number of extensions:

Standard Library

We introduce a standard library that defines a number of functions and classes that are added to the global scope before executing user programs. Part of the standard library is implemented in Python, and part is implemented directly in Lox. The standard library includes functions for conversion between strings and numbers, a number of mathematical functions such as floor or trigonometric functions, the input function for prompting user input, as well as lists, linked lists, and ranges (see below).

List Syntax

As a part of the of the standard library, we add a List class implemented using Python's list. As a more compact way to create lists, we add list syntax similar to Python: var myList = [1, 2, 3].

Foreach Loops and Iterators

We add foreach loops to the language that, similar to for loops, are converted to while loops upon parsing. The newly introduced syntax

foreach(i; list)
{
    print i;
}

is equivalent to:

var it = list.iterate();
while(it.hasItems())
{
    var i = it.get();
    print i;
    it.move();
}

That is, the object list must have a method iterate that returns an iterator, which is an object implementing methods called hasItems, get, and move. The standard library implements iterators for List, LinkedList and Range. The latter is a lazy iterable over an evenly spaced list of integers, similar to Python's range.

Usage

Plox has no dependencies beyond the Python standard library.

Run in interactive mode:

python -m plox.run

Run a Lox source file:

python -m plox.run example.lox