Losos

Losos is implementation of Lox language in Python 3.

Goals

• Python implementation should be as close as possible to Java version in book (see below)

• Losos 1.0 - working Python implementation of tree-walk interpreter

• Losos 1.x - extend Losos so next goals are possible

• Losos 2.0 - bytecode compiler written in Lox itself

• Use v1.x to run v2.0 to compile v2.0 source to bytecode

• Losos VM written in C++

Differences

• No tool for generating syntax tree classes (tool/GenerateAst.java).
• [expr.py] Visitor abstract class and all expression classes are not nested inside Expr: BinaryExpr instead of Expr.Binary, ExprVisitor instead of Expr.Visitor, etc.
• [parser.py] _ParseError class is not nested inside Parser class.
• [helpers.py] Whole new file with handy helpers.
• [losos.py] Losos class is instantiable and all methods are non-static.
• [losos.py] Reporter instance instead of _had_error property (see below).
• [losos.py] main responsibility is to parse args and call run_* methods, so I made them public and moved main to __main__.py.
• [losos.py] run_prompt method prints welcome message and will exit after CTRL+Z followed by RETURN.
• [losos.py] error, _report, runtimeError, etc moved to Reporter (see below).
• [reporter.py] In book, there was hadError static boolean flag inside top-level Lox class. We have instantiable Losos class, and we just cannot call static error method from it. This is minimal error reporter class. It's instance will be passed around, so classes used inside Losos (like Parser and Scanner) can report errors back. Later it may be turned into interface for swapping different error-reporting implementations. For now it's just a way to pass around basic information about encountered errors (just like mentioned flag in book).
• Java's char type: I added class Char (in helpers.py) for type-hinting single character (see #2 for reasoning).
• [runtimeerror.py] RuntimeError in Losos is named LososRuntimeError to avoid name collision with Python's built-in exception.
• Possibly other minor differences.