losos's Introduction

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.

losos's People

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losos's Issues

Add documentation

Add documentation, maybe references to corresponding book chapters.
https://craftinginterpreters.com/contents.html

Remove sys.exit() calls from losos.py

There are two places where Losos.run_file() method will call sys.exit().

Control should be passed back to main() function which in turn should return exit status code.

Only single place where sys.exit() can be used is __main__.py file, around main() function call.

Complete differences section in readme

Complete differences section in readme.

scanner.py: Decide what to do with char type

Near comments in form: # TODO CHAR
In files: scanner.py, helpers.py

Python do not have separate type for single character. In original Java implementation, some methods are expecting/returning char type. Goal for Losos 1.x is to be as close to original implementation as possible.

Creating char type is not for blindly keeping implementations similar, but can allow mypy for catching more bugs.

Following class enables mypy to check if we pass around actual character (empty string as null char):

class Char(str):
    def __new__(cls, s: str) -> 'Char':
        if len(s) > 1:
            raise ValueError("Only one character or empty string")
        return super(Char, cls).__new__(cls, s)

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acinis / losos Goto Github PK

losos's Introduction

Losos

Goals

Differences

losos's People

Contributors

Watchers

losos's Issues

Add documentation

Remove sys.exit() calls from losos.py

Complete differences section in readme

scanner.py: Decide what to do with char type

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