m2-pylox

This is a port of the tree-walking interpreter for the Lox programming language featured in the Crafting Interpreters book.

This project currently uses rye for dependency and project management.

Implemented chapters and challenges

• Chapter 4
  • C-Style comments (/* ... */)
  • Nested C-Style comments (/* ... /* ... */ ... */)
• Chapter 5
  • Reverse Polish Notation (RPN) AST printer
• Chapter 6
  • Comma operator ,
  • Ternary operator ?:
  • Error productions for binary operators missing left side expression
• Chapter 7
  • Non-number comparisons. Only allowed between expressions of the same type, allowing numbers, strings and booleans.
  • Convert addition operands to strings if only one of them is already a string.
  • Handle division by zero with a runtime error.
• Chapter 8
  • Throw runtime error on uninitialized variable access.
• Chapter 9
  • break statement.
• Chapter 10
  • Anonymous (lambda) functions.
• Chapter 11
  • Error on unused local variables.
• Chapter 12
  • Class methods (class prefix before function definition)
  • Getter methods (Function name followed by block)
• Chapter 13
  • Code reuse mechanism: Traits, implementation like PHP. See additional features section for more details.

Implementation differences

There are some changes in comparison with the reference implementation provided in the book.

• No AST generator: Two of the main issues the book's AST generator deals with can be easily be solved with standard library functionality: The data and contructor definition are simply dataclasses; the accept() definition is provided by inheritance, with the visit() method selecting the implementation based on the type of the passed parameter using functools.singledispatchmethod (which can easily be replaced by a dispatching function using a match or isinstance to select the correct function).
• Unsugared for loop: The for loop in this implementation isn't syntactic sugar for a block with a while loop; instead, it is a different statement type to allow better flexibility.

Additional features

• Native functions
  • input(prompt: String): Read a line from stdin and return it as a string, showing prompt before asking for input.
  • randint(min: Number, max: Number): Pick a random number between min and max, both included.
• continue statement.
• trait and use.
  • Declare a trait using trait and providing a class body; traits cannot inherit, but can be extended by using other traits.
  • Include traits in classes or other traits via the use statement, this copies the trait's methods over to the class/trait using it.
  • An use statement can indicate one or more traits separated by commas, e.g. use TraitA, TraitB, TraitC;.