loxrs_treewalk

loxrs tree-walk interpreter

Done: Ch.12 Classes

Note

I refactored a bit, but still this is a terrible implementation.

It would be easier to just slice an input string and that's actually done in the Part 3 of the book.

Example

Runnning a File

You can run a program such as this:

class Vec2 {
    fn init(x, y) {
        @.x = x;
        @.y = y;
    }

    fn add(another) {
        return Vec2(@.x + another.x, @.y + another.y);
    }
}

var x = Vec2(1, 2);
var y = Vec2(3, 4);
print x.add(y);

Some keywords have different names from the original.

Let's run it:

$ cargo run -- examples/for_readme.lox
(instance (class Vec2) ((x 4), (y 6)))

Debug output of AST

When -d or --debug is specified, you get debug output, which contains a pretty-printed AST:

$ cargo run -- examples/for_readme.lox --debug | sed -n '/^===== AST/,/^$/p'
===== AST =====
0 (class Vec2
    (defn init (x, y)
        (eval (set @ x x))
        (eval (set @ y y)))
    (defn add (another)
        (return (Vec2 ((+ (get x @) (get x another)), (+ (get y @) (get y another)))))))
1 (var x (Vec2 (1, 2)))
2 (var y (Vec2 (3, 4)))
3 (print ((get add x) (y)))

REPL

Of cource we also have a read–eval–print loop:

$ cargo run
Entered loxrs REPL (press q<Enter> or Ctrl-c to quit)
> var x = 3;
> print x;
3
>

Notes

Dependent crates

• itertools to multipeek
• anyhow and thiserror for CLI error handling

Differences from the original Lox

• implementation
  • return is dealt as Ok(Some(LoxObj)), not as an exception
• design
  • variable declaration requires initial value expression
  • while without parentheses

Skipped

• there's no for statement (maybe make range-based one instead later)
• can't return from a constructor

TODO

I won't do but these are left:

• Challenges
• Ch. 13 (inheritance)
• Better error context (source position tracking and error chain)
• Add += -= etc.