Crafting Interpreters - Part 13: Inheritance

This post is based on my notes from following the Crafting Interpreters book. The complete code and notes are available in this repository.

This one came with some ups and downs but those were all caused by "skill issue". Trying to understand why super wasn't defined until I realized that my class didn't even have a superclass. Then it was crashing when calling get_at, which I thought was when trying to get super so I went to debug if super was being defined and it was. Turns out it was when I called get_at to get this because I was giving it the wrong distance (forgot to subtract 1). Other than my skill issue while testing things, it was overall not hard to implement this.

EDIT: While doing this part I removed the call to the bind method that sets the value of this where the init method is called when making a new instance of a class. This made the REPL crash if I attempted to set the value of some property.

Challenges

  1. So I'm not feeling courageous as my master thesis is actually close to this challenge. The feature I would like to add is multiple inheritance, as I've been looking into this because of my master thesis. However, I don't feel confident in how this should be implemented as find_method would have to be revisited and a method resolution order would have to be put in place. This is how Python does it but I don't think it is intuitive when we are dealing with a lot of classes.

So instead I tried to make some kind of extension methods, which came out a little bit limited:

  • I didn't want to define a new syntax or tokens so I reused the syntax for setting the value of a property
  • The only validation is checking if we are setting it to a callable object, which can be a class or a function
  • If an instance was created before the extension method then it won't have that method

The code for this challenge can be found here.

  1. For this I knew I would have to change the way find_method works to get all of the possible methods and bind the value of both this and inner in each of them. I think that park should be fine, but my implementation was left with some flaws. To make it a function that doesn't do anything, I made it so it's set to a nativefn that receives no arguments and does nothing. This means passing arguments will make it error. I'm not sure what else wasn't properly done but the example in the book is working.

The code for this challenge can be found here.

  1. So going back to the features I thought were missing:

On features that I think are missing I would say the biggest one is arrays, had to make a class for having a list. Another would be anonymous functions, since function declarations aren't expressions. And functional programming functions such as map, reduce, and filter would be nice to have.

Anonymous functions were implemented in a previous challenge so I went with making a list type. I haven't made the functions map, reduce or filter. I haven't made any way of getting a value at a certain index yet, but I'm considering adding these features later on. It only supports being printed out (showing all elements), pushing to the end and popping the last element. I know this is just a stack but I wanted very basic features for now.

EDIT: Lists work as functions and can be called with an index to get the value at that index.

The code for this challenge can be found here.