30th October
Evaluating Symbols
Took a break once the parser was finished, getting back to it. Today was the first day I made some progress since finishing the parser.
Made a couple of small incremental additions to be able to evaluate booleans, integers,
and null.
Tree Walking Interpreter
You can read about the other approaches in the book. Tree walking interepreter is the easiest to implement & for the sake of learning that's the one we are going with.
A tree walking interpreter takes the AST from the parser and intereprets it "on the fly", without preprocessing or compliation.
Object Representation
We are representing all values as objects that we define. There will be no native types. Again, as it's easier and performance is not really a concern right now.
With that, we implement the foundation for our object system. We define Object, Integer,
Boolean and Null.
Garbage Collection
The book does not implement any garbage collection but I would like to know how it works. So once I'm finished with the interpreter I will extend the interpreter to support garbage collection.
Evaluating Expressions
With the data types defined, we can evaluate literal expressions and map them to their representation in the monkey programming language. This is quite easy and straightforward.
Evaluating Prefix Expressions
Once the above is complete, we can then move on to evaluating prefix expressions ! & -.
This is where I'm starting to see how minute decisions regarding the behavior of a prgramming
language are made. For example, in the monkey programming language !5 equates to false.
As we are defining 5 to be a truthy value, where this would throw an error in golang.
For evaluating prefix expressions, we first evaluate the right. This means we can
recursively evaluate expressions like !!true. Once, the right is evaluated, based
on the operator ! or -, the appropriate object is returned. If it's a ! operator
a boolean is returned, whereas for - operator we verify if the operand is an integer
and return the negated integer.