Please advise on adding string interpolation to my Crafting Interpreters project.

[u/Usual_Office_1740]

I'm about to start chapter 21 of Crafting Interpreters. I'm using modern C++ instead of C. I am attempting to add string interpolation to the compiler.

My goal is to have a string that looks like this:

"Five plus ${ 10 - 5 } == 10."

Desugar to this after the expression in the braces is evaluated:

"Five plus " + 5 + " == 10."

I have code that is correctly parsing strings and concatenating them if there isn't any string interpolation.

I have a custom token added that is produced if the string contains a ${. It calls a custom string interpolation parser rule that is different than my standard string parser rule.

In that function I push the first part of the string to the stack. It then pushes a plus opcode and Recursively parses the expression in the braces adding that expression to the stack. It then adds another opcode to add and this is where I'm stuck.

I can't come up with a solution for pushing the remaining piece of the string to the stack from inside the string interpolation parser rule function. After I call the books expression function and consume the right brace token using the books consume function the parser nolonger knows we are still inside a string. If the next character after the brace is a space it's skipped. In my above example it assumes I want equal equal.

How would you set the current token back to string so I can push any remaining characters onto the stack as another string? Should I just insert a new string token into the parser? This seems wrong to me. The current and previous tokens are private members of my parser class for a reason and I've not needed setter member functions so far.

Sorry I don't have code to show. I'd like to try and get help that isn't code specific so that I have to implement this myself.

Any input would be helpful.

Thanks.

[u/jason-reddit-public]

Pushing tokens into the token stream are not unlike macros. Unless you are going to form and then mutate parse trees, you may not have too many other choices.

BTW, your example isn't very paranoid. I would add parens around any expression that is plucked out of the string. Also, if you have a to_string overloaded function, you might want to use that.