G'day all.
Quoting Giuliano Vilela
- Keeping the whole AST in memory for the evalution phase seems overkill. Is there a better way?
In this day and age, it's not considered overkill to keep an entire program in memory in a tree form. Perl 5 does that, for example. However, Pascal is simple enough that it can be translated from within the parser. Quite a few influential Pascal compilers, including the simplest ones such as Pascal-P and Pascal-S, and some not-so-simple ones such as Turbo Pascal, did not even generate an AST, but compiled straight to P-code or assembly code from within the parser.
- The evalution, I think, would be a set of nice pure mutually recursive functions that do some pattern matching on the program AST. I would pass the current stack and heap for those functions to use and modify. Is the State monad a good fit for this task? Wouldn't the code become "too imperative"?
Interpretation of an imperative language is imperative. I wouldn't worry about it. You will probably end up using a few monad transformers, because you need to need at least I/O and a heap, and quite possibly a symbol table as well.
Obviously, to evaluate writeln I need to be in the IO monad. Here, my whole scheme went down. Do I really have to mix my own state (stack, heap) within the IO monad along my evaluation functions?
You really need to learn about monad transformers. Try this for starters: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.17.268 Good luck, and let us know how you go. Cheers, Andrew Bromage