[Haskell-beginners] Design questions for a Pascal interpreter

Peter Verswyvelen bugfact at gmail.com
Tue Apr 21 17:54:41 EDT 2009

Maybe you could get come inspiration from the BASIC interpreter written in

On Tue, Apr 21, 2009 at 2:14 AM, <ajb at spamcop.net> wrote:

> G'day all.
> Quoting Giuliano Vilela <giulianoxt at gmail.com>:
>  - 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=
> Good luck, and let us know how you go.
> Cheers,
> Andrew Bromage
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