Re: Newbie Dynamic Data Typing Help

[Hans Aberg]

On 5 Oct 2005, at 18:54, Gill,Michael J wrote:

> I'm a complete newbie to bison and language semantics in general.  I'm
> trying to extend the mfcalc.y example from the manual to allow
> dynamically typed variables but am at a loss as to how to implement
> them.  I'm currently saving var names and values in the symbol.c  
> linked
> list but with double vals only, I want to add char and char* types as
> dynamic types.  Any direction/help would be apprectiated.

If you write in C++, you can make a polymorphic class hierarchy,  
i.e., a root class object, and classes derived from a that. As a  
parser semantic type, one would use another class, maintaining a  
pointer object*, which may be combined with a reference count, in  
order to avoid unnecessary copying.

Now, if you program in C, you just translate this picture; vice  
versa, the C++ constructs were developed in order to automate the  
corresponding C constructs. C requires more programming by hand, but  
is easier to make optimized work in. So in C, you might have a class  
(in pseudocode)
  enum type { DOUBLE, STRING, OTHER, ... };

  struct data {
    type type_;
    union {
      double double_;
      char* string_;
      void* other_;
      ...
    }:
  };

#define YYSTYPE data

Then use the type_ value to extract the right kind of data, making  
sure that (de-)allocations takes place correctly, etc.

If you only need statically typed variables that can hold different  
values, then use the Bison feature %union, and instead of the type  
enum, use Bison statc type system to select the right union field.  
Use %destructor to cleanup during error recovery.

  Hans Aberg