Hans,
I'm not familiar with the statc type system in Bison, how does that
work? My current vars are implemented as pointers to symrec from
the .y
file and hold only doubles in their value fields. Is it enough to
simply add the types to the union in the definition of symrec and then
set them from the grammar actions based on the return type from yylex?
How about the exp type can it still be just double? The whole type
matching of tokens to grammar rules to values is confusing me.
Thanks again,
-Mike
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