Re: Extending Functions from Manual Example

[Hans Aberg]

On 10 Sep 2008, at 05:39, Jason Melbye wrote:
> I have been reading through the Bison manual the past couple days.   
> I really
> liked the three examples, especially the last - the multifunction
> calculator.  Since reading that I have been wondering if there was  
> a general
> way to describe a function that can take on any number of arguments  
> in the
> grammer rules?
>
> I suppose that I could just do something like,
>
> | FNCT '(' exp ')'   { $$ = (*($1->value.fnctptr))($3); }
> | FNCT '(' exp ',' exp ')'   { $$ = (*($1->value.fnctptr))($3,$5); }
> | FNCT '(' exp ',' exp ',' exp ')'   ...
>
>
> I could go on like that to all for as many arguments as I wanted  
> to / needed
> to.  However, I would also like to create a "plug-in" system where  
> users can
> create their own functions and add them to the application.  Thus a  
> avoiding
> a limit on arguments would be desirable.

You need to create recursive rules. If lists are non-empty, write:

expr_list:
    expr {
      ...
    }
  | expr_list "," expr {
      ...
    }
;

If empty lists are admitted, simply delete the first "expr" in the  
above.

Then use it:
  FNCT '(' expr ')'

Some prefer not using the '...' construct. So one can write:

%token LP "("
%token RP ")"

 FNCT "(" expr ")"

In this variation, LP and RP are macro names that he lexer d
should return, and "(" and ")" are just symbols used in the grammar  
code to make it readable, and in parser error messages.

  Hans