Re: Extending Functions from Manual Example

[Hans Aberg]

On 11 Sep 2008, at 05:17, Jason Melbye wrote:

> > Then use it:
> >  FNCT '(' expr ')'
> >
> > Some prefer not using the '...' construct. So one can write:
> >
> > %token LP "("
> > %token RP ")"
> >
> >  FNCT "(" expr ")"
>
> Should those FNCT '(' expr ')' / FNCT "(" expr ")" lines say FNCT '('
> expr_list ')' (or "(", ")" ) instead?  I'm still struggling to  
> construct the
> action that calls the function as well, passing all the arguments.   
> So say I
> had this:
>
> expr:    NUM
>          | expr '+' expr {$$ = $1 + $2; }
>          ... other basic math ...
>          | FNCT '(' expr_list ')' { what goes here? }
>
> and expr_list is as you suggest above.

It depends on what you return from the lexer (typically generated  
using Flex). If the lexer says
  return '(';
use that in the grammar. If it says
  return LP;
use LP or (if defined using %token as above) "(".

> I'm not sure how to actually call the function, because I'm passing  
> it a
> variable amount of parameters - as many as make up expr_list.

You need to create a data type that can hold the expr list. Then  
write code to compute the function application. Such data structures  
are called "closures", that is, they represent code that should be  
computed later. This is necessary for doing loops, too.

So the action code should look something like in pseudocode:
expr_list:
   expr {
     $$ = list($1);
   }
 | expr_list "," expr {
     $$ = append($1, $3);
   }
;

  Hans