Re: FYI: default %printer/%destructor

[Hans Aberg]

On 23 Nov 2006, at 00:13, Paul Eggert wrote:

> Hans Aberg <address@hidden> writes:
>
> > This can become cumbersome when writing:
>
> OK, thanks, I now understand the problem better: the nonterminal names
> can be quite long, and you want a shorter name in the action (which is
> typically small, so it's ok to have short local names).  In that case,
> the syntax could have an identifier after the '#', as you suggested,
> and this identifier would supersede the nonterminal's identifier
> within the action.

One is really identifying a parser stack location, from which  
semantic value, location, and possibly more is extracted, like  
perhaps token names (for, among other things, error messages) and  
token values (needed when implementing language definitions). This is  
why I am thinking in terms of OO like a 'struct' or 'class'.

One might extend the # notation to indicate actions. Then one might  
simplify
  expression:
      INTEGRAL_NUMBER#x                 { $$ = $x; }
    | '-' expression#x %prec NEGATION   { $$ = -$x; }
    | '(' expression#x ')'              { $$ = $x; }
    | expression#x '+' expression#y     { $$ = $x + $y; }
    | expression#x '-' expression#y     { $$ = $x - $y; }
    | expression#x '*' expression#y     { $$ = $x * $y; }
    | expression#x '/' expression#y     { $$ = $x / $y; }
    | expression#x '^' expression#y     { $$ = pow($x, $y); }
  ;
to
  expression:
      INTEGRAL_NUMBER#x                 ##identity
    | '-' expression#x %prec NEGATION   ##neg
    | '(' expression#x ')'              ##identity
    | expression#x '+' expression#y     ##add
    | expression#x '-' expression#y     ##sub
    | expression#x '*' expression#y     ##mul
    | expression#x '/' expression#y     ##div
    | expression#x '^' expression#y     ##pow
  ;

identity { $$ = $x; }
neg { $$ = -$x; }
add { $$ = $x + $y; }
sub { $$ = $x - $y; }
mul { $$ = $x * $y; }
div { $$ = $x / $y; }
pow { $$ = pow($x, $y); }

The idea is to put the action definitions, at need, somewhere where  
they do not make the grammar rules hard to read. In addition, action  
definitions used more than once, like "identity" above, need not be  
repeated in the grammar.

One must use a double hash "##", or another token than "#", as
  a # b
is ambiguous: it can mean either a variable with a parser stack  
(semantic) variable, or a variable with nothing followed by a named  
action.

  Hans Aberg