Re: FYI: default %printer/%destructor

[Hans Aberg]

On 23 Nov 2006, at 09:47, Paul Eggert wrote:

> > Maybe it's just me, but I prefer Hans' suggestion:
> >
> >   exp/sum: exp/term1 '+' exp/term2
> >
> > over
> >
> >   exp#sum: exp#term1 '+' exp#term2
> >
> > I can't think of any semantic reason to prefer one over the  
> > other.  The
> > slash is just a little easier on my eyes.
>
> I think "/" bugs me because it means "or" in ABNF, which is the
> standard grammatical notation used in Internet RFCs; see
> <http://www.ietf.org/rfc/rfc4234>.  I could live with "/",
> I suppose.

Here are the two variations, for comparisons:

  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); }
  ;

  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); }
  ;

One advantage of the first one, using "/", is that frees "#" for  
indicating actions, if it should be cobined with my other proposal.  
Then it will look like:

  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); }

I think this last variation makes the grammar standing out quite  
clearly.

One problem though is that the variable definitions end up at  
different places. Ten one might combine the different ideas to arrive  
at:

  expression:
      INTEGRAL_NUMBER/x                 #identity(x)
    | '-' expression/x %prec NEGATION   #neg(x)
    | '(' expression/x ')'              #identity(x)
    | expression/x '+' expression/y     #add(x, y)
    | expression/x '-' expression/y     #sub(x, y)
    | expression/x '*' expression/y     #mul(x, y)
    | expression/x '/' expression/y     #div(x, y)
    | expression/x '^' expression/y     #pow(x, y)
  ;

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

But I am not sure it adds something. - I put in this example, to see  
where things are heading. Apparently, the actions act as implicit  
functions, and the grammar variables insert values into those functions.

  Hans Aberg