Re: GLR: modification of $1

[Hans Aberg]

At 15:31 -0700 2003/10/02, Paul Hilfinger wrote:
>> But rather than this I'd prefer if $1 would be writable. This would
>> give the grammar writer more freedom in the actions, and require
>> fewer changes to an existing LALR grammar.
>
>Personally, I find this prospect a bit distasteful.  Part of it is
>knowing too much, I suppose: to me, writing ``$1 = E;'' suggests the
>computation of an inherited attribute, which of course it is not.  I
>would just as soon avoid such constructs in Bison files to ease the
>(some fictional future) transition to a full attribute system, when
>they would mean something else.

It has been discussed here before, in the case of non-GLR parser, people
wanting
to make special rule midaction tweaks which works under the special C stack
array implementation. In those cases, it then turns out that such
constructs can pose problems when stacks are implemented say under C++ not
using any simple array structure like under C. The problem is that the
midactions are implemented by introducing an anonymous rule, and one can
not in more general stack implementations expect values to survive on such
stacks. Thus, this is the same problem as in the case of the GLR parser:
One uses more general types of stack models, simply. Clearly the general
Bison framework should not rely on any particular C stack implementation
model which will not work under other languages, or other more general
implementations.

So this suggests that one, at least for now, should only admit a rule
midaction to assign to its $$ value, and it can only use the $n values
referring to the variables/tokens/midactions before it in the rule. If the
$n values can be implemented as const values, that seems to be an advantage
under this model.

If one wants $n to be assignable, I figure one will have to think through
more carefully how its going to be done and what's its worth. That sounds
as the usual featuritis problem: If one can get somebody doing it, fine,
but otherwise it will probably have a low priority.

  Hans Aberg