Re: merge function called AFTER user actions

[Joel E. Denny]

On Mon, 26 Sep 2005, Derek M Jones wrote:

> > > Does anybody know why the decision was made to have the %merge
> > > function called last?  This choice seems to be very suboptimal.
> >
> >
> > The merge function's job is to merge the semantic values constructed by the 
> > two actions.  If it were called before the actions, what would it merge?

My point here is that the current functionality seems natural and 
necessary for all GLR parsers.  Do you disagree?

> Ok, so let's change its name.
>
> %premerge  ??
> %initmerge ??

This pre-merge user function would be an extra feature. I don't foresee 
myself using it, so I really can't justify spending my own time 
implementing it and arguing for its acceptance.

> In my case I would have to save all sorts of information in
> order to undo the effects of the semantic actions that happen
> in both rules are reduced.  It is much simpler to perform a
> test (ie, will another rule also be matched for this token
> subsequence) in a rule.

Personally, I'd do my best to rework this logic.  Evaluate the two 
possibilities in the two semantic actions, and then select one (or both) 
of the values in the merge function.  That's how bison is intended to 
work.

Joel