[racket-dev] Merging nonterminals in union-language

By the way, when I was tinkering, I originally got my example working
*without* merging nts (lines 1967-1983). I only added it after
inspecting the output of `compiled-lang-lang'. The resulting
union-language had the same nonterminal in a language several times with
overlapping right-hand-sides, so I merged them

William Bowman

On Thu, Feb 28, 2013 at 02:05:59PM -0600, Robby Findler wrote:
> The reason I didn't do that is very much related to lines 1967-1983 in your
> diff.
> 
> That isn't a good idea: what you really want to do there is check to see if
> different patterns generate the same languages or not. But that's not
> something that is easily done (I'm guessing it is computable, but very very
> expensive. It may not be computable, tho, for all I know.)
> 
> One could allow unioning by just keeping all productions, and I guess I
> wouldn't mind an extension to define-union-language via a keyword that did
> that, tho.
> 
> In your case, is there not a way to refactor your grammars so that you
> don't need this capability? Could you maybe make a small, representative
> example to post to the list? I find that kind of thing to be quite helpful
> for me to understand what the best change to Redex is.
> 
> Robby
> 
> 
> 
> On Thu, Feb 28, 2013 at 1:46 PM, William J. Bowman
> <wjb at williamjbowman.com>wrote:
> 
> > Hello all,
> >
> > I've been hacking on some languages in Redex, and found myself
> > abstracting commons parts into base languages, and gradually building new
> > languages via `define-extended-language' and `define-union-language'.
> > Unfortunately, I hit a wall when I discovered `define-union-language'
> > doesn't like to union languages that define the same nonterminals.
> >
> > Consider this toy example: https://gist.github.com/bluephoenix47/5054403
> >
> > This seems like a sensible thing to want to do, so I forked racket wrote a
> > patch:
> >
> > https://github.com/bluephoenix47/racket/commit/0a7781b2be2643778f8d8d10d771ab1ce2dc622b
> >
> > Unfortunately, several Redex tests fail (http://sprunge.us/fPHU) because
> > they expect an error when languages which define the same nonterminals
> > are used in `define-union-language'.
> >
> > Is this *desired* behavior? If so, why? It seems very reasonable to want
> > to merge the nonterminals of languages.
> >
> > --
> >
> > William J. Bowman
> >
> > _________________________
> >   Racket Developers list:
> >   http://lists.racket-lang.org/dev
> >
> >
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