Hi Mark,

thank you very much for replying so quickly.

Am Mi., 14. Nov. 2018 um 20:11 Uhr schrieb Mark H Weaver <mhw@netris.org>:

> > The `ellipsis?' procedure in psyntax.ss does exactly this, but it
> > isn't available to user code. Re-implementing it is not possible
> > without accessing internal details like the special identifier #{
> > $sc-ellipsis }# and without resorting to hacks.
>
> Surprisingly, it actually _is_ possible to do it portably in any
> R[567]RS Scheme.  See 'match-check-ellipsis' near the end of
> modules/ice-9/match.upstream.scm.
>

This is why I wrote "without resorting to hacks". :-) Like `ellipsis?',
`bound-identifier=?' and `free-identifier=?' can also be portably
implemented in R5RS, and I had to use all these tricks to provide a
portable implementation of SRFI-148. But it is nicer to have
`bound-identifier?' and `free-identifier?' as procedures that can be called
by macro transformers easily. And so I think of `ellipsis?'.


> > Thus, I would like to ask to add `ellipsis?' to the list of procedures
> > exported by Guile (like `identifier?` or `bound-identifier=?` already
> > are).
>
> I'll need to think about how this could be exposed in the API.  It's not
> as simple as exporting that procedure.  The 'ellipsis?' procedure is not
> able to answer the question by looking only at the syntax object; it
> also needs the macro-expansion environment 'r', which you do not have.
> 'identifier?' and 'bound-identifier=?' only need the syntax objects.
>

I think I understand (please correct me if I am wrong): If we want to check
whether `e' is the current ellipsis, we cannot simply look up the binding
of `(datum->syntax e '$sc-expand)' because the wrap of `e' may be different
than the wrap of the, say, `syntax-case' or `syntax' form in which `e'
appears. So what we actually need is a procedure of two arguments:
`(ellipsis? e ctx)' returns `#t' if the identifier `e' is the current
ellipsis in the lexical environment of the identifier `ctx'.

-- Marc