bug#17096: Guile 1.8: syntax-case messed up when "void" is defined

[Mark H Weaver]

David Kastrup <address@hidden> writes:

> When running the following program
>
> (define void 6)
> (use-modules (ice-9 syncase))
> (define-syntax absurd
>   (lambda (x)
>     (syntax-case x ()
>       ((_ var) (syntax var)))))
>
> through Guile 1.8, I get the error
>
> ERROR: Wrong type to apply: 6
>
> The same appears to go through without problem (apart from the
> deprecation warning for the module) on Guile 2.0.
>
> Is there any workaround?  Apart from the obvious "don't define void"
> which is not a good option for the actual application at hand since the
> function is part of a user-level API.  And it's not like "void" is
> documented as being anything here...

It would be good if Andy or Ludovic would chime in here, because my
knowledge of Guile 1.8 is extremely limited, and frankly I have no
interest in learning more about this obsolete version of Guile.

However, I looked at the implementation of syntax-case in Guile 1.8.8
and in short, I think you're stuffed.

IIUC, the implementation freely inserts references to 'void' in the
expanded code, on the assumption that (void) will return the unspecified
value.  Unlike the version in Guile 2, this version of psyntax is not
integrated with Guile's module system, and the inserted top-level
references do not refer to 'void' from the (ice-9 syncase) module, but
rather from whatever module happens to be current when the expansions
are done.

At first I thought of monkey-patching the 'chi-void' procedure, which
produces these references, but alas it is not a top-level procedure, but
rather an internal one.

I'm afraid the best suggestion I can think of is to make your own local
copy of ice-9/syncase.scm and ice-9/psyntax.pp, under different
filenames and module names, to be used only when built with Guile 1.8.
In these files, make the following changes (assuming for now that you
name them scm/old-syncase.scm and scm/old-psyntax.pp):

* In old-syncase.scm, change the two occurrences of "void" to "%void".
* In old-syncase.scm, change "ice-9/psyntax.pp" to "scm/old-psyntax.pp".
* In old-psyntax.scm, change the only occurrence of "(quote void)"
  to "(quote %void)".

* Where you need to import syncase, do it as follows:
  (cond-expand (guile-2) (guile (use-modules (scm ly-syncase))))

* Drop support for Guile 1.8 in the first version of LilyPond that
  supports Guile 2.0, so you can clear out the horrible workarounds
  and compatibility cruft.

I'm sorry I don't have a better answer for you, and I realize that
LilyPond is in a very uncomfortable position w.r.t. Guile.  I'll try to
find some time in the next few months to help more with the porting
effort.

     Regards,
       Mark