empty syntax objects

[Julian Graham]

Hi all,

This is a weird one.  It looks like there's a problem with performing
certain operations on syntax objects that "wrap" the empty list -- I
think because such syntax objects are themselves indistinguishable
from the empty list.  I'm still not sure I fully understand what's
going on, but suffice it to say that if you have a syntax-case
expression that looks like:

(syntax-case x ()
  ((foo . rest)
   (datum->syntax #'rest 'foo)))

...you get an error if `foo' is expanded without anything to bind to `rest':

;;; key wrong-type-arg, throw_args (#f "Wrong type argument in
position ~A: ~S" (2 2) (2))

You can get the same error via the following code:

(datum->syntax #'() 'foo)

The initial error message didn't include any source information, so I
connected to the process with gdb and dug around in psyntax-pp.scm
until I located the following line (5459), which seems to be the
culprit:

(#{syntax-object-wrap\ 116}#
  (lambda (#{x\ 783}#) (vector-ref #{x\ 783}# 2)))

What's the right thing to do here?  Do you just have to be careful
with what you do with pattern variables that might end up getting
bound to no arguments?  R6RS isn't totally clear on the topic, but,
for what it's worth, the same code seems to work in MzScheme.


Regards,
Julian