[Larceny-users] Bug in syntax-rules expander (R6RS semantics changed?)

[David Rush]

Hi y'all -

I have been deeply buried in wrapping various C APIs, and the
repetitive bytevector trickery has led me to a bit of macrology. Now,
this is my first time *ever* using LET-SYNTAX, so maybe I am getting
something wrong here, but using PLT 4.2.1 the following macro expands
as I (almost) expect

; -*- scheme -*-

(define-syntax define-wrapped-field
  (syntax-rules ()
    ((define-wrapped-field (base field) spec0 spec+ ...)
     (let-syntax
         ((wrap-spec
           (syntax-rules ()
             ((wrap-spec (reader converter raw))
              (define reader
                (bvw-access-wrapper
                 (lambda (raw*) raw)
                 (lambda (init) (converter base 'field init)))
                ))
             ((wrap-spec (writer converter!))
              (define writer
                (bvw-update-wrapper
                 (lambda (bv val) (converter! base 'field bv val)))
                ))
             )))
       (begin
         (wrap-spec spec0)
         (wrap-spec spec+)
         ...
         'de-nada
         )))
    ))

; the above is used thusly

(define-wrapped-field (MQOD DynamicQName)
  (mqod-dynamic-queue-name bvw-z-string "unknown")
  (mqod-dynamic-queue-name bvw-z-string!)
  )

; -*- end scheme -*-

The expansion under PLT becomes

  (begin
    (define mqod-object-name
      (bvw-access-wrapper
       (lambda (raw*) "unknown")
       (lambda (init) (bvw-z-string MQOD 'ObjectName init))))
    (define mqod-object-name!
      (bvw-update-wrapper
       (lambda (bv val) (bvw-z-string! MQOD 'ObjectName bv val))))
    'de-nada))

which is totally expected. Larceny does comething else which looks
more or less like

((lambda ()
   ((lambda (\x2e;mqod-object-name\x7c;4 \x2e;mqod-object-name!\x7c;4)
     (begin
       (set! \x2e;mqod-object-name\x7c;4 ...)
       ...
       'de-nada
       ))
    (\x2e;unspecified)
    (\x2e;unspecified))))

which would be fine *I suppose* except that it does *not* end up
defining the new bindings at toplevel (in R5RS mode, I have not tried
the other modes). Is this an actual bug?

Additionally, the inclusion of the expression 'de-nada appears to be a
new requirement (both in Larceny and PLT). I recall that using BEGIN
to encapsulate multiple toplevel defines was pretty much the standard
way to handle this kind of thing in R[45]RS. Is this really the way it
is supposed to work? That is quite a pernicious minor change, if it
is.

david rush
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