Formal comment #109 (defect)

identifier-syntax is not a derived form
Reported by:	Andre van Tonder

Component:	libraries
Version:	5.91

Dependencies: Libraries (phase semantics) and free-identifier=? 
(p. 112)

Discussion

Identifier-syntax cannot currently be portably expressed as a derived
form. I know of two conforming implementations in which it can be so
expressed only with en extension to negative import levels. A third
implementation can express identifier-syntax without negative import
levels, but only by violating the stated semantics of
free-identifier=?.

Here is essentially how it must be espressed in two of these
implementations:

(library (core identifier-syntax)
   (export identifier-syntax)
   (import (for (r6rs syntax-case base) run expand)
           (for (r6rs base) expand
                            run
                            (meta -1)))  ;; FOR LAST OCCURRENCE OF SET!

   (define-syntax identifier-syntax
     (lambda (x)
       (syntax-case x (set!)
         ((_ e)
          (syntax (lambda (x)
                    (syntax-case x ()
                      (id (identifier? (syntax id)) (syntax e))
                      ((_ x (... ...))              (syntax (e x (... ...))))))))
         ((_ (id exp1)
             ((set! var val) exp2))
          (and (identifier? (syntax id))
               (identifier? (syntax var)))
          (syntax
           (make-variable-transformer
            (lambda (x)
              (syntax-case x (set!)   ;; THIS IS THE PROBLEMATIC OCCURRENCE
                ((set! var val)               (syntax exp2))
                ((id x (... ...))             (syntax (exp1 x (... ...))))
                (id (identifier? (syntax id)) (syntax exp1))))))))))
   )

A third implementation does not require the "(meta -1)". It does so by
modifying the semantics of free-identifier=?. In this implementation,
the current definition

"The free-identifier=? procedure returns #t if and only if the two
identifiers would resolve to the same binding if both were to appear
in the output of transformer outside of any bindings inserted by the
transformer." no longer holds.

Suggestion

Consider refining the semantics of phases, import levels and/or
free-identifier=? to make identifier-syntax expressible as a derived
form.

Alternatively, put free-identifier=? somewhere else than the "derived
forms" section.

RESPONSE:

We will revise R6RS to include support for negative levels, so that
`identifier-syntax' can indeed be implemented as a derived form.

See also the response to formal comment #92.