Formal comment #62 (defect)

Lexical scoping violation for internal define-syntax.
Reported by:	Andre van Tonder

Component:	macros
Version:	5.91

Component : Expansion process 

Pages : 27-28 

Dependencies: Sections 9.3 (Syntax definitions) 
Description

In the following example, the expansion algorithm, as described in
r6rs, will consider (bar x) an expression and throw a syntax violation
stating that expressions may not precede definitions.

    (let ((bar 1))
      (bar x)
      (define-syntax bar
        (syntax-rules ()
          ((bar x) (define x 1))))
      x)

This syntax error violates lexical scoping, specifically the sentence
"Keyword bindings established by define-syntax are visible throughout
the body in which they appear, ..." on p. 29. By lexical scoping, the
result should be 1. I think it can be made 1 if the expansion
algorithm is made multi-pass (see below).

I am unsure if this case is covered by the constraint on p.28, which
reads: "It is a syntax violation if the keyword that identifies one of
the body forms as a definition (derived or core) is redefined by the
same definition or a later definition in the same body." When (bar x)
is expanded, it is not yet known that bar will be a keyword. In any
case, bar is never "re"-defined.

Even if it is intended to be covered by the constraint, it will not be
detected by the algorithm in the first paragraph on page 28.

Suggestion

Perhaps lexical scoping with internal define-syntax can be better
handled by a multipass expansion algorithm instead of single-pass. On
each pass, the expansion process would restart, using syntax
definitions discovered on previous passes, until no more syntax
definitions are discovered.

It seems that such an algorithm would give the result 1 above. The
implied reprocessing of forms, however, may cause problems with macros
that update expand-time state (e.g. record definitions).

RESPONSE:

A multipass expansion algorithm is indeed potentially expensive and may
indeed cause problems with transformers that have side effects.  We will
attempt to clarify the constraint in the next draft but will not adopt
the multipass algorithm.