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Re: Introductory material on hygienic macros in dr scheme
Quoting "Pierpaolo BERNARDI":
> I have a question myself on macros.
> mzscheme 200alpha9 gives:
> > (let-syntax ((foo
> (syntax-rules ()
> ((_ x) (bar x)))))
> (define x 33))
> begin (possibly implicit): no expression after a sequence of internal
> definitions in: ((define x 33))
> While the same expression works in Chez, where the (define x ...) is
> top level.
> From reading the r5rs is not clear to me which behaviour is correct.
In the "internal definitions" section:
Definitions may occur at the beginning of a <body> (that is, the body
of a lambda, let, let*, letrec, let-syntax, or letrec-syntax expression
or that of a definition of an appropriate form). Such definitions are
known as internal definitions as opposed to the top level definitions
So it seems the `define' above has to be treated as an internal
definition. But I wouldn't bet against there being a contradictory
statement elsewhere in R5RS.
I think the Chez convention probably doesn't work for PLT Scheme --
long explanation below.
The Chez model seems to it interact confusingly (wrongly?) with
internal definitions. Here's the simplest example that I could manage:
((_ x) (define (x) (bar))))]
((_) (ack)))]) ;; <<<<<
(define (ack) 'inner-ack)
Which `ack' is the one marked by "<<<<<" ? If the `letrec-syntax'
expression produces a definition, then we have a two-definition
internal sequence, and `(define (ack) 5)' should bind the marked `ack'.
In other words, the above should be equivalent to
(lambda () (bar)))]
[ack (lambda () 'inner-ack)])
However, Chez produces 'outer-ack for the first expression, and
'inner-ack for the second. So the marked `ack' is actually captured by
the outer `ack', not not the internal definition.
Scale this up to the problem of definitions with `class' and `unit',
consider that `define-struct' generates both normal and syntax
definitions, etc., and I believe the above pattern will soon cause
problems in practice.