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On Tue, 9 Aug 2005, Keith Wright wrote:
This revision is more than a trival change. It seems to me that it would be a good idea to extend the discussion period to give more time to think about it.
I certainly agree that discussion is not finished and I will request an extension from the editor.
MzScheme also has two procedures called |datum->syntax-object| and |syntax-object->datum|. These conflict with your newly renamed |datum->syntax| and |syntax->datum|. I don't know why you changed the names, but it is a conflict, not a compatibility, because the MzScheme syntax objects are not lists and can not be accessed with |car| and |cdr|. It seems unlikely that programs written to work with the MzScheme system that used these procedures could run unchanged on the SRFI system or vice-versa---the data types are completely different!
Although, as long as you don't use |car|, |cdr|, etc., and just use syntax-case with the pattern matching idiom described in the syntax-case papers, you can ignore the difference. One may therefore, if one wishes, regard the fact that syntax objects are lists as an irrelevant implementation detail, and stick with a purely portable programming style, which the shorter names datum->syntax would prevent.
I must say that I personally also like the shorter datum->syntax better. However, with the longer version, /pure/ "portable syntax-case" macros will be portable between the canonical Chez implementation and the system described here. For example, the macros |loop| and |include| on
http://www.scheme.com/csug/syntax.html#g2138will work unmodified with this SRFI. I believe that they will also work unmodified on MzScheme.
As an aside, the canonical Chez implementation does allow |car| and |cdr| in some situations. See, on the above page, the uses of (car cmore) and (cdr cmore) and also (null? cmore):
(define-syntax cond (lambda (x) (syntax-case x () ((_ c1 c2 ...) (let f ((c1 (syntax c1)) (cmore (syntax (c2 ...)))) (if (null? cmore) (syntax-case c1 (else =>) ((else e1 e2 ...) (syntax (begin e1 e2 ...))) ((e0) (syntax (let ((t e0)) (if t t)))) ((e0 => e1) (syntax (let ((t e0)) (if t (e1 t))))) ((e0 e1 e2 ...) (syntax (if e0 (begin e1 e2 ...))))) (with-syntax ((rest (f (car cmore) (cdr cmore)))) (syntax-case c1 (=>) ((e0) (syntax (let ((t e0)) (if t t rest)))) ((e0 => e1) (syntax (let ((t e0)) (if t (e1 t) rest)))) ((e0 e1 e2 ...) (syntax (if e0 (begin e1 e2 ...) rest)))))))))))(However, Chez does not allow this on all syntax objects, which is confusing. Also, this macro will not work on MzScheme.)