Adrien Pierard wrote:

Hi.

...

I may be misunderstanding hygienic macros here, but as far as I can tell I am getting variable capture.

I don't use hygienic macros either but

...

This displays (7 8 9) when arg2 should be (1 2 3) (the value of a b c in the calling code, not the value in "bad").

Bigloo prints (7 8 9) and Gauche prints (1 2 3) Chicken, guile, mzscheme and petite all fail with macro error, or receive error.

You probably forgot to import "receive". Here is what happens in mzscheme:

(require (lib "8.ss" "srfi"))

(define-syntax bad

(syntax-rules () ((_ (arg1 ...) (arg2 ...)) (receive (a b c) (values arg1 ...) (newline) (write (list arg2 ...))))))

(receive (a b c) (values 1 2 3) (bad (7 8 9) (a b c)))

(1 2 3)

thanks for the replies.

that makes sense, but implies that both gambit and bigloo (see other reply) are using unsafe macro expansion. which seems like crazy talk - why would they purposefully make code unreliable?

as it happens, i am looking at rewriting the code with vectors, so will lose the values/receive code.

andrew

Date: Sat, 18 Aug 2007 23:29:19 -0400 (CLT) From: "andrew cooke" andrew@acooke.org

(define-syntax bad (syntax-rules () ((_ (arg1 ...) (arg2 ...)) (receive (a b c) (values arg1 ...) (newline) (write (list arg2 ...))))))

(receive (a b c) (values 1 2 3) (bad (7 8 9) (a b c)))

This displays (7 8 9) when arg2 should be (1 2 3) (the value of a b c in the calling code, not the value in "bad").

This is because Gambit runs two macro expanders over the code: the SYNTAX-CASE expander (hygienic), and then the evaluator's own non-hygienic expander. To SYNTAX-CASE, RECEIVE looks like a variable reference, not a binding form, so it doesn't recognize that A, B, and C are to be bound to distinct variables in the two distinct uses of RECEIVE. Then the evaluator's non-hygienic expander actually expands the RECEIVE to a call to CALL-WITH-VALUES (or the ridiculously octothorped version thereof), and the variables are captured.

Try this before running the example:

(define-syntax receive (syntax-rules () ((RECEIVE bvl expression body0 body1 ...) (CALL-WITH-VALUES (LAMBDA () expression) (LAMBDA bvl body0 body1 ...)))))

Then RECEIVE will be a hygienic macro, and the SYNTAX-CASE expander should rename the variables introduced by BAD accordingly.

Date: Sun, 19 Aug 2007 20:17:03 -0400 From: Marc Feeley feeley@iro.umontreal.ca

Could you please explain what you mean by "unsafe macro expansion"?

Presumably, he is referring to the non-hygienic macros like RECEIVE in the Gambit system, which are processed not by the hygienic SYNTAX-CASE expander but rather by the non-hygienic macro expander in the built-in evaluator.