Josef Wolf <[email protected]> writes:
> On Fri, Mar 19, 2010 at 05:15:57PM +0100, Andreas Rottmann wrote:
>> defmacros are bound to bite you in the proverbial ass real hard when you
>> combine them with modules, so it's best to avoid them like the
>> plague. At least that's my experience, FWIW.
>
> Thanks for the warning. I'd like to understand why this is dangerous, though.
> Do you have an example for me?
>
Yes, consider the following two modules:
;;---------------8<----------------
(define-module (test a)
#:export-syntax (test-a)
#:use-module (srfi srfi-34)
#:use-module (srfi srfi-35))
(define (funny-condition? c)
(and (message-condition? c)
(string-prefix? "FUNNY: " (condition-message c))))
(define-syntax test-a
(syntax-rules ()
((test-a expr)
(guard (c ((funny-condition? c) 'funny))
expr))))
(define-module (test b)
#:use-module (srfi srfi-34)
#:use-module (srfi srfi-35)
#:use-module (test a))
(test-a
(raise (condition (&message (message (string-append "FUNNY: "))))))
;;---------------8<----------------
Now running it causes this:
WARNING: (test b): imported module (srfi srfi-34) overrides core binding
`raise'
Throw to key `unbound-variable':
ERROR: In procedure module-lookup:
ERROR: Unbound variable: funny-condition?
Entering the debugger. Type `bt' for a backtrace or `c' to continue.
Now why doesn't this work, and return 'funny in module B's body? It's
because currently, the `guard' macro from SRFI-34 is implemented with
`define-macro', and it hence throws away (among other things) the
information what `funny-condition?' refers to when being expanded.
The fully expanded code of the `test-a' form now contains a free
reference to `funny-condition?', while it should have a bound reference
referring to the `funny-condition?' defined in module (test a).
As you can see, the use of a single defmacro can poison your otherwise
perfectly fine hygienic, referentially transparent, syntax-rules macros.
> Do common-lisp macros suffer from the same problems or is this a scheme
> specific problem?
>
I've just superficial knowledge of CL, but AFAIK, Common Lisp macros
still have similiar issues, but they are (largely) mitigated by the fact
that CL has a function namespace separate from the variable namespace.
> Please note that I am very new to lisp, so please don't beat me if I ask
> dumb questions :-)
>
They were actually very reasonable questions!
Regards, Rotty
--
Andreas Rottmann -- <http://rotty.yi.org/>
