On May 1, 2009, at 4:42 AM, Ramana Kumar wrote:
I finally discovered a way to make definitions depending on whether a
certain library is available:
(define make-engine
(call/cc
(lambda (k)
(with-exception-handler
(lambda (x) (k #f))
(lambda () (eval 'make-engine (environment '(scheme))))))))
(define-syntax if-engines
(lambda (o)
(define make-engine
(call/cc
(lambda (k)
(with-exception-handler
(lambda (x) (k #f))
(lambda () (eval 'make-engine (environment
'(scheme))))))))
(syntax-case o ()
((_ c) #'(if-engines c (if #f #f)))
((_ c a) (if make-engine #'c #'a)))))
I then go on to use if-engines like this: (if-engines (begin some
definitions which require make-engine) (begin alternative
definitions)).
So far so good.
As you can see I had to define make-engine twice, once for syntax and
once for runtime. I know there's a nonstandard way to define it only
once: (meta define make-engine ...) (actually, I'm not sure whether
you still have to define it for runtime if you do that...). Is there
another portable solution than defining it twice which would still let
me put all my code in one file?
I presume you're talking about Chez's meta keyword, which afaik makes
the definition available at macro expansion time and not at run time.
If you want a definition that's available in both times, and you want
to do it portably, it has to be put in a separate library. Too bad.
Anyways, does Chez support a Chez-specific library file extension
like foo.chez.sls? This is pretty much the de facto standard way of
solving problems like yours. So, you define a chez library like
;;; ramana/engines.chez.sls
(library (ramana engines)
(export make-engine if-engines)
(import (scheme))
(define-syntax if-engines
(syntax-rules ()
[(_ c a) c]
[(_ c) c])))
;;; ramana/engines.sls
(library (ramana engines)
(export if-engines) ;;; not exporting make-engine to get a
(import (rnrs)) ;;; syntax error if used incorrectly
(define-syntax if-engines
(syntax-rules ()
[(_ c a) a]
[(_ c) (begin)])))
That's less code than what you have and is simpler since it does
not involve eval, exceptions, and lengthy macros.
If Chez does not support this extension, well, you know where
LH-401 is. :-)
Aziz,,,
