From: Marco Gidde <marco.gidde@tiscali.de>
Date: 13 June 2006 10:59:43 BDT
To: gardeners@lispniks.com
Subject: Re: [Gardeners] python-on-lisp needs help
Reply-To: Tending the Lisp Garden <gardeners@lispniks.com>

Jeremy Smith <jeremy@decompiler.org> writes:

> Lu’s Oliveira wrote:
>>
>> Many (?) months ago, way before 0.9.0 (the first actual release of
>> CFFI), the behaviour of MEM-REF (and others) changed so that the type
>> translators are called (in this case, the :STRING type  
>> translator). So,
>> MEM-REF will call FOREIGN-STRING-TO-LISP for you.
>
> Thanks guys, that's great news. I've implemented the fix so it does it
> with the 'old' CFFI (the one I use!) which doesn't automatically do
> this, and the new one (which does), by using 'typep' to check for
> string-ness.
>
> I'm not sure if this is really the proper use of 'case' but Lisp  
> allows
> 'if' to specify any of 2 values, so I think it'll work.
>
> I'll upload it soon if it works with the newer CFFI. I haven't got CVS
> running at common-lisp.net yet, but I'll look into it when I have  
> time.
>
> (defun pygetarg (variable type)
>   "Helper function for handling python arguments, used by
> gen-pycallback. Should never be used directly from code - use defpyfun
> with its arg format"
>   (case type
>     (long (cffi:mem-ref variable :long))
>     ;If it's decoded by the new version of CFFI
>     (if (typep (cffi:mem-ref variable :string) 'string)
>           (string (cffi:mem-ref variable :string))))
>           (string (cffi:foreign-string-to-lisp (cffi:mem-ref variable
> :string))))

If this works for you it is not obvious (to me) why. If the
indentation indicates two IF branches, there is a parenthesis
mismatch. Try to MACROEXPAND the CASE form to see the resulting
code. I would recommend something like

(defun pygetarg (variable type)
   "Helper function for handling python arguments, used by
gen-pycallback. Should never be used directly from code - use defpyfun
with its arg format"
   (case type
     (long (cffi:mem-ref variable :long))
     (string (if (typep (cffi:mem-ref variable :string) 'string)
		(cffi:mem-ref variable :string)
		(cffi:foreign-string-to-lisp (cffi:mem-ref variable :string))))))

Also using ECASE instead of CASE might be appropriate.

As Luis already pointed out switching to a more current version of
CFFI is probably a good idea. And mentioning this dependency on a
specific version on the python-on-lisp home page might also motivate
others to update their ancient CFFI version :-)


Regards,

Marco

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