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From: Pascal Costanza on 14 Jun 2010 08:02
On 14/06/2010 01:27, Paul Griffioen wrote:
> On Mon, 14 Jun 2010 00:41:51 +0200, Pascal Costanza wrote:
>
>> On 13/06/2010 20:24, Paul Griffioen wrote:
>>>
>>> Why do compilers warn about unused keyword in methods when (call-next-
>>> method) is used? For example when compiling
>>>
>>> (defclass some-class () ())
>>>
>>> (defmethod foo ((object some-class) some-arg&key some-key)
>>> (call-next-method))
>>>
>>> SBCL and LispWorks warn about keyword some-key (haven't tried other
>>> compilers). The keyword is passed to the next method so declaring it as
>>> unused seems wrong. And they don't warn about argument some-arg.
>>>
>>> Is there any reason for this? And is there a way to get rid of these
>>> warnings?
>>
>> If there is no need to use the keyword argument in the method body, it's
>> better to just omit it. You can just say this:
>>
>> (defmethod foo ((object some-class) some-arg&key)
>> ...)
>>
>> This doesn't work if the defgeneric form also mentions the particular
>> keyword argument in question, but maybe it shouldn't either, since it
>> doesn't seem to be necessary for all methods.
>>
>> The exact rules when you have to mention a keyword argument and when not
>> are in the HyperSpec in Section 7.6.5. Note that keyword arguments
>> require extra processing, so not mentioning them may be preferable. If
>> all else fails, (declare (ignore some-key)) always does the trick.
>>
>>
>> Pascal
>
> Thanks for the explanation and the reference. I didn't know these rules.
>
> In my case the keyword is mentioned in the generic function so I have to
> mention it in the method. I'm not sure if I want to remove the keywords
> in the generic function. Why do you say that the keyword doesn't seem
> necessary? I want the method to accept it but the default handling is
> sufficient.

In that case it's ok to have it in the defgeneric form.

> I was uncertain about an ignore declaration (or the ignorable that
> Raffael Cavallaro mentions) because the regular argument doesn't need it.
> Is such a declaration always valid? Because the keyword is used, although
> not visible. The declaration seems a bit strange when I read the code,
> but maybe I just have to get used to it.

You just have to get used to it. ;)

'ignore is good when you know you don't want to use the variable. If you
then happen to use it anyway, you will get a warning, which may be helpful.

'ignorable is good when you don't care. I think this case occurs rarer
than 'ignore in manually written code, but can be very useful in macros
that generate function or method definitions.


Pascal

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