[vwnc] Finding out whether a class’ instance understands a method
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[vwnc] Finding out whether a class’ instance understands a method
 
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Re: [vwnc] Finding out whether a class' instance understands a method
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Re: [vwnc] Finding out whether a class’ instance understands a method
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Re: [vwnc] Finding out whether a class' instance understands a method
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Re: [vwnc] Finding out whether a class' instance understands a method
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Re: [vwnc] Finding out whether a class' instance understands a method
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Re: [vwnc] Finding out whether a class' instance understands a method
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No, you don’t want to implement #respondsTo: on the class!
#respondsTo: is implemented in Object and answers whether the receiver can respond to the argument message. Classes inherit this behavior to answer whether they respond to a particular message. You don’t want to override this in Class to answer instance behavior, because then you can’t find out whether the class responds to something.
I like the idea of combining “respondsTo” and “canUnderstand” to similar names – so, using #instancesRespondTo: implemented in Behavior or ClassDescription answers whether instances of the receiver respond to the indicated selector. This, of course, now covers sending “instancesRespondTo:” to a metaclass to see whether the class can respond… Don’t you just love the complications and convolutions of self-referential systems?
Cheers!
From: [hidden email] [mailto:[hidden email]] On Behalf Of Victor
>
- The method should really be called "instancesCanUnderstand:" For a second I thought that it's easy enough to write a new class method #respondsTo: However, this would create a maintenance nightmare as VW moves to a new version. So, maybe this should be left to Cincom?
Guys?
Víctor
_______________________________________________ vwnc mailing list [hidden email] http://lists.cs.uiuc.edu/mailman/listinfo/vwnc |
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Re: [vwnc] Finding out whether a class'instance understands a method
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Runar, if you can, use polymorphism... it's much easier,
faster, and easier to maintain than using respondsTo: or any of those
mechanisms. In its most rudimentary form, you'd have something like
this:
(someObject respondsTo: #blah) ifTrue: [someObject
something] ifFalse: [someObject somethingElse]
This transforms to
Object>>blah
^self somethingElse
ObjectThatUnderstoodBlahFromBefore>>blah
^self something
Therefore, the code above simplifies to
someObject blah
Each implementation of #blah takes care of the
receiver. The #respondsTo: stuff does not end up reimplementing what the
VM was designed to do. Also, it's easier to maintain because if you tag
every object that could receive #blah with an implementation of #blah, checking
implementors of #blah tells you all the objects that will be sent #blah at the
send site. This, without running any code.
With #respondsTo:, #isKindOf: and the like you only know
the objects that go through the send site which understand #blah. You do
not know all the others that do go through the send site but do not understand
#blah. One could think of nil, but is that really the only one? Why
not nil, the empty string and the empty array? This complicates matters
because it makes code unmaintainable. If the actions for #blah need to be
updated, which objects should now implement #blah when previously they
didn't? Using #isKindOf: is even worse because it flat out prohibits
polymorphism, and thus cuts flexibility away. To top it off, all this does
is to simulate what the VM would do. And, of course, doing this
work in the image is nowhere near as fast.
My 2 cents...
Andres. From: [hidden email] [mailto:[hidden email]] On Behalf Of [hidden email] Sent: Friday, March 27, 2009 10:39 AM To: [hidden email]; [hidden email]; [hidden email] Cc: [hidden email] Subject: Re: [vwnc] Finding out whether a class'instance understands a method No, you dont want to implement #respondsTo: on the class!
#respondsTo: is implemented in Object and answers whether the receiver can respond to the argument message. Classes inherit this behavior to answer whether they respond to a particular message. You dont want to override this in Class to answer instance behavior, because then you cant find out whether the class responds to something.
I like the idea of combining respondsTo and canUnderstand to similar names so, using #instancesRespondTo: implemented in Behavior or ClassDescription answers whether instances of the receiver respond to the indicated selector. This, of course, now covers sending instancesRespondTo: to a metaclass to see whether the class can respond Dont you just love the complications and convolutions of self-referential systems?
Cheers!
From:
[hidden email] [mailto:[hidden email]] On Behalf Of Victor
>
- The method should really be called "instancesCanUnderstand:" For a second I thought that it's easy enough to write a new class method #respondsTo: However, this would create a maintenance nightmare as VW moves to a new version. So, maybe this should be left to Cincom?
Guys?
Víctor
_______________________________________________ vwnc mailing list [hidden email] http://lists.cs.uiuc.edu/mailman/listinfo/vwnc |
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Re: [vwnc] Finding out whether a class'instance understands a method
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For known situations this works nicely. However, we have an
application development framework which often
gets lists of "domain attributes" which it has to apply. Often you can assume they are correct, but there are times when you cannot assume that. I would shoot-on-sight an application level programmer who did it, but in lower level things you really need to at times. Its the fact that I do it so seldom that makes me forget "canUnderstand" ! Valloud, Andres wrote:
-- Dennis Smith +1 416.798.7948 Cherniak Software Development Corporation Fax: +1 416.798.0948 509-2001 Sheppard Avenue East [hidden email] Toronto, ON M2J 4Z8 <a class="moz-txt-link-freetext" href="sip:dennis@CherniakSoftware.com">sip:dennis@... Canada http://www.CherniakSoftware.com Entrance off Yorkland Blvd south of Sheppard Ave east of the DVP _______________________________________________ vwnc mailing list [hidden email] http://lists.cs.uiuc.edu/mailman/listinfo/vwnc |
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Re: [vwnc] Finding out whether a class'instance understands a method
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Yeah... sometimes isKindOf: is actually "correct"...
for example, when dealing with exceptions, isKindOf: is more or less
natural. As you say though, I'd rather avoid getting shot on sight
:). From: [hidden email] [mailto:[hidden email]] On Behalf Of Dennis Smith Sent: Friday, March 27, 2009 11:06 AM Cc: VW NC Subject: Re: [vwnc] Finding out whether a class'instance understands a method gets lists of "domain attributes" which it has to apply. Often you can assume they are correct, but there are times when you cannot assume that. I would shoot-on-sight an application level programmer who did it, but in lower level things you really need to at times. Its the fact that I do it so seldom that makes me forget "canUnderstand" ! Valloud, Andres wrote:
-- Dennis Smith +1 416.798.7948 Cherniak Software Development Corporation Fax: +1 416.798.0948 509-2001 Sheppard Avenue East [hidden email] Toronto, ON M2J 4Z8 <A class=moz-txt-link-freetext href="sip:dennis@CherniakSoftware.com">sip:dennis@... Canada http://www.CherniakSoftware.com Entrance off Yorkland Blvd south of Sheppard Ave east of the DVP _______________________________________________ vwnc mailing list [hidden email] http://lists.cs.uiuc.edu/mailman/listinfo/vwnc |
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Re: [vwnc] Finding out whether a class'instance understands a method
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Valloud, Andres wrote: I have come close -- had one that wanted to assign a title to each business-level class in a hierarchy of about 5 or 6, had title (self isKindOf: XXclass) ifTrue: [^'...]. (self isKindOf: YYclass) ifTrue: [^'...]. ... Makes for a nice mess if you add a subclass :( -- AND its a ton more code!).
-- Dennis Smith +1 416.798.7948 Cherniak Software Development Corporation Fax: +1 416.798.0948 509-2001 Sheppard Avenue East [hidden email] Toronto, ON M2J 4Z8 <a class="moz-txt-link-freetext" href="sip:dennis@CherniakSoftware.com">sip:dennis@... Canada http://www.CherniakSoftware.com Entrance off Yorkland Blvd south of Sheppard Ave east of the DVP _______________________________________________ vwnc mailing list [hidden email] http://lists.cs.uiuc.edu/mailman/listinfo/vwnc |
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Re: [vwnc] Finding out whether a class'instance understands a method
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Re: [vwnc] Finding out whether a class'instance understands a method
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Re: [vwnc] Finding out whether a class'instance understands a method
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