[squeak-dev] Re: intersect: when intsersects: is false

Louis LaBrunda Lou at Keystone-Software.com
Mon Jun 21 19:14:55 UTC 2010

On Mon, 21 Jun 2010 21:02:29 +0200, Frank Shearar
<frank.shearar at angband.za.org> wrote:

>On 2010/06/21 16:03, Louis LaBrunda wrote:
>>>>> snip...
>>>>> One could just
>>>>> leave it as it is, because the precondition for answering a "correct"
>>>>> rectangle is that the receiver and the argument intersect. If they don't, the
>>>>> answer is undefined, which means that whatever is answered is correct in a
>>>>> mathematical sense.
>>>>> snip...
>>>> I urge some caution here.  If "whatever is answered is correct in a
>>>> mathematical sense" is true then returning any rectangle like (100 at 100)
>>>> corner: (150 at 150) would be correct, clearly that is not the case.  In
>>>> Smalltalk, nil is undefined, nil is probably the "correct" answer.
>>> Yes, this is tempting.
>>> However from a pragmatic point of view that would mean that:
>>> - either every call must be protected with an intersects: test or (...
>>> intersect ...) ifNil: [...]
>>> - or UndefinedObject must understand Rectangle protocol
>>> Answering an empty rectangle seems more simple with this respect.
>>> Nicolas
>> As I don't play with rectangles much, I will defer to your better judgment.
>> Mostly I was concerned about the "whatever is answered is correct in a
>> mathematical sense" statement but maybe I was being too picky.  And why I
>> quoted "correct" when I said nil was probably the "correct" answer. Correct
>> or not an empty rectangle is probably the "better" answer.
>It does occur to me that perhaps the behaviour's deliberate. Reading 
>Andreas' comment in the method, it's an optimisation of a method that 
>goes back to Smalltalk-76 [1]!
>GNU Smalltalk defines the behaviour of #intersect: [1] as returning nil 
>for non-overlapping Rectangles [2]. (So we're already incompatible with 
>another dialect!) My brief Googling didn't reveal what the other 
>Smalltalks do.

VA Smalltalk V8 returns:
 100 @ 100 corner: 50 @ 50

I don't think I like it but that is what it does.  I will be interested to
see what ends up in Squeak and pass that along to Instantiations.

Louis LaBrunda
Keystone Software Corp.
SkypeMe callto://PhotonDemon
mailto:Lou at Keystone-Software.com http://www.Keystone-Software.com

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