[squeak-dev] Re: Object>>#is:?

Colin Putney cputney at wiresong.ca
Fri Mar 5 04:37:36 UTC 2010

On 2010-03-04, at 1:18 PM, Stéphane Rollandin wrote:

> I got it; see my answer to Juan. I guess I'm just programming in bad style: I do indeed consider that Object is part of my packages (or, more accurately, that Object is not a forbidden place for my package to go in).

Hear hear. I've been wondering why people are so enthusiastic about #is: - good to see I'm not the only one. 

I think #isA: is fine, as an easy way to do #isKindOf: without a direct class reference. Being able to avoid class references makes it easier to avoid dependencies, which makes it easier to have a modular system.

Juan's implementation of #is: puzzles me though. It replaces polymorphic dispatch with boolean logic. Good OO design generally goes in the opposite direction. 

Furthermore, Juan's version of #is: makes it more difficult to modularize the system. If I write a package that needs to add the concept of "greenness" to the system, I can add #isGreen extension methods wherever I want, without breaking any existing code. Somebody else can add #isPurple methods without breaking my code. But if we both need to override #is:, we have a gratuitous incompatibility.

Note that #is: may work well in Cuis, but that's because Cuis is *not* a modular system. 

Finally, I also want to point out that "simpler" and "fewer methods" are not the same thing. Methods that answer booleans are dead simple to understand, no matter how many of them there are. A single #is: method increases incomplexity as the number of tests it encompasses increases.

So, consider this moral support for Stéphane, since he seems to be beset on all sides. Also,

+0 for #isA:
-1 for #is:


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