[squeak-dev] Re: Object>>#is:? (was: Re: PackageDependencyTest)

Juan Vuletich juan at jvuletich.org
Thu Mar 4 23:05:24 UTC 2010

Andreas Raab wrote:
> On 3/4/2010 12:33 PM, Juan Vuletich wrote:
>> I believe #isKindOf: is really bad. Reasons for this are given in
>> http://userweb.cs.utexas.edu/~wcook/Drafts/2009/essay.pdf . This was
>> also discussed here, you can google for the name of the pdf in the
>> archives.
>> In the Cuis implementation of Object>>#is: there is no call to
>> #isKindOf:. The idea is to ask for a protocol, not for inheritance.
> But inheritance implies protocol. There is nothing wrong with asking 
> for protocol conformance based on inheritance; it is the same as 
> implementing isNumber in class Number for the purpose of having 
> subclasses inherit that.
> The implementation that I've proposed is making that point. Thus (42 
> is: #Number) returns true, since 42 conforms to the Number protocol. 
> If you had some other class that conforms to the number protocol 
> without subclassing it, it is free to implement, e.g.,
> ComplexNumericRepresentative>>is: aSymbol
>     "I conform to the number protocol"
>     ^aSymbol == #Number or:[super is: aSymbol]
> As a consequence, the #is: test is not an inheritance test at all.
> Cheers,
>   - Andreas

What you say is right. The reasons for me not doing it that way are 
perhaps subtle and aesthetic. I prefer making it as clear as possible 
that this is not about inheritance but protocol. I also think that my 
approach is easier to explain and understand. Besides I don't like it 
looking like there are "special" or "privileged" classes or hierarchies. 
Perhaps more important, I prefer not having "implicit" protocol 
conformance without anybody declaring (for example) that #BorderedMorph 
is now considered a protocol. In addition, I believe that this should be 
used only when really needed. In general, a better design might make all 
these queries unneeded. So, forcing people to declare their protocols 
might discourage abuse.

Anyway, I think both solutions are acceptable and it comes down to taste.

Juan Vuletich

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