Python from Wise Guy's Viewpoint

[Joachim Durchholz]

Pascal Costanza wrote:

> Joachim Durchholz wrote:
> 
>> Oh, you're trolling for an inter-language flame fest...
>> well, anyway:
>>
>>> 3. no multimethods (why? Guido did not know Lisp, so he did not know 
>>>    about them) You now have to suffer from visitor patterns, etc. like
>>>     lowly Java monkeys.
>>
>> Multimethods suck.
> 
> Do they suck more or less than the Visitor pattern?

Well, the visitor pattern is worse.
Generics would be better though.

> So how do you implement an equality operator correctly with only single 
> dynamic dispatch?

Good question.

In practice, you don't use dispatch, you use some built-in mechanism.

Even more in practice, all equality operators that I have seen tended to 
compare more or less than one wanted to have compared, at least for 
complicated types with large hidden internal structures, or different 
equivalent internal structures. I have seen many cases where people 
implemented several equality operators - of course, with different 
names, and for most cases, I'm under the impression they weren't even 
aware that it was equality that they were implementing :-)

Examples are:
Lisp with its multitude of equality predicates nicely exposes the 
problems, and provides a solution.
Various string representations (7-bit Ascii, 8-bit Ascii, various 
Unicode flavors). Do you want to compare representations or contents? Do 
you need a code table to compare?
Various number representation: do you want to make 1 different from 1.0, 
or do you want to have them equal?

I think that dynamic dispatch is an interesting answer, but not to 
equality :-)

Regards,
Jo