[Python-Dev] type categories

[Andrew Koenig]

>> I can see how it could be done using some additional syntax similar to
>> what ML uses, e.g.:

Guido> def f(a: Cat1):
Guido>    ...code for Cat1...
Guido> else f(a: Cat2):
Guido>    ...code for Cat2...
Guido> else f(a: Cat3):
Guido>    ...code for Cat3...

Greg> As long as all the implementations have to be in one place,
Greg> this is equivalent to

Greg>   def f(a):
Greg>     if belongstocategory(a, Cat1):
Greg>       ...
Greg>     elif belongstocategory(a, Cat2):
Greg>       ...
Greg>     elif belongstocategory(a, Cat3):
Greg>       ...

Greg> so you're not gaining much from the new syntax.

I'm not so sure.  The code is alreasy somewhat simpler here, and it
would be substantially simpler in examples such as

        def arctan(x):
            ...
        else arctan(y, x):
            ...

>> It might also be possible to modify a multimethod dynamically,
>> e.g. later one could write:
>> 
>> def f4(a: Cat4):
>> ...code for Cat4...
>> 
>> f.add(f4)

Greg> This sort of scheme makes me uneasy, because it means that any module
Greg> can change the behaviour of any call of f() in any other
Greg> module.

It makes me uneasy because the behavior of programs might depend on the
order in which modules are loaded.  That's why I didn't suggest a way
of defining the variations on f in separate places.

-- 
Andrew Koenig, ark@research.att.com, http://www.research.att.com/info/ark