Re: Polymorphism, genericity, etc.

[Chris Bitmead <chris@tech.com.au>]

Maciej Stachowiak wrote:

> I don't buy it. Having "integer_list" inherit from "list" is 
> the kind of mistake beginning OO-programmers make (assuming 
> here that a "list" is a list of some root "object" type which 
> is a supertype of anything). 

I think Meyer's example if I remember correct (and which went through a
troublesome period of fashion when C++ lacked templates) is that the
item you are putting in a list inherits from list_member. So if you want
to put dogs in a list you change dog to inherit from list_member. And
then you have the problem of getting the object back out again without
casting.

> Covariant
> formal parameters (as opposed to return types) are forbidden by 
> most languages
> because they are __not_ type-safe.

In Eiffel they used to be type-safe because global checks were done.
Very ugly in a theoretical sense. I've got a feeling they might have
some other solution now.


> Actually, Java's built-in array types exhibit the covariance 
> problem I described above, one of several glaring design flaws 
> in the Java language. In other words, it will allow you to pass 
> a String[] where an Object[] is expected, and throw run-time 
> type errors if you then try to assign a non-String to an 
> element of the array.
> 
> So much for the benefits of static typing.

Yep, if you're going to do type-safe, you need to do it properly. And
when you do it properly the language can get pretty complicated.

-- 
Chris Bitmead
mailto:chris@tech.com.au