Re: [Caml-list] Restricting Method Overriding/Redefinition in Subclass

[skaller <skaller@users.sourceforge.net>]

On Wed, 2004-08-18 at 22:04, chris.danx wrote:

>  Separating subtyping from subclassing is 
> something I'd read about (in the context of polymorphism and 
> subclassing) and thought was very useful. 

You aren't going far enough. Subtyping is a *semantic* notion.
Whether a class satisfies the 'is a' relation or not is the issue.

In Ocaml, if a class has a type which is not an *Ocaml* 
subtype of another it probably isn't a subtype semantically ***

The converse is not true. Just because Ocaml thinks your
class has a subtype of another classes type does NOT
mean it actually is a subtype -- you still need to check
the methods to make sure the 'is a' property holds. 
If you override some method and give it an incompatible
behaviour you don't have a subtype (you'll just get weird
bugs!)

*** It is possible to have a class which acts semantically
'as' another but is not ruled by Ocaml as a subtype.
A trivial example of this is when you have two methods
called 'size' in one class and 'length' in another
that do the same job -- but happen to be encoded with
distinct names. You'll have to use a wrapper to fix this
without invading one of the classes (and then probably
breaking something else).

-- 
John Skaller, mailto:skaller@users.sf.net
voice: 061-2-9660-0850, 
snail: PO BOX 401 Glebe NSW 2037 Australia
Checkout the Felix programming language http://felix.sf.net



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