Can a class define a default superclass function?

[Graham Klyne]

At 00:59 03/05/2003 -0700, Ashley Yakeley wrote:
>In article <5.1.0.14.2.20030501153714.00b77c40@127.0.0.1>,
>  Graham Klyne <gk@ninebynine.org> wrote:
>
> > It appears that it is not possible for a subclass to define a default
> > for one of its superclass functions.
>
>Correct. This is a common desire I think especially among people used to
>OOP, but there doesn't seem to be a clean way of doing it.

FWIW, I can live with this OK, using the device I mentioned 
previously.  But I thought I might be missing something, which, in a sense, 
I was...

>  For instance:
>
>  module M where
>
>   class C a where
>    foo :: a -> a
>
>   class (C a) => D a where
>    foo = id
>
>   instance C Char
>
>  module N where
>   import M
>
>   instance D Char
>
>
>When compiling M, the compiler doesn't know instance D Char, so would
>presumably complain (warning) that foo is not defined in C and give it
>an undefined default.
>
>Perhaps it could work if it were a condition that instances of C and D
>be defined in the same module, and also if missing members in instances
>were errors rather than warnings (a good idea anyway IMO).

... I hadn't fully appreciated that an instance declaration without a 
corresponding function definition would be OK.

But maybe there's still a problem if one allows inheritance of function 
definitions:  if the "instance C Char" indeed had a definition of "foo", 
then the definition of "foo :: Char -> Char" could vary in unexpected ways 
depending on which modules were imported?

So, on reflection, it seems we have that being able to declare an 
*existing* data type as an instance (as opposed to OO style with static 
typing, where one typically must declare a new datatype to inherit from a 
supertype, I think),  inheriting default definitions from a superclass can 
create undesired sensitivity to the visibility of module definitions.

#g


-------------------
Graham Klyne
<GK@NineByNine.org>
PGP: 0FAA 69FF C083 000B A2E9  A131 01B9 1C7A DBCA CB5E