Re: Eiffel and Java + Ada dispatching

[Vincent Celier <vcelier@direct.ca>]

Don Harrison wrote:

> I'd have to say that I never instantiate anything in Ada without explicitly
> providing actual parameters. I think it makes it easier to see what's going
> on at the instantiation site without having to check the generic declaration
> to see that a default isn't unwittingly being used. So, rather than being
> disappointed that Eiffel doesn't have default actual parameters, I'm glad
> that it doesn't.
> 

Default parameters are very useful when low-level interfaces need to
migrate.

To remove a generic formal:
   1- Add a default: upwardly change, no client code should be
      affected,
   2- Ask all client to remove the obsolete parameter in their
instantiations
   3- When all clients have done 2, remove the generic formal.

To add a new generic formal:
   1- Add the new generic formal, with a default,
   2- Ask all clients to add an actual parameter in all instantiatrions,
   3- Remove the default.

This work well for constants of global types, and for subprograms
with parameters and return values of global types.
For values of generic private types, it is sometimes possible to add a
new generic
constant, if there is already another generic constant.

For example, in the following spec:

   generic
      type Item is private;
      Nil_Item : in Item;
   package Whatever_Generic is ...

it is possible to introduce a new generic constant of type Item:

   generic
      type Item is private;
      Nil_Item : in Item;
      Default_Item : in Item := Nil_Item;
   package Whatever_Generic is ...


However, it is not possible to default generic formal types and
variables. Other
techniques should then be used.

> Don.
> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> Don Harrison             donh@syd.csa.com.au

-- Vincent Celier,
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