Mixins (was Re: Ada 200X Assertions)

["Mark Lundquist" <mlundquist2@attbi.com>]

"Matthew Heaney" <mheaney@on2.com> wrote in message
news:u0t8rui7mdo870@corp.supernews.com...
>
> Well, you could argue there are two mixin idioms: the one you describe
here,
> and another using access discriminants.
>

Interesting thought...

Your 2nd example is the classic "multiple views" idiom.   I had never
thought of calling it a "mixin" before (and Lutz had asked specifically
about generic mixins), but a mixin isn't a bad way of thinking of it.  My
linked-list example is a special case of this, because you normally wouldn't
override the operations of the mixed-in type -- Root_Link or whatever I
called it -- you'd just use it as is, without specialization.  In that way
it's more "mixin-like" than the general case of multiple views.

Now that you've got me thinking of this as a mixin, I may start calling it a
"composition mixin" idiom (to distinguish it from the generic mixin).

Taking the linked list example as I wrote it, it's also trivial to
"genericize" that code and make a generic mixin out of it.  That'll work
with anything that doesn't have unknown discriminants (the formal type for
the "base" type of the mixin can't be indefinite since it is a component of
the list element type).

-- mark