Re: Which compiler is correct?

[bobduff@world.std.com (Robert A Duff)]

In article <x7vyb86jpr2.fsf@pogner.demon.co.uk>,
Simon Wright  <simon@pogner.demon.co.uk> wrote:
>a.c's body can see the full view of type a.x
>a.c's body can only see the partial view of type a.b.y
>the partial view of type a.b.y says it's a new a.x
>so how come the a.x bits of a.b.y aren't visible to a.c's body?

Because the RM says so.  ;-)

>(I'm sure it's true if you say so; I just think most people, after
>they've seen this sort of problem and fumbled their way to a
>resolution a few times, will have a private spellbook from which they
>will select incantations until they find one that works. So what's
>new, you say)

The idea here, is that when you say "type New_Type is new Old_Type ...",
the properties of New_Type are frozen at that point.  EXCEPT when this
declaration is inside where Old_Type is declared -- then, at the
earliest place within New_Type's immediate scope where it can see some
additional information about Old_Type, then that information becomes
visible for New_Type.  Usually, this doesn't happen.  When it does
happen, it's either at the beginning of a private part of a child
package, or at the beginning of the body of a physically-nested package.

Yes, this leads to surprising consequences, like where you're at a place
that *knows* New_Type is derived from Old_Type, and *knows* that
Old_Type has a component Foo, but nonetheless, Foo is not a visible
component of New_Type.

This can be confusing, but at least the confusion happens at compile
time, not at run time.  :-)

- Bob