Re: making a son benefit access ton an instanciation of a generic package required as a formal parameter to his father and instanciated by him

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2018-01-21 18:30, Mehdi Saada wrote:
> Child package, right.
> Sorry, seems like I mixed things up a bit.
> My setup is below, but I think it's already done, and I misunderstood from mispelling the exception ("zero_division" instead of "zero_denominator". Stupid me.
> 
> with P1_G;
> generic
>     with package P1 is new P1_G(<>);
>     Max : Positive;
> package P2 is [...] end P2.
> package body P2_G is [...] end P2_G;
> 
> generic
> package P2_G.Child is [...] end P2_G.Child; -- child package
> package body P2_G.Child
> [...] -- I wanted exceptions declared in P1_G (or P1, I dont know the details here) to be visible here, but it OBVIOUSLY already the case.
> end P2_G.Child.

For visibility you always use "use", which is logical, isn't it?

Now some remarks:

1. It is a bad idea to declare anything in a generic package which is 
not a subject parametrization. Exception is a simple value. It has 
nothing to seek in a generic. Especially because each instance of the 
generic will produce a new exception. Make a parent package instead:

    package My_Stuff is
       Baz : exception; -- One exception for all
    end My_Stuff;

    generic
       ...
    package My_Stuff.P1_G is ...

2. There are strange, if not outright stupid, rules about visibility of 
generic formal parameters in generic children. Strangeness of these 
rules is multiplied by compiler bugs infesting implementations of 
generics. Only a language lawyer call tell what is just an effect of 
these rules and what of the bugs. A pragmatical approach to resolve the 
mess without drowning there is to rename all stuff coming from the 
formals you would like to use later in children. E.g.

    generic
       with package P1_G is ...
    package P2 is
       Baz : exception renames P1_G.Baz; -- It won't get lost

3. Remember that Ada did visibility of renamings wrong. A renaming can 
hide the renamed object! So you must be careful if both original 
exception and its renaming become visible, you will see none. They hide 
each other.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de