Re: Generic package parameters not externally visible : what's the rational ?

[Ludovic Brenta <ludovic@ludovic-brenta.org>]

Yannick Duchêne wrote on comp.lang.ada:
> Hello ghosts out there,
>
> 24h without even a single post on comp.lang.ada... I hope every body
> is Ok.
>
> Still the time for my two cents of the day : parameters of generic
> packages are not externally visible. I use to read in a few places on
> the web, this is so to preserve type privacy, but without any examples
> which could illustrate this view.
>
> What's the rational behind this ?
>
> I like to do something like this to work around this :
>
> generic
>
>    Parameter_Type is (<>);
>
>    -- Other generic parameters ...
>
> package Generic_Package is
>
>    -- Give external access to generic parameters which are not visible
> otherwise.
>
>    subtype Formal_Parameter_Type is Parameter_Type
>
>    -- The same for other generic parameters ...
>
>    -- Remaining of the package ...
>
> end Generic_Package;
>
> I feel it is oftenly useful to have external access to generic
> parameters : it is more clear and more consistent than making
> reference to stuffs from different source (finally, the package
> instance is a place where all these references are available, so why
> not use these). I feel this is especially useful when other generic
> packages are instantiated using the same parameters used to
> instantiate another generic packages.
>
> That's why I was wondering about the rational which is underlying to
> this restriction (the one which disallows a generic parameter to be
> externally visible).
>
> Have a nice time (if every one is Ok).

(I'm OK, thank you, and qute busy with the Debian Policy for Ada and
gnat-4.4....)

I think this is at least consistent: the values of subprogram
parameters are also invisible outside the subprogram. Making "in out"
generic formal variables visible outside the generic would be
dangerous IMHO. As for generic formal types, subprograms, packages and
constants... why not.

--
Ludovic Brenta.