Re: Package's private parts and protected types

[Martin <martin.dowie@btopenworld.com>]

On Feb 8, 4:30 am, Hibou57 (Yannick Duchêne)
<yannick_duch...@yahoo.fr> wrote:
> Hi all out there,
>
> Whenever a protected type is declared, all of its specification has to
> be defined at once and not private stuff can be delay (like "type ...
> is private" would allow). If this protected type is to hold something
> which is not to be public, the only way to do as far I'm able to do,
> is to wrap it in a private type as a record component.
>
> package P
>
>    type A_Type is limited private;
>    -- Must be limited, to be able to hold a limited component.
>
>    function Value (A : A_Type) return Value_Type;
>    -- Value_Type defined somewhere else.
>
>    procedure Do_Something (A : in out A_Type);
>    -- Potentially blocking.
>    -- Obviously, there are comments to assert such things,
>    -- but I would prefer it to be formally stated.
>
> private
>
>    protected type B_Type is
>    -- Cannot move it in the public part, due
>    -- to a feature (Set_Value) which must not be public.
>       function Value return Value_Type;
>       -- Accessed via the corresponding method on A_Type.
>       procedue Set_Value (New_Value : in Value_Type);
>       -- The A_Type does not have such a corresponding
>       -- method : we want it to remains private.
>       procedure Do_Something;
>       -- Accessed via the corresponding method on A_Type.
>       -- B_Type being protected, this means this is
>       -- a potentially blocking operation.
>    end B_Type;
>
>    type A_Type is limited record
>       B : B_Type;
>    end record;
>
> end P;
>
> Then, implementation of methods on A_Type will then simply pass
> control to the corresponding ones of its B component. All of this,
> just to hide something which is not to be part of the public
> specification.
>
> I don't like it, because it does not any more publicly shows that
> things relies on a protected object.
>
> Was this a desired consequence when this part of Ada was designed ?
>
> Do someone know a reason ?
>
> Is there something I'm doing here I should not do ?

This is true of task types too.

We take the view that if something was a task/protected or not should
be hidden from a user (e.g. we may change the behaviour from being
task based/not protected, so we abstract away that information). The
associated 'rule' for callers is that they can't assume that calls to
any operation don't block - but that's true of calling pretty much any
package anyway.

I wonder if there is room for "pragma (Not_)Blocking"?...Not just as a
visual aid but maybe it could also be used extend the usage of "pragma
Detect_Blocking" to wrappers for "foreign languages".

Cheers
-- Martin