Re: Can't export object of private type

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

Matthew Heaney wrote in message ...
[...]
|> Just wondering.. Would private child packages have been a better
|> alternative to nested packages? [I have a pathological aversion to nested
|> packages.]
|
|If you have a choice between a child and a nested package, then use a
|child.  The reason, of course, is to push module dependencies into the
|child.  By moving the dependency to a child, you only have to recompile
|the subtree, if that module changes.


In fact, in the planetary example I gave, the inner nested package was
unnecessary (I simply fell foul of a GNAT 3.11p bug, which causes the need
for a declaration of an overriding abstract subprogram before the body of
the subprogram*).

It may be advantageous to use child packages, under some (unlikely)
circumstances.

[...]
|This is basically why always say, Don't implement a singleton as a type,
|unless you have a compelling need to.  It just creates module
|dependencies that wouldn't be there otherwise.


I would personally agree with this injunction, in general.  In Ada, it
produces a simpler result, except for the polymorphism, which can be
simulated as required by a big 'case' statement (as I demonstrated in an
earlier example).  Of course, this is slightly kludgeous, and may, in some
(perhaps many) cases, be so onerous as to make the type-based solution
preferable.

There is, however, another kind of situation that the package-based approach
does not solve.  This is where a base type (with its promised operations)
already exists, and cannot be (or is not to be) revoked (e.g. because it is
already used elsewhere, or because there will be some non-singleton types
derived from it).  The best solution to this situation is (unfortunately)
the introduction of the 'single record extension' object declaration to the
language (which I would hope could be done at the next revision, barring a
better alternative or a crushing objection).

-------------------------------------
Nick Roberts
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* this bug will be fixed in version 3.12