Re: Dynamic binding of packages

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

John G. Volan <johnvolan@sprintmail.com> wrote in article
<336FDF8B.73AF@sprintmail.com> a lengthy example of using a tagged type to
provide multiple implementations of a single interface.

The use of tagged types ('classes' in C++ and most other languages) in this
way is well known, and your example illustrates the principle nicely. In
many situations, this approach works fine, and often has further
advantages, too.

However - and I am going to ask you to trust me on this (unless you really
want me post a truly humungous example ;-) - for certain real-life
situations, it is just too clumsy, because the binding - the connection
between interface and implementation - is centred around a type rather than
a package.

This type-centred abstraction means that all abstract elements of the
interface has to be in terms of subprograms with that type as its parameter
(or one of its parameters). So, one loses the direct use of all the many
other language elements Ada allows in a package specification. This is not
a fundamental problem, it is just a problem of convenience. But, the
inconvenience is sometimes so great as to constitute a barrier.

So, I would still argue that (perhaps in a future revision) the Ada
language needs a standard mechanism for the dynamic binding of packages.

Nick.