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From: matteo.bordin@gmail.com
Newsgroups: comp.lang.ada
Subject: Re: prohibit certain generic instantiations in Ada 2005
Date: 12 Feb 2006 00:55:52 -0800
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Date: 2006-02-12T00:55:52-08:00
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As I explained in a prewieus post, my int/float example was just
didactic. I want that kind of checking because I want to use generics
as a generative environment for a component-based approach. If I can
map to the source code the knowledge that some template instantiations
are not allowed, then I can let the compiler check for me if the user
of the library I am writing is doing something wrong. For example,
imagine I want to compose a car component with an Engine and a
Transmission. Electric engine can be coupled only with automatic
transmission, while gasoline engine can work with both automatic and
manual transmission. Then the Car component is:

generic
    type My_Engine is new Engine with private;
    type My_Transmission is new Transmission with private;
package Car is
....

Then I want to prohibit:

package My_Car is new Car(My_Engine => Electric_Engine, My_Transmission
=> Manual_Tansmission).

As you can see there is a reason to do that. And there is other reasons
to want static (compile-time) checking insteam of run-time exceptions.