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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Operation can be dispatching in only one type
Date: Mon, 7 Dec 2009 18:19:11 -0600
Organization: Jacob Sparre Andersen
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Date: 2009-12-07T18:19:11-06:00
List-Id: <comp.lang.ada>

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1gcigitaii0u0.1psu2vj52e66g$.dlg@40tude.net...
> On Fri, 4 Dec 2009 20:45:19 -0600, Randy Brukardt wrote:
>
>> Writing in Ada 2012 as proposed today
>> (and that might change before it gets standardized):
>>
>>      procedure Do_Something (Window : Claw.Basic_Window_Type; ...)
>>              with Pre => Is_Valid (Window);
>
> Why not to allow such constraints for subtypes? E.g.
>
>   subtype Valid_Window_Type is Window_Type when Is_Valid;
>
> then simply:
>
>   procedure Do_Something (Window : Valid_Window_Type; ...)

That's also under consideration, but there are some problems with it. One 
problem is that such types are very confusing in array indexes/slices (do 
the values all participate or just the ones that pass the predicate?) There 
are a couple of others as well.

Another issue is that not all preconditions/postconditions can be written 
this way. For one thing, a precondition can depend on multiple parameters at 
once. Another issues is that the entry condition and exit conditions may be 
different for a parameter. For instance, from Claw:

     procedure Create (Window : Basic_Window_Type; ...)
         with Pre   => not Is_Valid (Window),
                 Post => Is_Valid (Window);

So subtypes cannot completely replace pre and post conditions, but they can 
be a complement to them.

                                Randy.