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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: type conversion
Date: Mon, 13 Oct 2014 08:26:48 +0100
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Date: 2014-10-13T08:26:48+01:00
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Georg Bauhaus <bauhaus@futureapps.invalid> writes:

> On 12.10.14 11:37, Pascal Obry wrote:
>> To control the message or the exception to be raised one can use the
>> Predicate_Failure aspect:
>>
>> subtype Integer_String is new String
>>     with Dynamic_Predicate => Integer'Value (Integer_String) in Integer,
>>          Predicate_Failure => "not an integer string";
>
> Is Predicate_Failure a new aspect in GNAT? My RM does not list it,
> and GNAT GPL 2014 rejects it. GNAT does, however, accept
>
>    subtype Integer_String is String
>      with Dynamic_Predicate => Integer'Value (Integer_String) in Integer
>            or else raise Assertion_Error with "not an integer string";
>
> (And warns that one should use 'Valid instead of a membership test,
> even though there is no object to which one could apply 'Valid.)

Unfortunately using 'Valid would make no difference, because if
Integer_String is say "foo" you get 

   Exception name: CONSTRAINT_ERROR
   Message: bad input for 'Value: "foo"