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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: function Is_Open (File : File_Type) return Boolean; :Text_io
Date: Wed, 28 Oct 2015 12:58:30 -0500
Organization: JSA Research & Innovation
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Xref: news.eternal-september.org comp.lang.ada:28091
Date: 2015-10-28T12:58:30-05:00
List-Id: <comp.lang.ada>

"Georg Bauhaus" <bauhaus@futureapps.invalid> wrote in message 
news:n0qa3q$th8$1@dont-email.me...
...
> "Better than Assertion_Error?" I understand we are going to get
>
>    subtype Open_File is File with Predicate =>
>       Is_Open(Open_File) or else raise Status_Error with "...";

You need to use the aspect Predicate_Failure here, as opposed to raising an 
exception in the predicate expression. (That's different than in a 
precondition.) In this case (and replacing the Gnat-only "predicate" 
aspect):

    subtype Open_File is File with Dynamic_Predicate =>
       Is_Open(Open_File), Predicate_Failure => raise Status_Error with 
"...";

 Else bad things might happen to membership tests:

    if My_File in Open_File then
        Call_Subprogram_That_Requires_an_Open_File (My_File);
    end if;

If My_File is not open, this membership and your subtype would raise 
Status_Error, not False. (This was a tough nut to crack for the ARG. And 
GNAT only recently implemented Predicate_Failure, so with older GNATs you 
have to leave the exception alone.)

                                            Randy.