From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.4
X-Google-Thread: 103376,c4bd2a19251049b1
X-Google-NewGroupId: yes
X-Google-Attributes: gida07f3367d7,domainid0,public,usenet
X-Google-Language: ENGLISH,ASCII-7-bit
Path: 
 g2news1.google.com!news4.google.com!feeder.news-service.com!feeder.erje.net!news2.arglkargh.de!news.tornevall.net!.POSTED!not-for-mail
From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: best practice: error handling
Date: Tue, 31 May 2011 11:08:38 -0700
Organization: TornevallNET - http://news.tornevall.net
Message-ID: <is3de7$br4$1@tornado.tornevall.net>
References: <0b95a2a1-6e3d-4ad1-a832-e3099a9bce37@v8g2000yqb.googlegroups.com>
NNTP-Posting-Host: 7aca3cd5544eb47e58cee04a34c3c90c
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-Trace: fb18559fc996858afa4cf76bb0e540bf
X-Complaints-To: abuse@tornevall.net
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US;
 rv:1.9.2.17) Gecko/20110424 Thunderbird/3.1.10
X-Complaints-Language: Spoken language is english or swedish - NOT ITALIAN,
 FRENCH, GERMAN OR ANY OTHER LANGUAGE!
In-Reply-To: 
 <0b95a2a1-6e3d-4ad1-a832-e3099a9bce37@v8g2000yqb.googlegroups.com>
X-UserIDNumber: 1738
X-Validate-Post: 
 http://news.tornevall.net/validate.php?trace=fb18559fc996858afa4cf76bb0e540bf
X-Complaints-Italiano: Non abbiamo padronanza della lingua italiana - se
 mandate una email scrivete solo in Inglese, grazie
X-Posting-User: 0243687135df8c4b260dd4a9a93c79bd
Xref: g2news1.google.com comp.lang.ada:19598
Date: 2011-05-31T11:08:38-07:00
List-Id: <comp.lang.ada>

On 05/31/2011 07:01 AM, milouz wrote:
>
> I'm wondering about the best way to handle errors in Ada when calling
> a function/procedure that may fails.
>
> For example, when programming on Unix systems in C, the called
> functions usually return a zero or positive value on success and -1 on
> failure, while setting a global variable (errno) to give some
> informations about the failure.

When programming in C, returned error codes are often ignored, leading to errors 
and security vulnerabilities. Global variables are never a good idea, especially 
in a concurrent language. This approach should almost never be used.

If the "error" is exceptional, then an exception should be raised.

If the "error" is unexceptional, then a signal that cannot be ignored/misused 
should be used.

For an example of the latter, consider an Index function that returns the index 
of a substring ("pattern") in a larger string ("source"). It will be fairly 
common for the pattern not to occur in the source, so raising an exception is 
not right here.

The Index functions in Ada.Strings.Fixed return Natural, with a value of zero 
returned if the pattern is not found in the source. However, this can be 
ignored/misused; calling code can omit the check for zero and use the returned 
value as if it were a valid index into the string. Sometimes this will cause an 
exception, but there are cases where it will not; such cases can be hard to find.

The correct way to handle this is for the function to return a variant record:

type Index_Result (Found : Boolean := False) is record
    case Found is
    when False =>
       null;
    when True =>
       Index : Positive;
    end case;
end record;

function Index (Source : in String; Pattern : in String) return Index_Result;

Now the signal (the discriminant) cannot be ignored, and the Index Component 
cannot be misused.

-- 
Jeff Carter
"Clear? Why, a 4-yr-old child could understand this
report. Run out and find me a 4-yr-old child. I can't
make head or tail out of it."
Duck Soup
94