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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Ada and "early return" - opinion/practice question
Date: Tue, 16 Mar 2021 10:46:13 +0100
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On 2021-03-16 09:24, John McCabe wrote:
> On 15/03/2021 17:31, Stephen Leake wrote:
>> John McCabe <john@mccabe.org.uk> writes:
>>
> <..snip..>
> 
>>> For example, you have a C++ function (pseudo code sort of thing):
>>>
>>> <sometype> fn(<some parameters>)
>>> {
>>>     if (<some undesirable condition 1>)
>>>     {
>>>         return <something bad happened 1>;
>>>     }
>>>
>>>     if (<some undesirable condition 2>)
>>>     {
>>>         return <something bad 2>;
>>>     }
>>>
>>>     if (<some undesirable condition 3>)
>>>     {
>>>         return <something bad 3>;
>>>     }
>>>
>>>     // Only get here if everything's good...
>>>     <do some real stuff>
>>>     return <something good>;
>>> }
>>>
> 
> <snip>
> 
>> I would tend to use an exception for "something bad", but that depends
>> on the overall design.
> 
> Thinking back on this point, would you do it a style where you try to do
> what needs to be done then handle the exception when it fails? For example,
> in this style
> 
> begin
>     <try to do a good thing>
> exception
>     when blah blah
> end
> 
> Or do you mean, rather than return when the undesirable condition 
> occurs, do
> something like:
> 
> if <some undesirable condition 1> then
>     raise <some exception 1>
> end if;
> 
> In other words, does each undesirable condition, when it occurs, throw a
> [potentially] different exception?

It is a difficult choice. One would expect a different exception for 
each case. Unfortunately Ada lacks important features to support that 
design:

1. Exception contracts. If you have many different exceptions it becomes 
increasingly difficult to track them.

2. There is no way to group exceptions in a tree-like or relational 
structure. You cannot name a group and handle its exceptions as whole. 
You have no operations on such groups etc.

A rather pragmatic design is to use a minimal set of exceptions packing 
structured information into the exception message. A quite fragile approach.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de