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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada and "early return" - opinion/practice question
Date: Tue, 16 Mar 2021 11:46:06 +0100
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On 3/16/21 9:24 AM, John McCabe wrote:
> 
> 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

That depends on a lot of things. In some cases, Bad_Condition means that 
Do_Good_Things will produce a wrong answer rather than fail, so this construct 
cannot be used.

If Do_Good_Things will raise an exception for every Bad_Condition, then it 
depends on how fine-grained those exceptions are and how helpful you want to be 
to the caller. If a different exception is raised for each Bad_Condition, then 
this is equivalent to explicitly raising an exception for each Bad_Condition. If 
you get the same exception for all, you may reduced to telling your caller that 
there's something wrong with the values supplied.

> 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?

I lean toward using fine-grained exceptions, so I'd probably use a different 
exception for each, and document when each exception is raised, or at least 
provide a different exception msg for each.

On the other hand, these kinds of tests can often become preconditions in Ada 
12, in which case you get a single exception, possibly with a generic msg that 
the precondition failed.

Why would you write "raise <some exception 1>" and then talk about "throwing"?
There's no throwing in Ada. That only happens in error-prone languages.

-- 
Jeff Carter
"If you think you got a nasty taunting this time,
you ain't heard nothing yet!"
Monty Python and the Holy Grail
23