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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: Ada and "early return" - opinion/practice question
Date: Tue, 16 Mar 2021 02:13:54 -0700
Organization: Aioe.org NNTP Server
Message-ID: <865z1rea0d.fsf@stephe-leake.org>
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John McCabe <"john@nospam.mccabe.org.uk"> writes:

> 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

No, your example fn returns a status code to the caller for "something
bad", so the Ada fn should raise an exception, and the caller (or some
function up the call chain) should handle it.

> 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;

Yes, but with helpful info in the exception message:

    raise <some exception 1> with "explanation";

For example, if the problem is that some number is out of bounds, the
"explanation" should include both the number and the bounds. And if the
bounds can be adjusted by a user option, the option name.

<rant>
The most frustrating error message ever is from Windows, when a program
is missing some DLL or other file; the error message says:

    the file cannot be found.

_Which_ file???!!! the program knows exactly what file name it is looking
for, and what directories it is looking in; why isn't it telling me?

Sigh.
</rant>

I often spend a lot of effort to improve error messages; they help a lot
when debugging, especially a few years later.

Sometimes I would like to pass data with the exception, as other
languages support, but not often.

-- 
-- Stephe