Re: Ada 2012 Constraints (WRT an Ada IR)

["Randy Brukardt" <randy@rrsoftware.com>]

<Stefan.Lucks@uni-weimar.de> wrote in message 
news:alpine.DEB.2.20.1612161308590.29766@debian...
...
>If you never call the subprogram with First > Second, it will not raise
>the exception. And if the exception is not raised, anyway, then why, on
>earth, whould you care which exception is not raised?

Only one reason: for compatibility with a previous version of the same 
routine that didn't have the precondition.

The embedded raise expression exists to ease the transition from not having 
preconditions to having them. Without it, one could not take existing code 
like Text_IO or Ada.Containers.Vectors and write it with preconditions. 
Allowing preconditions to be usable only on new code would slow the 
adoption.

Ideally, no one would ever call Ada.Text_IO.Put with a closed file. By 
making that a precondition, we allow tools to detect the mistake so that 
hopefully no one ever is handling Status_Error. But if some existing code is 
handling Status_Error, we can't raise Assertion_Error instead and expect 
that to be OK.

There shouldn't be much need to specify the exception with preconditions for 
new code. Maybe if one needs to be similar to related existing code, but 
that's about it.

                                      Randy.