Re: Ada 2012 Constraints (WRT an Ada IR)

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

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:o28mpn$1doo$1@gioia.aioe.org...
> On 07/12/2016 00:09, Randy Brukardt wrote:
...
> 2. This is a mixture of two independent things:
>
> 2.a an exception contract. Delete can raise Constraint_Error

Which is being made explicit.

> 2.b implementation. Delete must raise Constraint_Error at least when 
> Has_Element (Position) is false and in who knows which other cases and it 
> may raise Constraint_Error but not required to in some other undefined 
> cases...

This happens because Ada doesn't (yet) have exception contracts (which apply 
to the body, not the precondition). If it did, it wouldn't include 
Constraint_Error (there being no other correct reason for raising 
Constraint_Error from delete).

I want Ada to have exception contracts, precisely for reusable code like the 
containers. But there is a lot of opposition, mainly because experience with 
them in other languages (notably Java) isn't good. I believe that is because 
exception contracts mainly help code that is used by many projects (and thus 
the contract cannot change in any way without notice), and not so much 
within a single project (when changing details of contracts might be more 
work than benefit). In any case, Ada exception contracts would have to be 
optional, and the usage problems seem to happen when they are manditory.

..
> Precondition would be a contract that does all potential callers, not a 
> very good thing to do, really.

Of course it is a good thing to do, the alternative is to let callers call 
with anything they like (within the type system), which is obvious nonsense. 
Not everything can be described as a type (object state, for instance).

                                 Randy.