Re: Container reqs

[Jeffrey Carter <jrcarter@acm.org>]

Stephen Leake wrote:
> 
> PragmARC does _not_ support indefinite types, which are a large part
> of Ada (strings, unconstrained variant records, tagged types with
> unconstrained discriminants). In addition, the comment says "no
> scalars"; I'm not clear why.

That is correct. Raw indefinite types are not reasonable types to store
in a data structure. If you need to store strings, you should be using
an abstraction such as (Un)Bounded_String. Similar abstractions should
be used for other indefinite types.

The problem with scalars is that there is a range check required on copy
in for a scalar parameter of mode in out. The PragmARCs pass
uninitialized objects of type Element to the To parameter of Assign. If
Element is a scalar subtype (such as Positive) and an object's
uninitialized value is out of the subtype's range (-7), Constraint_Error
would be raised. For scalar types that cannot have an out of range value
(Integer), this is not an issue, but for a simple-minded rule, no
scalars is easily stated. It is a simple matter to wrap a scalar in a
record if it's really needed, but experience shows that data structures
are almost always used for more complex types. It's a reasonable trade
off for being able to handle limited types without access types.

> So I don't think PragmARC meets the criteria of "simple instantiation
> for all reasonable Ada types".
> 
> Perhaps we disagree on the meaning of "reasonable" here.

So it would seem. I think SAL doesn't provide simple instantiation for
any Ada type.

-- 
Jeffrey R. Carter
PragmAda Software Engineering