Re: Ada Enumerations

[kst@king.cts.com (Keith Thompson)]

Robert Dewar (dewar@merv.cs.nyu.edu) wrote:
> Keith says
> 
> <<attribute (but not 'Enum_Val), but it's not portable.  The 'Valid
> attribute should be usable to validate external data, but straightforward
> use of it is erroneous by RM95-13.9.1; see also AI95-00167.>>
> 
> The 'Valid attribute *is* usable for this purpose.

I should have been more specific.  At least one sensible usage of 'Valid
is not usable, given the current language definition.

Suppose an external interface provides integer values which are expected
to take on any of a specified set of values.  I declare an enumeration
type with a representation clause that matches the externally imposed
representation.  I then use Unchecked_Conversion to convert from the
integer type to my enumeration type.  Since I can't trust the external
interface 100%, I use 'Valid to check whether I have a valid value.

Unfortunately, RM95-13.9.1(12) says that a call to an instance of
Unchecked_Conversion is erroneous if the result is scalar and has an
invalid representation, so I get unpredictable behavior before I even
have a chance to use 'Valid.  I can work around this by wrapping the
scalar in a record, but I shouldn't have to.

It's true that most or all implementations will behave "sensibly" (as
the AARM vaguely encourages them to do), but I'd rather avoid erroneous
execution altogether than depend on this.

See also the minutes of the November ARG meeting in St. Louis.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com <*>
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San Diego, California, USA
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