James Giles wrote in message ...
>Martin Dowie wrote:
>> James Giles wrote:
>...
>>> In most Ada implementations, as for most other languages, all
>>> the bit patterns in the representation of an INTEGER data type
>>> are valid integer values.
>>
>> That's not entirely true, the standard states Integer must include the
>> range �2**15+1 .. +2**15�1 - thus (usually) leaving -2**15 as a
>> possible 'default uninitialized' value.
>
>Well I allowed for that possibility.  I made no absolute
>assertion.  I said "most Ada implementations".  In fact,
>most language definitions *allow* implementations
>to implement integers with a lot of flexibility.  Most
>actual implementations use all the bit patterns their
>underlying hardware allows and *don't* use any of
>them for NOI (Not An Integer).

Exceptioins include WATFOR and WATFIV, PL/C.
In BASIC, all variables have an initial value of zero.
In IBM's PL/I [PC and mainframe] provision is made
to detect uninitialized fixed-point decimal variables via hardware,
and this has been the case of their mainframe PL/Is since 1965.
The pre-decessor of WATFOR (IBM 704?) used hardware
(parity bit set wrong) for uninitialized variables.

>  And, let's not forget
>character data types (does your actual implementation
>have a value that means NOC - Not A Character?).
>What about booleans?  Ada also has fixed point types,
>enumerations, records, etc.  Does your actual implementation
>catch uses of undefined variables for all those?  Indeed,
>does it even use NANs from the floating point hardware to
>detect uninitialized REALs?
>
>It's true: nearly all languages permit implementations
>to internally represent data with additional memory
>for the purpose of detecting and reporting uses of
>undefined variables (and for other reasons - like the
>value of a variable that was defined, but the expression
>involved an unhandled exception).  In formal semantics,
>the idea is called a lifted domain.  Applying it in software
>is expensive at run-time.

Not necessarily.  It isn't in the case of Salford Fortran
(now Silverfrost), nor in WATFOR and WATFIV, PL/C,
and fixed-point decimal in IBM's PL/I compilers.