Re: language problem

[cjh@petsd.UUCP]

[]
In article <997@petsd.UUCP> joe@petsd.UUCP (Joseph M. Orost) writes:
>In article <8703261730.AA28726@taurus> amiram@TAURUS.BITNET.UUCP writes:
>>A colleague of mine, Yossi Veler of AITECH has come up with the following
>>program in Ada, which seems to create a serious problem.
>>
>>procedure boolsub is
>>   subtype bool is boolean range true..true;
>>   type arr is array(1..10) of bool;
>>   a : arr := (1..10 => true); -- this seems like the only legal value
>>begin
>>   a := not a;
>>-- Here a(1)=a(2)=...=a(10)= FALSE !!!! No exception occurs etc.
>>   a := (1..10 => false);
>>-- This does cause an exception
>>end boolsub;
>>
>>The program seems legal : we inspected the LRM and also the implementers
>>guide, and we ran it on both the DDC and VERDIX compilers. It seems that
>>a combination of innocent features in Ada produces a result that seems to
>>contradict with the basic philosophy of the language, that is an object
>>posseses a value which is not in the appropriate type.
>
>Our compiler (C3Ada R00-01.02/Beta) correctly raises CONSTRAINT_ERROR on the
>statement "a := not a;".
>
Why is this statement illegal?  Well... what is the type of the
expression "not a" ?  It can't be "arr" because its components do not
satisfy the subrange constraint.  And there isn't any available type
with boolean components.  The type "arr" may look like a subtype of
"ARRAY(1..10) of boolean", but it isn't.  On arrays, subtype
constraints affect the bounds of indexes, not the values of
components.

Regards,
Chris

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