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| Re: null as value in OCL? [message #45783 is a reply to message #45753] |
Tue, 04 December 2007 14:50   |
Eclipse User |
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Originally posted by: cdamus.ca.ibm.com
Hi, Philipp,
OCL has a keyword for the literal null value (the "instance" of OclVoid):
null
So, null can be returned as a value for expressions of any type except a
Collection type, in which case an empty collection is more appropriate.
e.g.,
context Person
def: employerName : String =
if
self.employers->notEmpty()
then
self.employers->any(true).name
else
null
endif
Doing simply "self.employers->any(true).name" would result in OclInvalid
(see below) instead of null if there were no employers.
In your second question, if the value of a is null, then
a.b
results in OclInvalid (the singleton valud of the Invalid type, indicating
failed evaluation) and
a.b.c
also results in OclInvalid, as accessing features of OclInvalid is not
allowed, except to ask for .oclIsUndefined() or .oclIsInvalid().
HTH,
Christian
Philipp W. Kutter wrote:
> I have an OCL semantics question.
>
> How do I treat cases, where I want to return "null"?
>
>
>
> And as a second question:
> What is the result of
>
> a.b
>
> and
>
> a.b.c
>
> if "a" already results in oclUndefined?
>
> Best Regards, Philipp
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| Re: null as value in OCL? [message #46209 is a reply to message #45783] |
Mon, 10 December 2007 09:35 |
 Philipp Kutter
Messages: 306
Registered: July 2009 |
Senior Member |
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Hi, Christian.
This was very useful.
Thus I assume it is better to write
a = null
instead of
a.oclIsUndefined()
since the second would as well be true, if a is invalid.
Best Regards, Philipp
Christian W. Damus wrote:
> Hi, Philipp,
>
> OCL has a keyword for the literal null value (the "instance" of OclVoid):
>
> null
>
> So, null can be returned as a value for expressions of any type except a
> Collection type, in which case an empty collection is more appropriate.
> e.g.,
>
> context Person
> def: employerName : String =
> if
> self.employers->notEmpty()
> then
> self.employers->any(true).name
> else
> null
> endif
>
> Doing simply "self.employers->any(true).name" would result in OclInvalid
> (see below) instead of null if there were no employers.
>
> In your second question, if the value of a is null, then
>
> a.b
>
> results in OclInvalid (the singleton valud of the Invalid type, indicating
> failed evaluation) and
>
> a.b.c
>
> also results in OclInvalid, as accessing features of OclInvalid is not
> allowed, except to ask for .oclIsUndefined() or .oclIsInvalid().
>
> HTH,
>
> Christian
>
>
> Philipp W. Kutter wrote:
>
>> I have an OCL semantics question.
>>
>> How do I treat cases, where I want to return "null"?
>>
>>
>>
>> And as a second question:
>> What is the result of
>>
>> a.b
>>
>> and
>>
>> a.b.c
>>
>> if "a" already results in oclUndefined?
>>
>> Best Regards, Philipp
>
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| Re: null as value in OCL? [message #46269 is a reply to message #46209] |
Mon, 10 December 2007 13:13 |
| Eclipse User |
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Originally posted by: cdamus.ca.ibm.com
Hi, Philipp,
Yes, you raise a very good point. I can imagine that there might be cases
where this is an important distinction to make. Although, I'm not sure how
often that would arise. It's a bit like trying to account in your Java API
design for possible programming errors in client code (as signalled by
run-time exceptions); not usually worth the trouble.
Testing specifically for null might result in an attempt to use an
OclInvalid, unless you also test for a.oclIsInvalid().
cW
Philipp W. Kutter wrote:
> Hi, Christian.
> This was very useful.
>
> Thus I assume it is better to write
>
> a = null
>
> instead of
>
> a.oclIsUndefined()
>
> since the second would as well be true, if a is invalid.
>
> Best Regards, Philipp
-----8<-----
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