Re[2]: Optional attributes



Date view Thread view Subject view Author view Attachment view

From: John Daniels (jd@syntropy.co.uk)
Date: Tue 18 May 2004 - 20:26:49 BST


Hi Joaquin,

> We are already distinguishing between the absence of a slot and an empty
> slot, which is good.

I like clear distinctions, but I'm not convinced this distinction is
helping.

All I'm trying to do is make sure there's a simple way to distinguish
two cases:

1) when I want to say "every X must have a valid value for
its attribute p" (and I don't count "null" - whatever it means - as a
valid value) and

2) when I want to say "it's fine to have an X without having a value
for its p attribute".

Pretty simple stuff (I thought).

--John

====
Original message
From: Joaquin Miller <jm-miller-at@sbcglobal.net>
Date: 18 May 2004
Subject: Optional attributes
----
Apologies if i jerked that chain too hard.

Next, we can discuss whether null is a value.

One possible decision is that null is not a value; instead, the word, 
'null' is used to mention the absence of any value.

In that case, it is often necessary for tools to use, in the 
implementation, some distinguished item to indicate the absence of any 
value.  (An example is when the design does not permit removal and addition 
of slots, and a slot is to contain an integer.  In that case, the 
representation of some outlying integer value might be used to indicate, 
not that integer, but the absence of any value.)  We need to distinguish 
model or concept from tool implementation, in order to discuss this.

We are already distinguishing between the absence of a slot and an empty 
slot, which is good.

[Don't ask me whether I am using 'slot' in the sense that it is used in the 
formal semantics proposed for UML by the 2U submitters, or am using it in 
the sense of a memory location in an implementation.  I'm trying to dodge 
that.  I'm not even sure whether the 2U "semantics" (i quote the word from 
what they wrote) is a formal semantics, or about a class of 
implementations.  I stand by to have my chain jerked now.  (I bet we will 
also benefit from distinguishing between the concept, empty slot, and 
implementations of that concept.)]

Cordially,

Joaquin



At 10:40 AM 5/18/2004, you wrote:
>Hi Joaquin,
>To clarify what I meant by quotation marks: you can intepret each of my 
>"null" as a 'null'
>I have learnt my lesson and promise never to misuse quotation marks again :-)
>
>Cheers,
>    Shane
>Joaquin Miller wrote:
>
>>The use of the term, 'null value,' makes me nervous.  And i reckon i 
>>don't know why folks put quotation marks around 'null.'  (Are those scare 
>>quotes, suggesting we don't mean null in the literal sense?)
>>
>>I expect we could make better progress on this if we could all be sure we 
>>share a precise (chain jerk intended) understanding of what we mean when 
>>we write 'null'  (and what we intend to convey when we write ' "null"  ').
>>
>>The MOF specification authors took a lot of care to make sure that the 
>>meaning of 'null' is single, that 'null' is used consistently, and that 
>>it was clear that (as used in MOF) null is not a value.
>>
>>http://www.omg.org/cgi-bin/doc?formal/00-04-03
>>
>>Cordially,
>>
>>Joaquin
>
>
>
>
>
>To remove yourself from this list please mail puml-list-request@cs.york.ac.uk
>with a message containing the word "unsubscribe".





To remove yourself from this list please mail puml-list-request@cs.york.ac.uk
with a message containing the word "unsubscribe".

Date view Thread view Subject view Author view Attachment view