enumeration and defaults

[John Cowan]

Simon St.Laurent scripsit:

> I've found myself in a somewhat odd situation, where I'd like to be able to
> include empty (i.e., no value) as a choice in a list of enumerated
> possibilities.  It doesn't seem possible.  (Empty is not a token.)
> 
> It might be nice to declare:
> <!ATTLIST myElement
> 	myAtt (0|1|2) #IMPLIED>
> 
> but it isn't clear to me what the implications are.  If I just write:
> <myElement />
> 
> I haven't provided a 'wrong' value for myAtt, but I haven't in fact
> provided a value that matches an entry in the list.

That is all right: in fact, it's commonplace.  A declaration like

	<!ATTLIST OL
		compact (compact) #IMPLIED>

such as is found in the DTD for XHTML 1.0 Transitional,
means that either <OL> or <OL compact="compact"> is legal.

> I may simply be lacking a key set of assumptions, but I feel like I could
> read the spec either way.  Does no default value mean that there is no
> value to check against constraints, or does it imply a null value that will
> violate constraints of this sort?

The former.

-- 
John Cowan                                   cowan at ccil.org
       I am a member of a civilization. --David Brin


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