xml - DTD required elements ordering

Question

I want to have list of required elements in any order defined in dtd, but have no idea how I could do this.

For example, I have following definition:

<!ELEMENT parent (child1, child2, child3)>

This dtd declaration will successfully validate following piece of xml:

<parent>
   <child1></child1>
   <child2></child2>
   <child3></child3>
</parent>

But in following case xml will not pass validation:

<parent>
   <child2></child2>
   <child1></child1>
   <child3></child3>
</parent>

One of the possible solution is to declare

<!ELEMENT parent (child1 | child2 | child3)>

But in this case one of the childs might be missing though validation will be successful.

I need correct dtd element declaration for the case when list of required elements can be present in any order.

Answer 1

ChrisF is wrong to say you can't do this (but kudos for checking the spec!); DevNull [now known as Daniel Haley] is right to say you can.

There is, however, one complication: the content model given by DevNull for parent violates the determinism rules of XML. Informally, these rules say the parser must know, without lookahead, which token in the content model each element in the document matches. If in a parent element the parser sees a child1, it can't know without lookahead whether it has just matched the first or second occurrence of child1 in DevNull's content model. This is an error (but as it happens it's an error that processors are not obligated to report -- it's an error that means "all bets are off, the processor may do anything here, including fail to notice anything is wrong").

A content model for parent that obeys the determinism rules can be formed by factoring out common prefixes, thus:

<!ELEMENT parent ( 
                   (child1, ( (child2,child3)
                            | (child3,child2)))
                 | (child2, ( (child1,child3)
                            | (child3,child1)))
                 | (child3, ( (child1,child2)
                            | (child2,child1)))
                 ) >

This is less convenient than declarations for this model can be in other schema languages, and for more than three elements it's error prone and extremely tedious to do by hand. But it's not impossible to declare the content model you want with DTDs. Or perhaps I should say it's impossible only for DTD authors who are incapable of putting up with any inconvenience.

Note also that unless the sequence of children carries some information, some design authorities argue strongly that it's better to fix a sequence.