W3C home > Mailing lists > Public > w3c-sgml-wg@w3.org > June 1997

Re: Parameter entities vs. GI name groups

From: Michael Sperberg-McQueen <U35395@UICVM.UIC.EDU>
Date: Fri, 20 Jun 97 11:56:06 CDT
Message-Id: <199706201819.OAA03677@www10.w3.org>
To: W3C SGML Working Group <w3c-sgml-wg@w3.org>
This is just to register (again) my views on parameter entities in XML.

1 The claim that they are hard to implement is simply bogus.  I can't
believe I'm hearing it from people whose technical judgement I take
seriously.

The only thing I can see that's hard to implement about PEs in 8879 is
the odd requirement that they can begin and end at *almost any* but *not
quite any* white space; this requires (a) that in a yacc/lex parser the
parameter entities be expanded by the lexer, but that (b) the parser
include various ad hoc rules for checking that (i) the PE begins and
ends in legal places and (ii) the PE does not cross any forbidden
boundaries (in a content model, its parentheses match; it doesn't begin
and end in different declarations; etc.); also (c) that the yacc
grammar be encrufted with ps nonterminals and the like.


All of this is gone in XML.  To implement PEs as defined in XML-lang
970331, all that's needed is a very simple pattern, similar to that
required by matching parentheses:

* In the parser, when transcribing the grammar, replace each %
expression in the right-hand side of a rule with a single non-terminal
with a name beginning with the prefix 'pe_'.  Define the pe_
non-terminal as

  pe_foo : PE_START optional_s foo_expression optional_s PE_END

where 'foo_expression' is the expression governed by the % operator,
after recursive substitution of nested %-expressions.

* In the lexer, insert rules like the following

"%"{Name}";"    { yyin = OpenEntity(yytext);
                  /* i.e. set yyin to the appropriate external data
                     stream or internal buffer; push old yyin onto stack
                  */
                  return PE_START;
                }
<<EOF>>         { yyin = CloseEntity(); /* pop the entity stack */
                  if (yyin==NULL) {
                     yyterminate(); /* if we just closed the outer
                                       entity */
                  } else {
                     return PE_END;
                  }
                }

That's all.  So what part of this is supposed to be so hard that a
computer science graduate student is supposed to need more than fifteen
minutes for it?  (Oh, right.  Doing this means finding and reading
the relevant part of the flex manual.  OK:  45 minutes for finding the
place in the manual; 20 minutes to read and grasp it; 10 minutes to
write the code.)

This is not rocket science; it's not even news, since this came up in
April on xml-dev, when Norbert Mikula was having trouble making NXP's
parser generator handle the PE rules.  He solved it, and since for
better or worse he *is* a graduate student in computer science, I think
the evidence shows (a) that PEs do not present impossible implementation
loads, and (b) the canary in the mineshaft is still alive and chirping.

Frankly, I think the fact that an amateur like myself can figure
this out qualifies it for 'not-that-hard' status.  I studied
comparative literature, for crying out loud!  Can PEs really be
easy for a fluffy like me and hard for hardened veterans like Tim
Bray?

2 Losing PEs means giving up entirely on our second design goal:  "XML
shall support a wide variety of applications", because it means XML will
be usable primarily as a delivery mechanism for material maintained
outside XML.  I haven't seen any reason to give up on this design goal.

3 Losing PEs entirely means:

  - no conditional marked sections in DTDs or elsewhere (and thus no
serious customizable DTDs in XML, only frozen snapshots of Full-SGML
DTDs after application of the customization layer)
  - no method for embedding DTD modules in DTDs constructed of such
modules (the TEI is not the only one, but the TEI is the one I am
thinking about right now)
  - no method of embedding the ISO entity sets in a suitable XMLified
version

The proponents of this meataxe have yet to point out as an advantage
that losing PEs reduces our work load: we no longer have to bother
making an XML version of ISOLat1, ISOLat2, etc., because they won't be
usable anyway.

In that case, do we need *general entities* any more, either?

And all this because PE_START ... PE_END bracketing is too hard?

Perhaps we should get rid of start-tag and end-tag bracketing, too?
Then we could just call it troff and be done with it.

4 The effect of simplifying PEs varies with the proposal:

4a Lose the 8879 restrictions.  A non-starter:  valid XML documents,
including their prologs, need to be valid SGML.  I agree that this
would be an improvement for 8879, since the rules in 8879 don't seem
to me to achieve their goal of preventing obfuscation (they can
always be evaded by adding another level of obfuscation, so in fact
to the extent people do want to do bizarre and reprehensible things
in the DTD, the 8879 rules make matters worse by preventing simple,
straightforward expression of bizarre and reprehensible constructs,
while allowing bizarre and convoluted expressions of those same
constructs -- a lesson not likely to be lost on the devious minds who
want to do those things in the first place).  I assume it's too late for
the TC, though (we missed our chance:  if we had had the % notation in
XML-ling-961114, we would have noticed that the TC could help us out
here).

4b Restrict parameter entity references to occurring *between*
declarations, or as conditional-section keywords, and require PEs to
contain some integral number of declarations, or the keywords 'INCLUDE'
or 'IGNORE'.  I believe this is James Clark's suggestion.

This amounts to restricting the % operator to productions
29 (markupdecl) and 56-57 (conditional sections) and losing it in

  39 (where PEs can currently provide for indirection in the element's
generic identifier, the tag-omissibility needed by most Full-SGML
systems, and the content specification),
  42-45 (PEs within content models)
  46-47 (gi, attribute definition, parts of the attribute definition)
  52 (notation names)
  53 (enumerated types within an attribute's Atttype)
  54 (attribute default and whether it's FIXED or not)
  63 (entity name and replacement text)
  67 (notation name on entity declarations)

This approach means XML can at least use XMLified versions of the ISO
entity sets, but I can't imagine any serious DTD maintenance taking
place in XML.  Any DTD which needs easy customization or maintenance
(and I don't know of any other kind except toy DTDs) will have to be in
Full-SGML, not XML; as a result, no authoring or markup management or
data enrichment can usefully take place in XML, only in Full SGML.

There goes goal 2.  This approach does have the advantage of making
XML a usable language for delivery of material on the Web, and a
reasonable toy language for markup.  Without this, XML isn't even a
good toy language.

4c Restrict the use of PEs more than 8879 does, and more than XML
currently does, but not as much as 4b; I think this is Bernhard
Weichel's suggestion.

This is at least less damaging than the other options, and I've never
wanted to use a parameter entity for the notation name on an entity
declaration (though now that I think of it, that might solve a problem
I'm now having in a production system ... ).  So maybe restricting the
use of PEs to (say) productions 39, 42-47, and 54 wouldn't be a
catastrophe.  But I don't think it will satisfy those who claim
(simultaneously!) that PEs are too complicated, but that the various
namespace proposals we've seen are *not* (!), and (b) it reduces the
regularity of the grammar, where right now you can virtually always use
PEs to replace (a) any single token, and (b) any major top-level part of
a declaration.

5 Losing PEs in the syntax means losing DTDs in practice.

This one's a multi-step argument; bear with me.

If we lose PEs, I am confident that no DTD designer of sound mind and
not forced to it with whips and cattle prods will ever attempt to build
a DTD in XML except by automatic translation from a Full-SGML DTD.

Since XML DTDs won't be customizable, except in trivial and error-prone
ways, XML won't be much use as a format for document maintenance.

Since XML won't be much use for document maintenance, it will be used
primarily (in my case, solely, just like HTML) for document delivery on
the Web.

Since serious information publishers will tend to validate documents
before serving them on the Web, XML document instances for which DTDs
actually exist will tend to have been validated against that (Full-SGML)
DTD before publication; strictly-XML processors will hardly ever need to
validate any documents, because strictly-XML processors will hardly ever
see documents being changed or maintained, only frozen documents
converted from a full-SGML system.

Since most XML documents will be prevalidated, and most XML processors
will choose to rely on that prevalidation, the document type declaration
in an XML document will not be of much more use than the document type
declaration in an HTML document today.  It will, that is, be purely
decorative (a way to detect the cognoscenti among information providers)
and fundamentally irrelevant to processing.

This is a very serious topic, and I dissent from Eve's view that we need
to gather usage experience before including PEs in XML.  If PEs are not
in version 1.0, there won't be any usage that requires PEs, ever,
because we will have driven all such usage to Full SGML or over to other
non-standard subsets of SGML, and set up a neon sign rivaling those in
Times Square, saying XML DOES NOT CARE ABOUT DOCUMENT MAINTENANCE OR
VALIDATION.  DTDS IN XML ARE STRICTLY DECORATIVE.  SO FORGET XML, ALL
YOU WHO ARE LOOKING FOR WAYS TO *WORK* WITH YOUR DOCUMENTS.

Me, I think DTDs are not merely decorative.  If the XML spec loses PEs,
it will (for my purposes) be XML, not the DTD, which becomes decorative.

XML should be a real language for real documents. That means it has
to have real, serious DTDs.  That means PEs.  Without it, I don't see
that we've got ourselves a serious improvement over the current system:
maintain in SGML, down translate, deliver in &web-markup-lang-of-month;.

Apologies to those who don't like the sight of grown men screaming in
agony on the network.

-C. M. Sperberg-McQueen

*****************************************************************
* "SGML doesn't have lookahead because ŹN.N.┘ couldn't make     *
* lookahead work in his parser.  And now XML is going to lose   *
* parameter entities because ... ?"                             *
*                        -Name withheld to protect the Guilty   *
*****************************************************************
Received on Friday, 20 June 1997 14:19:40 EDT

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