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RE: Comments on Extending and Versioning Languages: Strategies

From: Dave Orchard <orchard@pacificspirit.com>
Date: Fri, 28 Mar 2008 15:24:50 -0700
To: <noah_mendelsohn@us.ibm.com>
Cc: <www-tag@w3.org>
Message-ID: <02ff01c89122$8a086260$6d01a8c0@amer.bea.com>

I have incorporated all of these changes in the latest versions, including what we talked about on the phone today.

Cheers,
Dave
 

> -----Original Message-----
> From: noah_mendelsohn@us.ibm.com [mailto:noah_mendelsohn@us.ibm.com] 
> Sent: Monday, February 25, 2008 5:44 PM
> To: David Orchard
> Cc: www-tag@w3.org
> Subject: Comments on Extending and Versioning Languages: Strategies 
> 
> In preparation for the F2F, I'm doing another readthrough of 
> the Versioning Strategies document [1].  Here are some 
> comments, in document not priority order:
> 
> --------
> 
> Section 1.2:
> 
> > Some typical backwards- and forwards-compatible changes:
> > 
> >     *  adding optional components ( in XML, this is 
> generally elements 
> > and/or attributes)
> 
> Optional elements are only compatible changes if their 
> presence doesn't modify or conflict with the semantics of an 
> existing element.  If I add a component that changes the 
> semantics of some other component (e.g. ignore this, 
> mustUnderstand, currencyUnit=Pesos) then the change is often 
> not compatible (and especially not forwards compatible), even 
> if the new component is syntactically optional.
> 
> --------
> 
> Section 2:
> 
> > In broad terms, the strategies to versioning fall into a numberof
> classes
> 
> (editorial) I think that should be:  "In broad terms, strategies for 
> versioning fall into a number of classes" 
> 
> -or-
> 
> "In broad terms, strategies for versioning may be grouped 
> into a number of 
> classes" 
> 
> 
> --------
> 
> Section 2:
> 
> > Different application domains will choose different approaches. 
> 
> (Editorial) I don't think a domain is something that 
> typically makes a 
> choice.  Suggest something along the lines of:
> 
> "Different choices may be appropriate for different applications"
> 
> -or-
> 
> "Different choices may be appropriate for different 
> application domains."
> 
> 
> --------
> 
> Section 2:
> 
> > The dependencies makes it imperative to plan for versioning 
> from the 
> start.
> 
> Two concerns:
> 
> 1) It's not clear what dependencies you're talking about,
> 
> 2) (grammar) "dependencies" is plural, so I think the phrase 
> should be 
> "The dependencies >make< it imperative
> 
> --------
> Section 2.1:
> 
> > "Big bang" is a very coarse-grained approach to versioning. It 
> > establishes a single version identifier, either a version 
> > number or namespace name, for an entire text. 
> 
> I'm confused about this at a few levels.  First, it seems to 
> imply that 
> using a single namespace for all of the content in one 
> particular instance 
> document (a text) is a mistake.  I don't think that's what 
> you meant.  I 
> suspect that the point you're making really doesn't have to 
> do directly 
> with namespaces or version identifiers, but has to do with 
> policies for 
> knowing how much of a document a consumer can continue to 
> safely process 
> given that some of the content is not what was expected.  
> That issue can 
> arise, and there are solutions that are possible, without any 
> reference to 
> either namespaces or version identifiers, I think.  Just as 
> one example, 
> most any syntactic marker that indicates that processing of 
> contents is 
> optional (such as mustUnderstand="false" in SOAP) seems to achieve 
> non-big-bang versioning without any mention of either namespaces or 
> version ids.
> 
> If I'm right about this, then the remainder of 2.1 needs to 
> be rewritten 
> to define big bang in a way that's less focussed on version 
> identifiers.
> 
> -----------
> Section 2.2 starts with a hyperlink to "Forwards compatible". 
>  Shouldn't 
> that hyperlink be where the term is first used?  Note that 
> there is text 
> that seems quite close to another definition of forwards and backward 
> compatible in the bullets immediately under the start of section 2.
> 
> -----------
> Section 2.2.2:
> 
> > Forwards compatible evolution of a language means that 
> > producers of texts in language should be able 
> 
> (typo) should be "in >a< language"
> 
> ----------
> Section 2.2.2:
> 
> > A supreme example of the benefits of extensibility is HTML. 
> 
> Seems a bit strong.  Suggest:
> 
> "A good example of the benefits of extensibility is HTML. "
> 
> ----------
> Section 2.2.2.1:
> 
> Formatting bug.  The header is indendented under the Good 
> Practice Note 
> above.  This is a bit in either the XSL or CSS stylesheets.  
> I've found 
> that this happens if a GPN is the last thing in a section.  
> You can beat 
> it by putting an empty paragraph in the XML just after the 
> GPN, and ahead 
> of the header for 2.2.2.1. 
> 
> ---------
> Section 2.2.2.1:
> 
> > "By the definition of Extensibility, there is a mapping from 
> > all texts with additional syntax to texts without."
> 
> This formulation keeps coming up, and I still believe it's much too 
> limiting.  Version 1 need not know how to map all extension 
> content to 
> some text in which the extension content doesn't appear:  it 
> must have 
> some default rule for interpreting the extension content.  
> That rule need 
> not be to ignore it entirely for all applications.  For 
> example, there's 
> no reason a version 1 storage application shouldn't store the entire 
> contents of a document it receives, including markup it doesn't fully 
> understand.  Of course, it will need to understand any markup that 
> controls the storage operation, but other content it can 
> blindly store. 
> Maybe it does more than that:  maybe by default it removes 
> whitespace from 
> extension content and stores the results.  Many important and 
> interesting 
> applications do extensibility this way.  I'm really reluctant 
> to imply 
> that the only default processing rule is a mapping that makes the 
> extension content disappear.
> 
> ------------
> Section 2.2.2.1:
> 
> > "Must Accept Unknowns Rule: Consumers MUST accept any text 
> > portion that they do not recognize."
> 
> This is way to strong in my opinion.  Let's say I decide to make an 
> extension to XML that allows attributes to be quoted with 
> backquotes as 
> well as forward quotes:
> 
>         <element a=`backquotedattr`/>
> 
> Are you really saying that XML violates good practice because XML 
> processors will reject this "text that it does not 
> recognize".  Taken to 
> its extreme, this GPN seems to imply that an XML processor 
> should quietly 
> accept a FORTRAN program as just one giant "text that it doesn't 
> recognize".
> 
> In fact, almost all extensible languages have syntactic rules 
> that are 
> fixed and that cannot be changed without breaking 
> compatibility.  Within 
> those rules, extensible languages encourage processors to accept and 
> provide some default interpretation to content that wasn't 
> specified in 
> detail in the original specification.  (Note that the first 
> versions of 
> HTML do allow for <img> just as they allow for <frog> and 
> <banana>; what 
> they don't do is call out those tags specifically or given them any 
> distinguished interpretation).
> 
> -----------
> Section 2.2.2.1:
> 
> > Preserve existing information Rule: An Extensible Language MUST
> > require that any texts with extensions MUST be compatible with 
> > a text without the extensions.
> 
> For the reasons stated above, I don't think this is right in 
> all cases 
> either.  I think an extensible language must provide for a default 
> interpretation of any extensions that may be present.  Why that 
> interpretation should be equivalent to some text without the 
> extension 
> isn't motivated in the finding at all.  In fact, I think SOAP 
> is a counter 
> example, if you're willing to grant that headers are extensions.  I'm 
> fairly sure that if in SOAP I have a header that's 
> mustUnderstand="false", 
> I need not process it locally, but I think that if I'm an 
> intermediary I 
> in general MUST relay that header downstream, even though I don't 
> understand it.  If the GPN requires that the message be 
> equivalent to one 
> without the header, how can I do that?  So, that's another 
> example of a 
> default processing rule for extension content:  don't ignore 
> it, relay it.
> 
> Ah... I see now that you acknowledge this use case:
> 
> > Must Accept and Preserve Unknowns Rule (Must Accept variant 2):
> > Consumers MUST accept and preserve any text portion that they 
> > do not recognize.
> 
> I feel pretty strongly that this flat out contradicts the 
> first GPN quoted 
> above.  Either the text is equivalent to one without the 
> extension OR you 
> must preserve it.  I don't think you can have it both ways.  The HTTP 
> proxy in your example is not acting as if the document had 
> some equivalent 
> without the extensions.  I would drop that first GPN, 
> probably replacing 
> it with one that requires a default processing rule for 
> extension content.
> 
> -----------
> Section 2.2.2.1
> 
> > In tree based languages, which includes all markup languages,
> 
> I can't invent a markup language that would, for example, 
> represent graphs 
> more general than trees?
> 
> -----------
> Section 2.2.2.1: GENERAL COMMENT
> 
> I think that we usually use Good Practice Notes for things that you 
> usually SHOULD do.  Some of the GPN's in this finding seem to be 
> alternatives that are advertised as being more or less equally good 
> choices.  I think you need to do some explaining as to which 
> GPNs are to 
> be followed as good practice more or less all the time, and which are 
> intended as alternatives.  For example "Must Accept ALL" vs. 
> "Must accept 
> container" seem to be just two choices, and they are mutually 
> exclusive. 
> 
> ----------
> Section 2.2.2.3
> 
> > Languages MUST provide a substitution model for version 
> identifiers for 
> forwards-compatible evolution.
> 
> (do we put MUST's in GPN's?   Uusually, SHOULD feels better in a GPN. 
> Maybe it's OK, not sure)
> 
> Anyway, as an example of this you give:
> 
> > There could be an algorithmic approach. For version numbers, 
> > one could say that version numbers will only have a "major" 
> > change if there is an incompatible change. For example, version
> > 1.1 of a language is by definition compatible with version 1.0 
> > and version 2.0 is incompatible. 
> 
> I understand why this is a common strategy and sometimes a 
> good one.  I'm 
> less clear on why it's an example of a substitution rule.  
> What's being 
> substituted for what when I say "gee, I can't process this document 
> because it's got a newer major version number than I understand?"
> 
> --------
> Section 3.0:
> 
> > there are some key requirements the language designer consider 
> > in choosing a strategy and design.
> 
> (typo) there are some key requirements the language designer >should< 
> consider in choosing a strategy and design.
> 
> --------
> Section 3.0:
> 
> > It is sometimes desirable to prevent 3rd parties from extending
> > languages, but it does happen. 
> 
> > An example may be a tightly constrained security environment 
> > where distributed authoring is considered a "bug" rather 
> than a feature.
> 
> The first sentence doesn't parse unambiquously.  Is it 
> "preventing" that 
> does happen, or "extending".  On first reading, I assumed you meant 
> extending, then when I saw the next sentence I thought probably not. 
> Either way, I think the sentence should be reworded.  How about:
> 
> "Allowing 3rd parties to extend a language can  be extremely 
> valuable for 
> applying the language to new use cases, but in other situations such 
> extensibility may be undesirable.  For example, there may be 
> situations in 
> which security concerns dictate that the language specification be 
> centrally maintained."
> 
> --------
> Section 3.3:
> 
> > If so, a substitution mechanism is required for forwards 
> compatibility.
> 
> Suggest:  "If so, a default processing rule is required for forwards 
> compatibility."  (same reason as discussed earlier)
> 
> ------
> 
> I've skimmed the rest of the draft, but not reviewed it in as 
> much detail. 
>  I think the general nature of my comments would be similar.  
> I hope they 
> are helpful.  Thank you.
> 
> Noah
> 
> [1]    
> http://www.w3.org/2001/tag/doc/versioning-strategies-20070917.html 
> 
> --------------------------------------
> Noah Mendelsohn 
> IBM Corporation
> One Rogers Street
> Cambridge, MA 02142
> 1-617-693-4036
> --------------------------------------
> 
> 
> 
> 
> 
Received on Friday, 28 March 2008 22:25:40 GMT

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