Marc de Graauw - Random Notes

Larry Masinter believes there is a risk in making ‘the “meaning” of [a] language depend on operational behavior‘.

In discussions like this there should be a sharp distinction between natural and computer languages. Larry says meaning ‘depends on what the speaker intends and how the listener will interpret the utterance’ - that’s a valid viewpoint for natural languages, but for markup (html, xml etc.) or programming languages it’s really stretching definitions. There is no speaker, and no direct intentions - unless software has intentions. For markup document instances, there are usually three indirect intentions: those of the language designer, the software builder and the end-user producing the instance. I’m not sure an approach based on intentions is workable for computer languages at all - whose intention does the document reflect? How do we know the intentions of end-user, software builder and language designer do not conflict? How do we know their intentions at all?

Besides, semantics in this way can be described in prose or first order logic. I don’t think first order logic is appropriate for most language design (it’s not human readable, only logician-readable), and prose may be fine but is often inexact. I spoke at length at Balisage 2008 on this issue, and discussed it with David Orchard and others, and I believe an operational approach is often appropriate for computer languages. Why? It certainly is problematic for natural language semantics. There may be an utterance, and a meaning, without any behavior - I can say ‘North Korea tested an A-bomb’, and that has a meaning, even if no-one exhibits any behavior whatsoever after my words. For computer languages, it’s different. It’s possible to describe operational behavior as testable conditions - if I send a system test message A, it should do X. And that makes operational semantics a nearly perfect fit for computer languages - the semantics can be tested with a test suite. That’s enough. No need for real-time behavior after receiving each message.

Larry writes: ‘…standards organizations are in the business of defining languages … and not in … telling organizations and participants … how they are supposed to behave’. That’s far too lenient. If I render text marked <b> as italic, and <i> as bold, I’m not following your spec. Period. If I implement a language specification, and claim conformance, I’m not free to do as I choose. This is even more true in other contexts - business, healthcare, insurance. There organizations exchanging documents sign contracts, and are legally bound to do certain things upon receiving documents - paying after ordering, for instance. Larry’s free-for-all approach to language definitions does not apply to the real world. It’s only true that behavior should not be constrained any further than necessary - but without behavioral consequences, document exchange is meaningless indeed.

Obsolete, please see the latest version

Version 1

An attempt to define syntactical and semantical compatiblity of versions in a formal way. Much derives from the writings of David Orchard, especially the parts on syntactical forward and backward compatibility (though my terminology differs).

1. Let U be a set of (specifications of) software processable languages {L1, L2, … Ln}
   1. This axiomatization does not concern itself with natural language
2. The extension of a language Lx is a set of conforming document instances ELx = {Dx1, Dx2, … Dxn}
   1. Iff ELx = ELy then Lx and Ly are extensionally equivalent
      • Lx and Ly may still be different: they may define different semantics for the same syntactical construct
   2. Iff ELx ⊂ ELy then Lx is an extensional sublanguage of Ly
   3. Iff ELx ⊃ ELy then Lx is an extensional superlanguage of Ly
   4. D is the set of all possible documents; or the union of all ELx where Lx ∈ U
3. For each Lx ∈ U there is a set of processors Px = {Px1, Px2, … Pxn} which implement Lx
   1. Each Pxy exhibits behaviour as defined in Lx
   2. Processors can produce and consume documents
   3. Each Pxy produces only documents it can consume itself
   4. At consumption, Pxy may accept or reject documents
4. The behaviour of a processor Pxy of language Lx is a function Bxy
   1. The function Bxy takes as argument a document, and its value is a processor state
      • We assume a single processor state before each function execution, alternatively we could assume a <state, document> tuple as function argument
   2. If for two processors Pxy and Pxz for language Lx for a document d Bxy(d) = Bxz(d) then the two processors behave similar for d
      • Two processor states for language Lx are deemed equivalent if a human with thorough knowledge of language specification Lx considers the states equivalent. Details may vary insofar as the language specification allows it.
      • Processor equivalence is not intended to be formally or computably decidable; though in some cases it could be.
   3. If ∀d ( d ∈ ELx → Bxy(d) = Bxz(d) ) then Pxy and Pxz are behaviourally equivalent for Lx
      • If two processors behave the same for every document which belongs to a language Lx, the processors are behaviourally equivalent for Lx.
5. An ideal language specifies all aspects of desired processor behaviour completely and unambiguously; assume all languages in U are ideal
6. A processor Px is an exemplary processor of a language Lx if it fully implements language specification Lx; assume all processors for all languages in U are exemplary
   1. Because they are (defined to be) exemplary, every two processors for a language Lx are behaviourally equivalent
   2. ELx = { d is a document ∧ Px accepts d }
   3. The complement of ELx is the set of everything (normally, every document) which is rejected by Px
   4. The make set MLx = { d is a document ∧ Px can produce d }
7. A language Lx is syntactically compatible with Ly iff MLx ⊂ ELy and MLy ⊂ ELx
   • Two languages are syntactically compatible if they accept the documents produced by each other.
   1. A language Ln+1 is syntactically backward compatible with Ln iff MLn ⊂ ELn+1 and Ln+1 is a successor of Ln
      • A language change is syntactically backward compatible if a new receiver accepts all documents produced by an older sender.
   2. A language Ln is syntactically forward compatible with Ln+1 iff MLn+1 ⊂ ELn and Ln+1 is a successor of Ln
      • A language change is syntactically forward compatible if an old receiver accepts all documents produced by a new sender.
8. A document d can be a member of the extension of any number of languages
   1. Px is an (exemplary) processor of Lx, Py is an (exemplary) processor of language Ly
   2. Two langauges Lx and Ly are semantically equivalent iff ELx = ELy ∧ ∀d ( (d ∈ ELx ) → Bx(d) = By(d) )
      • If two languages Lx and Ly take the same documents as input, and their exemplary processors behave the same for every document, the languages are semantically equivalent.
      • Two languages can only be compared if their exemplary processors are similar enough to be compared.
      • Not every two languages can be compared.
      • “Semantic” should not be interpreted in the sense of “formal semantics”.
9. The semantical equivalence set of a document d for Lx = { y ∈ ELx | Bx(d) = Bx(y) }
   1. Or: SLx,d = { y ∈ ELx | Bx(d) = Bx(y) }
      • The semantical equivalence set of a document d is the set of documents which make a processor behave the same as d
      • Semantical equivalence occurs for expressions which are semantically equivalent, such as i = i + 1 and i += 1 for C, or different attribute order in XML etc.
   2. d ∈ SLx,d
   3. Any two semantical equivalence sets of Lx are necessarily disjunct
      • If z ∈ SLx,e were also z ∈ SLx,d then every member of SLx,e would be in SLx,d and vice versa and thus SLx,d = SLx,e
10. A language Ly is a semantical superlanguage of Lx iff ∀d ( d ∈ MLx → By(d) = Bx(d) )
    1. For all documents produced by Px, Py behaves the same as Px
       • Equivalence in this case should be decided based on Lx; if Ly makes behavioural distinctions which are not mentioned in Lx, behaviour is still the same as far as Lx is concerned
    2. It follows: ∀d ( d ∈ MLx → ∃SLy,d ( SLy,d ⊂ ELy ∧ ( SLx,d ∩ MLx ) ⊂ SLy,d ∧ By(d) = Bx(d) ) )
       • For any document produced by Px, the part of its semantical equivalence set which Px can actually produce, is a subset of the semantical equivalence set of Py for this document
    3. For all d ∈ ELx ∧ d ∉ MLx there may be many equivalence sets in Ly for which By(d) ≠ Bx(d)
       • In other words: for documents accepted but not produced by Px, Ly may define additional behaviours
    4. Lx is a semantical sublanguage of Ly iff Ly is a semantical superlanguage of Lx
11. A language Ln+1 is semantically backward compatible with Ln iff Ln+1 is a semantical superlanguage of Ln and Ln+1 is a successor of Ln
    1. An old sender may expect a newer, but semantically backward compatible, receiver to behave as the sender intended
    2. A language Ln is semantically forward compatible with Ln+1 iff Ln+1 is a semantical sublanguage of Ln and Ln+1 is a successor of Ln
    3. Semantic forward compatibility is only possible if a language loses semantics; i.e. it’s processors exhibit less functionality, and produce less diverse documents
    4. A processor cannot understand what it does not know about yet

Update: I learned from the TAG list that Dan Connolly already proposed using #it or #this for the same purpose, and  Tim Berners-Lee proposed using #i to refer to oneself in a similar way. My idea therefore was not very original, and since I regularly read the TAG list and similar sources, it’s even possible I read the idea somewhere and (much) later thought of it as one of my own - though if this happened it was certainly unintentional.

There is a very simple solution to the entire hash-versus-slash debate: whenever you would want to identify anything with a hashless URI, suffix it with #referent. The meaning of x#referent is: I identify whatever x is about. And x is simply an information resource (about x#referent).

The httpRange-14 debate is about what hashless URI’s (without a #) refer to: can they refer only to documents (information resources) or to anything, i.e. persons or cars or concepts. Is it meaningful to say http://www.marcdegraauw.com/marcdegraauw/ refers to ‘Marc de Graauw’. Or does it now identify both a web page and a person, and is this meaningful and/or desirable?

Hash URI’s aren’t thought to be much of a problem in this respect. They have some drawbacks however. It may be desirable to retrieve an entire information resource which describes what the referent of the URI is. And putting all identifiers in one large file makes the file large. Norman Walsh did this: http://norman.walsh.name/knows/who#norman-walsh identifies Norman Walsh, and the ‘who’ file got big. So Norm switched to hashless URI’s: http://norman.walsh.name/knows/who/norman-walsh identifies Norman Walsh. The httpRange-14 solution requires Norm to answer to a GET on this URI with a 303 redirect, in this case to http://norman.walsh.name/knows/who/norman-walsh.html, which does not identify Norman, but simply is an information resource.

If we use the #referent convention, I can say: http://www.marcdegraauw.com/marcdegraauw.html#referent identifies me. And http://www.marcdegraauw.com/marcdegraauw.html is simply an information resource, which is about me. Problem solved.

If I put http://www.marcdegraauw.com/marcdegraauw.html#referent in a browser, I will simply get the entire http://www.marcdegraauw.com/marcdegraauw.html resource, which is a human readable resource about http://www.marcdegraauw.com/marcdegraauw.html#referent. Semantic Web software which understands the #referent convention will know http://www.marcdegraauw.com/marcdegraauw.html#referent refers to a non-information resource (except when web pages are about other web pages) and http://www.marcdegraauw.com/marcdegraauw.html is simply an information resource. Chances of collision of the #referent fragment identifier are very small (Semantic Web jokers who do this intentionally apart) and even in the case of collision with existing #referent fragment identifiers the collision seems pretty harmless. The only thing the #referent convention does not solve is all the existing hashless URI’s out there which (are purported to) identify non-information resources.
In Semantic Web architecture, there is no need ever for hashless URI’s. The #referent convention is easier, more explicit about what is meant, retrieves a nice descriptive human-readable information resource in a browser, along with all necesssary rdf metadata for Semantic Web applications.

I just ran into a disaster scenario which Mark Baker recently described as the way things should be: a new message exchange without schema validation. He writes: “If the message can be understood, then it should be processed” and in a comment “I say we just junk the practice of only processing ‘valid’ documents … and let the determination of obvious constraints … be done by the software responsible for processing that value.” I’ll show this is unworkable, undesirable and impossible (in that order).

I’ve got an application out there which reads XML sent to my customer. The XML format is terrible, and old - it predates XML Schema. So there is no schema, just an Excel file with “AN…10″ style descriptions and value lists. It is built into my software, and works pretty well - my code does the validation, and the incoming files are always processed fine.

Now a second party is going to send XML in the same format. Since there is no schema, we started testing in the obvious way - entering data on their website, exporting to XML, send it over, import in my app, see what goes wrong, fix, start over. We have had an awful lot of those cycles so far, and no error-proof XML yet. Given a common schema, we could have a decent start the first cycle. Check unworkable.

So I wrote a RelaxNG schema for the XML. It turned out there where hidden errors which my software did not notice. For instance, there is a code field, and if it has some specific value, such as ‘D7′, my customer must be alerted immediately. My code checks for ‘D7′ and alerts the users if it comes in. The new sender sent ‘d7′ instead. My software did not see the ‘D7′ code and gave no signal. I wouldn’t have caught this so easily without the schema - I would have caught it in the final test rounds, but it is so much easier to catch those errors early, which schema’s can do. Check undesirable.

Next look at an element with value ‘01022007′. According to Mark, if it can be understood, it should be processed. And indeed I can enter ‘Feb 1, 2007′ in the database. Or did the programmer serialize the American MM/DD/YYYY format as MMDDYYYY and is it ‘Jan 2, 2007′? Look at the value ‘100,001′ - perfectly understandable, one hundred thousand and one hundred - or is this one hundred and one thousandth, with a decimal comma? Questions like that may not be common in an American context, but in Europe they arise all the time - on the continent we use DD-MM-YYYY dates and decimal comma’s, but given the amount of American software MM/DD/YYYY dates and decimal points occur everywhere. The point is the values can apparently be understood, but aren’t in fact. One cannot catch those errors with processing logic because the values are perfectly acceptable to the software. Check impossible.

In exchanges, making agreements and checking if those agreements are met is essential. Schema’s wouldn’t always catch the third kind of errors either, but they provide a way to avoid the misinterpretations. The schema is the common agreement - unless one prefers to fall back to prose - and once we have it, not using it for validation seems pointless. Mark Baker makes some good points on over-restrictive validation rules, but throws out the baby with the bathwater.

Published Subject Identifiers face a couple of serious problems, as do all URI-based identifier schemes. A recent post of Lars Marius Garshol reminded me of the - pardon the pun - subject. I was pretty occupied with PSI’s some time ago, maybe now is the moment to write down some of my reservations about PSI’s. PSI’s are URI’s which uniquely identify something, which is - ideally - described on the web page you see when you browse to the URI - read Lars’ post for a real introduction.

First, PSI’s solve the wrong problem. The idea of using some schema to uniquely identify things is hardly novel. Any larger collection of names or id’s for whatever sooner or later faces the problem of telling whether two names point to the same whatever or not. So we’ve got ISBN numbers for books, codes for asteroids and social security numbers for people. They are supposed to designate a single individual or single concept. The problem with all those identifier schemes is not the idea, but the fact that they get polluted over time. Through software glitches and human error and - mostly - intentional fraud a single social security number may point to two or more persons and two or more social security numbers may point to a single person. I can’t speak for non-Dutch realms, but the problem here is very real, and I assume given human nature it is not much different elsewhere. So the real problem is not inventing a identification scheme, the problem is avoiding pollution. This may seem like unfair criticism - no, PSI’s don’t solve famine and war either - but it does set PSI’s in the right light - they are not a panacea for identity problems.

Second, PSI’s are supposed to help identification for both computers - they will compare URI’s, and conclude two things are the same if their URI’s are equivalent - and for humans, through an associated web page. The trouble is what to put on the web page. Let’s make a PSI for me, using my social securitiy number: http://www.sofinummer.nl/0123.456.789. Now what can we put on the web page? We could say “This page identifies the person with the Dutch social security number 0123.456.789″ - but that is hardly additional information. If we elaborate - “This page identifies Marc de Graauw, the son of Joop de Graauw and Mieke Hoendervangers, who was born on the 6th of March 1961 in Tilburg, the Netherlands, the person with the Dutch social security number 0123.456.789″ we get into trouble. I could find out for instance I was not actually born in Tilburg, but my that my parents for some reason falsely reported this as my birthplace to the authorities. Now even if this were the case, 0123.456.789 would still be my social security number, and it would identify me, not someone else. But if we look at the page, we have to conclude http://www.sofinummer.nl/0123.456.789 identifies nobody, since nobody fits all the criteria listed. The same goes for any other fact we could list - I could find out I was born on another day, to other parents et cetera. The only truly reliable information, the one piece we cannot change, is “This page identifies the person who has been given the Dutch social security number 0123.456.789 by the Dutch Tax Authority”, which hardly is no information at all beyond the social security number itself. All we’ve achieved is prepending my social security number with http://www.sofinummer.nl/, and this simple addition won’t solve any real-world problem. The problem is highlighted by Lars’ example of a PSI, the date page for my birthday, http://psi.semagia.com/iso8601/1961-03-06. This page has no information whatsoever which could not be conveyed with a simple standardized date format, such ‘1961-03-06′ in ISO 8601.

Third, in a real-world scenario, establishing identity statements across concepts from diverse ontologies is the problem to solve. Getting everybody to use the same single identifier for a concept is not feasible. Take an example such as Chimezie Ogbuji’s work on a Problem-Oriented Medical Record Ontology, where it says about ‘Person’:

cpr:person = foaf:Person and galen:Person and rim:EntityPerson and dol:rational-agent

In FOAF a person is defined with:”Something is a foaf:Person if it is a person. We don’t nitpic about whethet they’re alive, dead, real or imaginary.”

HL7’s RIM defines Person as:

“A subtype of LivingSubject representing a human being.”

and defines LivingSubject as:
“A subtype of Entity representing an organism or complex animal, alive or not.”

In FOAF persons can be imaginary and ‘not real’, in the RIM they cannot. Now Chimezie wisely uses and which is an intersection in OWL, so his cpr:Person does not include imaginary persons. And for patient records, which are his concern, it doesn’t matter: we don’t treat Donald Duck for bird flu, so for medical care the entire problem is theoretical. But what about PSI’s: could we ever reconcile the concepts behind foaf:Person and rim:EntityPerson? Probably not: there are a lot of contexts where imaginary persons make sense. So if we make two PSI’s, foaf:Person and rim:EntityPerson, our subjects won’t merge, even when - in a certain context such as medical care - they should. Or we could forbid the use of foaf:Person in the medical realm, but this seems to harsh: the FOAF approach to personal information is certainly useful in medical care.

Identity of concepts is context-dependent. The definitions behind the concepts don’t matter much. Trying to find a universal definition for any complex concept such as ‘person’ will only lead to endless semantic war. Usually natural language words will do for a definition (but you do need disambiguation for homonyms). Way more important than trying to establish a single new id system with new definitions, are ways to make sensible context-dependent equivalences between existing id systems.

15 Sep 2008: Comments are closed

Mark Baker misses an important distinction in “Validation Considered Harmful” when he writes:

“Today’s sacred cow is document validation, such as is performed by technologies such as DTDs, and more recently XML Schema and RelaxNG.

Surprisingly though, we’re not picking on any one particular validation technology. XML Schema has been getting its fair share of bad press, and rightly so, but for different reasons than we’re going to talk about here. We believe that virtually all forms of validation, as commonly practiced, are harmful; an anathema to use at Web scale.”

Dare Obasanjo replied in “Versioning does not make validation irrelevant“:

“Let’s say we have a purchase order format which in v1 has a element which can have a value of "U.S. dollars" or "Canadian dollars" then in v2 we now support any valid currency. What happens if a v2 document is sent to a v1 client? Is it a good idea for such a client to muddle along even though it can't handle the specified currency format?"