# One way to define the XPath data model

[6 April 2010; addenda and copy editing 7-8 April 2010]

After discovering earlier this year that the definition of the XPath 1.0 data model falls short of the goal of guaranteeing the desired properties to all instances of the data model, I’ve been spending some time experimenting with alternative definitions, trying to see what must be specified a priori and what properties can be left to follow from others.

It’s no particular surprise that the data model can be defined in a variety of different ways. I’ve worked out three with a certain degree of precision. Here is one, which is not the usual way of defining things. For simplicity, it ignores attributes and namespace nodes; it’s easy enough to add them in once the foundations are a bit firmer.

Assume a non-empty finite set S and two binary relations R and Q on S, with the following properties [Some constraints are shown here as deleted: they were included in the first version of this list but later proved to be redundant; see below] :

1. R is functional, acyclic, and injective (i.e. for any x and y, R(x) = R(y) implies x = y).
2. There is exactly one member of S which is not in the domain of R (i.e. R(e) has no value), and exactly one which is not in the range of R (i.e. there is one element e such that for no element f do we have e = R(f)).
3. Q is transitive and acyclic.
4. The transitive reduction of Q is functional and injective.
5. It will be observed that R essentially places the elements of S in a sequence without duplicates. For all elements e, f, g, h of S, if Q includes the pairs (e, f) and (g, h) and if g falls between e and f in the sequence defined by R (or, more formally, if the transitive closure of R contains the pairs (e, f), (e, g), and (g, f)), then h also falls between e and f in that sequence.
6. The transitive closure of the inverse of R (i.e. R-1*) contains Q as a subset.
7. The single element of S which is not in the domain of R is also neither in the domain nor the range of Q.

It turns out that if we have any relations R and Q defined on some set S, then we have an instance of the XPath 1.0 data model. The nodes in the model instance, the axes defining their interrelations, and so on can all be defined in terms of S, R, and Q.

For the moment, I’ll leave the details as an exercise for the reader. (I also realize, as I’m about to click “Publish”, that I have not actually checked to see whether the set of constraints given above is minimal. I started with a short list and added constraints until S, R, and Q sufficed to determine a unique data model instance, but I have not checked to see whether any of the later additions rendered any of the earlier additions unnecessary. So points for any reader who identifies redundant constraints in the list given above.)

[When I did check for minimality, it turned out that several of the constraints included in the list above are redundant. The fact that relation R is functional and injective, for example, follows from the others shown. Actually it follows from a subset of them. The deletions above show one way of reducing the number of a priori constraints: they all follow from the others and can be dropped. None of the remaining items follows from the others; if any of them are deleted, the constraints no longer suffice to ensure the properties required by XPath.]

# Alloy as logical desk calculator

[26 March 2010]

Long ago I used a wonderful file-oriented database system called Watfile, which was designed as a sort of desk-calculator for data. It was designed for personal use, not industrial-strength data management, and its designers successfully resisted the temptation to add more features and more power at the cost of a more complex user interface. Watfile was to a full enterprise-class database as a desk calculator of the 1960s was to … oh, perhaps to Fortran. For suitable problems, the ease of setup far outweighed any considerations of power or completeness.

The experience of using Watfile for data manipulation tasks established in my mind the class of ‘desk-calculator-like’ packages for various kinds of problem.

Today I experimented with Alloy as a sort of logical desk calculator, and I’m happy to report that it passed the test with flying colors.

For reasons I won’t go into here, I’ve wondered a bit recently what it might look like to apply the technique of distinctive-feature analysis (originally developed for phonological descriptions of sound systems of language) to writing systems. When I sat down a few months ago with pencil and paper to see if I could devise a smallish set of typographic features which could (say) distinguish the twenty-six letters of the alphabet as I was taught it in first grade, I rapidly found that working solely with pen and paper made me impatient: it was too tedious to look at the set of features already identified and see which letters could not yet be distinguished (because they had the same value for all the features in question).

When I came back to this problem this afternoon, I thought for a few minutes about what questions I’d like to be able to ask the machine. Given a specified set of graphemes (as a first exercise, I chose the lower-case alphabet) and a specified set of binary features (does the letter have an ascender? a descender? a vertical stroke? a full or partial circle or bowl? Is the stroke to the left of the bowl? …), with information about which graphemes have the feature in question, I want to be able to ask, first, whether a particular set of features suffices to distinguish each individual grapheme? Or are there two or more graphemes which have the same value for all features in the set? And of course, if there are such sets of indistinct graphemes, what are they?

It occurred to me to solve the problem in Prolog; it would take just a relatively simple set of Prolog predicates to do what I wanted. But as I was preparing to launch X Windows, so that I could launch Prolog, I realized that I already had the Alloy Analyzer running. And so I wrote the predicates I wanted in Alloy instead of Prolog, to see whether it would work.

The upshot is: yes, it worked, and it was probably a bit easier to do than it would have been in Prolog. When I was thinking about how to set up the problem in Prolog, I found myself wondering about the best data representation to choose, and so on, almost as much as about the structure of the problem. I won’t say that Alloy posed no analogous problems — I did have to think for a moment or two about the best way to describe graphemes and distinctive features. But the high level of abstraction afforded by Alloy made the decision feel less binding, and made me feel a bit more comfortable experimenting. (It sounds strange to put it this way: after all, Prolog’s high level of abstraction is one of its great design strengths. But Prolog is also designed to be an efficient and effective programming language, which means that some details are exposed which have only procedural significance, and sometimes you find yourself thinking about them, even in situations where questions of execution efficiency don’t arise.

In very short order, I found it possible to define a suitably abstract representation of graphemes and features, specify some useful functions and predicates for asking the questions described above, and specify a small set of features (ten) which have a certain degree of typographic plausibility and which suffice to distinguish the graphemes in question. (Ten binary features for twenty-six graphemes may seem high, given that the theoretical minimum is only five, and that ten bits suffice to distinguish a thousand objects, not just twenty-six. But writing, like natural language, has some redundancy. Feature sets used to analyse natural language sound systems are also often very inefficient.) The visualization tools did not prove very helpful, but the Evaluator feature of the Alloy Analyzer was a great help.

If I pursue this work any further, I probably will put it into Prolog, where the interactive interface for expression evaluation is perhaps a bit more convenient than in Alloy. But it’s nice to know that Alloy can be used for this kind of problem, too.

Interested readers can find both the generic definitions and the specific graphemes and features for lower-case Latin letters (as used in Anglophone countries) on the Black Mesa Technologies web site.

# The axes of XPath

[25 March 2010; error noticed by Dimitre Novatchev fixed 29 March 2010]

Steve DeRose and I have been discussing the XPath [1.0] data model recently (among other things), and in the course of the discussion an interesting observation has emerged.

it’s obvious that some of the axes in XPath expressions are inverses of each other, and also that some are transitive closures of others (or, going the other way, that some are transitive reductions of others). What surprised me a little was that (if for the moment you leave out of account the `self` and the `XYZ-or-self` axes, the attribute axis, and the namespace axis [and also `preceding` and `following`) all of the XPath axes fit naturally into a pattern that can be represented by three squares. (Will table markup work here? I wonder.) The first square represents the up/down axes:

Up/down
` parent ` ` ancestor `
` child ` ` descendant `

The next square covers sibling relations. Unlike `parent` and `child`, which are just short-hand for single steps along the up or down axis, XPath provides no syntactic sugar for `preceding-sibling :: * [1]` and `following-sibling :: * [1]`, so I’ve invented the names “nextsib” and “prevsib” (marked with a star here to signal that they are invented):

Sideways
*prevsib ` preceding-sibling `
*nextsib ` following-sibling `

The third square describes overall document order; again, I’ve invented names for the single-step relations [note that the names used here for the transitive relations are given by XPath 2.0; XPath 1.0 doesn’t provide notation for them]:

Overall document order
*prevnode ` >> `
*nextnode ` << `

[In the first version of this post, the right-hand columns were labeled `preceding` and `following`, but Dimitre Novatchev reminded me gently that these axes do not in fact correspond to document order: `preceding` excludes andestors and `following` excludes descendants. That’s a plausible exclusion, since no one in their right mind would say that chapter one of Moby Dick precedes the first paragraph of Moby Dick. Contains, yes; precedes, no. In fact, I remember getting into an argument with Phil Wadler about this, early on in the days of the XML Query working group, not realizing (a) that the document ordering he was describing was actually prescribed by XPath 1.0, nor (b) that saying that ancestors precede their descendants in document order didn’t mean that the ancestors would have to be present on the `preceding` axis. Thank you, Dimitre! And sorry, Phil!]

In each table row, the relation on the right is the positive transitive closure of the one on the left, and the one on the left is the transitive reduction of the one on the right.

In each table column, the relations in the top and bottom rows are inverses of each other.

The tables make it easy to see that it suffices to take a single pair of relations on nodes as primitive (e.g. `child` [or better `first-child`] and `nextsib`, or `parent` and `prevsib`); everything else in the tree can be defined in terms of the two primitive relations. (It’s not completely clear to me yet whether any two relations will do as long as they are from different tables, or not. Taking `nextnode` and `parent` seems to work, as does the pair `nextnode` and `child` but `nextnode` and `first-child` seems to pose problems — why can `child` be replaced by `first-child` in some situations but not others? Hmm.)

There seem to be implications for the formalization of the data model (which is how we got here in the first place), but maybe also for teaching new users how to think about or learn XPath.

# A small gotcha in Alloy’s closure operator

[24 March 2010]

Consider the following Alloy model:

```sig Node {}
one sig gl { r, s : Node -> Node }{ s = *r }
run {}```

It has no instances, and the Alloy Analyzer comments laconically “Predicate may be inconsistent.” But the similar model below does have instances. Why?

```sig Node {}
one sig gl { r, s : univ -> univ }{ s = *r }
run {}```

I ran into this phenomenon today, when I was trying to do some work relating to the definition of document order in the XPath data model. It’s convenient to have both a relation like the successor relation of arithmetic on the natural numbers, which gives you the next item in document order, and a relation like the less-than-or-equals relation of arithmetic, which is the reflexive transitive closure of the successor relation. And if you want to have both, you will want to specify, as is done above, that the one is the reflexive transitive closure of the other. And when you do, it’s a bit alarming to be told (and mostly very quickly) that nothing in your model has any instances at all.

It took me a couple hours to reduce the problem to the terms shown above (there were several complications which looked more likely to be the cause of the trouble, and which took time to eliminate), and then a little more time to realize that declaring the relations in question as `Node -> Node` or as `univ -> univ` made a difference.

I invite the interested reader, if a user of Alloy, to consider the two models and explain why one has instances and the other doesn’t. No peeking at what follows until you have a theory.

Found it? If you found it without spending a couple hours on it, my hat’s off to you.

The `*` operator produces the reflexive transitive closure of a binary relation `r` by essentially taking the union of the positive transitive closure of `r` and the identity relation `iden`. That is, for all relations `r`, `*r` is defined as `^r + iden`.

The problem is that `iden` covers everything in the model, including the `gl` (globals) object and the automatically included integers. And the upshot is that `s` cannot satisfy the constraint `s = *r` while also satisfying its declaration (`Node -> Node`).

In his book Software Abstractions, Daniel Jackson remarks on the oddity of * including ‘irrelevant’ tuples in the relation, but (as he points out) it almost never matters, because in many context the irrelevant tuples are dropped out during evaluation of the containing expression. The consequence is that it’s possible to work with Alloy (as I have obviously been doing) with a mental model in which * is somehow smart enough to add only those tuples which are ‘relevant’ to the relation whose closure is being taken. That mental model proves to be an over-complexification.

One reason I like Alloy a lot is that it allows the user to operate at a fairly high level of abstraction, if you can just find the right level. Working in Alloy presents fewer of the small issues of syntax and data typing and so on that can bedevil attempts to explore problems even in high-level programming languages, and so you mostly get to your results a lot faster. But I guess no formal system is ever completely free of cases where the user stares blankly at the screen for some indefinite period of time, trying to figure out why the system is producing such a counter-intuitive and obviously wrong result.

In the words of the old MVS error message: `Probable user error. Solution: correct and re-submit.`

# How formal can you get?

[26 January 2010; updated a URI 28 Jan 2010]

In a recent comment on an earlier post here, David Carlisle wrote of proofs concerning properties of the XPath 1.0 data model

Itâ€™s not clear how formal any such proof could be, given that the XPath model and even more so, XML itself are defined so informally.

It’s true that the XPath spec defines the data model in prose and not in formulas. But on the whole it’s relatively formal and explicit prose, and I would expect it to be relatively easy to translate the prose into a formal notation.

As a test of that proposition, I recently improved a shining hour or four by translating section 5 of the XPath 1.0 spec into Alloy, the modeling tool developed by Daniel Jackson and others in the MIT Software Design Group. (I would describe Alloy briefly here, but I’ve written about Alloy often enough in this blog that I won’t describe it again now; regular readers of this blog will already know about it, and those who don’t can search the Web for information, or search this blog for what I’ve said about it before.)

The full Alloy model of XPath 1.0 is available from the Black Mesa Technologies web site. It will be of most interest, of course, for those familiar with Alloy, or wanting to learn more about it. But Alloy’s formalism is simple enough that anyone with a taste for formal methods will find it easy going, and the document paraphrases all the important things in English prose.

The net result, I think, is that there are (unsurprisingly) some places where the definition of the XPath 1.0 data model could or should be more explicit, and a few places where it seems a rule or two given explicitly is strictly speaking redundant, but by and large the definition is pretty clean and seems to mean what its creators meant it to mean. By and large, the definition of the data model is formulated without reliance on the XML spec (there are a few places where this separation could be cleaner), so that the informality of the XML spec (or what I prefer to think of as its programmatic promiscuity regarding models) does not in fact make it hard to formalize the XPath 1.0 data model.

In the current version, there are a couple of rough bits that I hope to sand down before I use this model in any other contexts. The ones I’m currently aware of are these:

• The definition of the parent relation does not require that whenever parent(c,p) is true, either child(p,c) or attribute(p,c) is true. The XPath 1.0 spec doesn’t say this explicitly, but I think its authors probably felt that it would be pedantic to say something like that for a relation with a name like parent.
• Similarly, the relations parent and ch are not guaranteed acyclic, though I think the original spec assumes that the names parent and child make clear that the relations should be acyclic. (But see the song “I’m my own Grandpa” and the Alloy models illustrating it, developed in Daniel Jackson’s Software Abstractions and shipped with Alloy in the samples directory.)
• The predicate precedes intended to model document order is defined recursively; this is not legal in Alloy. So a conventional relation on nodes will need to be defined instead, with appropriate constraints. It is still not clear whether the rules given in the spec suffice to make the order total (at least in the absence of multiply occurring children); if they don’t, I’d like to propose an additional predicate which has that effect.