Serialisation, abstraction and XML applications

Steven Pemberton, CWI and W3C, Amsterdam

About me

Researcher at CWI in Amsterdam (first non-military internet site in Europe - 1988, whole of Europe connected to USA with 64kb link!)

Co-designed the programming language ABC, that was later used as the basis for Python

At the end of the 80's built a system that you would now call a browser.

Organised 2 workshops at the first Web conference in 1994

Chaired the first style and internationalization workshops at W3C.

Chaired HTML WG for the best part of a decade.

Co-author of HTML4, CSS, XHTML, XML Events, XForms, RDFa, etc

Forms co-chair at W3C


The Views System In the late 80's I was building with a group an 'application environment'. This system had an extensible markup language, vector graphics, style sheets, a DOM, client-side you would call it a browser (it didn't use TCP/IP though). It ran on an Atari ST (amongst others).


This system had a fairly regular data system, numbers, strings, tuples, lists, unions etc.

One day when I was explaining the data types to someone, he asked "Why didn't you use SGML?"

I gave the wrong answer, because I realised later, I hadn't understood the question.

(Understanding the question is a skill in itself)

No external representations

The point of the Views system was that there were no (standard) external representations. The name sort of suggests that anyway.

We regarded the internal data as a parse-tree, that could have as many external representations -- serialisations -- as you liked.

Parsing is easy, and the relationship between serialisation and parsing is fairly close.

The person asking the question was confusing the internal form with the (necessary) external form.

Computer scientists

The problem with letting computer scientists design stuff is that we often lose sight of the correct abstractions. We are so close to the implementation that we make the mistake of conflating the abstraction with its implementation.


Let me give an example: the C programming language

C conflates the concept of 'character' with 'unit of storage'. When I was first learning C, I immediately thought "Abstraction error"!

I am presently still fighting with this abstraction error, trying to upgrade a program so that it will accept Unicode characters. It is surprisingly hard to disentangle the conflation.

Character sets

I believe that the current observable confusion between 'character set' and 'character encoding' is attributable at least in part to C's conflation.

It is quite possible that if C (and consequently Unix) hadn't had this confusion between character and storage unit, that we wouldn't have needed UTF-8 (although the paucity of metadata in Unix files holds some of the blame).


Python inherited the same abstraction error from C, again making something that ought to be really easy (handling all available characters) really hard.

Only recently has Python finally fixed this.



RGB is another example of an abstraction error.

Apart from the stupidity of exposing everyday folk to hexadecimal numbers (an abstraction error in itself), RGB is a description of an implementation of colours, not a description of colours (a sure sign of an abstraction error).

For instance: yellow is a colour, at a particular frequency. However, there is no yellow in this slide (there really isn't): what you see is an optical illusion caused by the way our eyes work. When you play two piano notes you hear both, but when you 'play' two colours, our eyes think they see a diffent colour.

Screens do not display yellow, but other devices (printers for instance) do (printers on the other hand may have no green). A good abstraction for colour would talk in colours and not in implementations of them. (HSL is an example).

Programming Languages

Programming languages largely suffer from abstraction errors as well.


In the 50's, when (so-called) high-level languages first started emerging, computers cost in the millions. Nearly no one bought computers, nearly everyone leased them.

When you leased a computer in those days, you would get programmers for free to go with it. Programmers were essentially free (in comparison with the cost of the computer).

Nowadays it is exactly the reverse of course. Computers are essentially free. It is the programmers who are expensive.

The design of programming languages

What this meant was that the computer's time was expensive.

TypingSo a programmer would write the program, copy it to special paper, give it to a typist, who would type it out, then give the result to another typist who would then type it out again to verify that it had been typed correctly the first time.

Why all this extra work? Because it was much cheaper to let 3 people do this work, than to let the computer discover the errors for you.

The Design of Programming Languages

And so programming languages were designed around the needs of the computer, not the programmer. It was much cheaper to let the programmer spend lots of time producing a program than to let the computer do some of the work for you.

Programming languages were designed so that you can tell the computer what to do, in its terms, not what you want to achieve in yours.

Almost all present-day programming languages still bear the marks of this design. They still talk in terms of the computer.


By the 1970's computers were becoming two orders of magnitude cheaper, and programmers weren't: the cost of software was starting to hurt.

The DoD did some research and discovered that 90% of the cost of software production was in debugging.

Interestingly, Fred Brookes in his book "The Mythical Man Month" reported that the number of bugs in a program is not linear with the length of a program but quadratic:

b ∝ l1.5

Which means: if a program is ten times as long, it has 30 times as many bugs, which means it costs 30 times as much to make.

Conversely, a program that is 10 times smaller costs 3% of the larger program.

The DoD's response

The DoD's response was to organise a competition to design a new programming language, which became Ada.


Here is the essence of the code used for the Views clock example.

type clock = (h, m, s)
displayed as 
   circled(combined(hhand; mhand; shand; decor))
   shand = line(slength) rotated (s × 6)
   mhand = line(mlength) rotated (m × 6)
   hhand = line(hlength) rotated (h × 30 + m ÷ 2)
   decor = ...
   slength = ...
clock c
c.s = system:seconds mod 60
c.m = (system:seconds div 60) mod 60
c.h = (system:seconds div 3600) mod 24

A clock in C

A clock in C, 4000+ lines


XForms originally designed as a replacement for HTML Forms.

The first version did roughly that, but thanks to generality in the design we realised that with small changes it could do much more: you could use it for applications.

The resultant design


What this concretely means is that the data is physically separated from the controls in the form. The data is placed in the head of the document, and the controls bind to the data.

<html xmlns="">
   <model xmlns="">
         <data xmlns="">
<body> ...

Or initialised from external sources

What this concretely means is that the data is physically separated from the controls in the form. The data is placed in the head of the document, and the controls bind to the data.

<html xmlns="">
   <model xmlns="">
      <instance src="html://www..."/>
<body> ...

Controls and initial values

Controls in the body refer to values in the data instance(s) using XPath expressions:

<input ref="year">...
<input ref="event[1]/title/@language">...

The controls can be initialised by putting values in the data.


Relationships between, and restrictions on, values can be specified in the model, allowing dependent values to be calculated automatically and data checking to be performed on the client rather than on the server.

<bind nodeset="year" constraint=". &gt; 1752"/>
<bind nodeset="state" required="../country = 'USA'"/>
<bind nodeset="age" calculate="../thisyear - ../birthdate/year"/> <bind nodeset="birthdate" type="date"/>


Values can be exposed in the document itself, using an output control:

The result for the year <output ref="year"/> is ...

Input and output


Intent-based Controls

Controls are intent-based, by expressing what the control should do, rather than how it should look. So a control like this:

<select1 ref="colour">

can be represented in different ways depending purely on styling.

THe same control three times, each with a different styling

Controls are abstract

Here are three identical controls, just styled differently


Initial experience

Particularly in the use of fixed strings rather than (potentially) calculated values for such things as the submission URI.

As a consequence this restricted what was possible with the language.

XForms 1.0 → 1.1

As a consequence, XForms 1.1 addressed these shortcomings

The resultant language turned out to be far more than a forms language, but a declarative application language.

Since XForms has input, output, and a processing engine, XForms is Turing-complete, and much more than just forms is now possible with the language.


Because you specify what you are trying to achieve and not how to achieve it, there is far less administration to worry about in programs. This means: shorter programs.

How much shorter?

One correspondent who was converting a large collections of apps for a company from Javascript to XForms reported that the XForms were about ¼ the size.

So that means we should expect the production time and cost to reduce by one eighth, about an order of magnitude. And indeed we do.

Data point: 150 person years becomes 10!

A certain company makes BIG machines (walk in): user interface is very demanding — traditionally needed 5 years, 30 people.

With XForms this became: 1 year, 10 people.

Do the sums. Assume one person costs 100k a year. Then this has gone from a 15M cost to a 1M cost. They have saved 14 million! (And 4 years)

Data point: A true story

Manager: I want you to come back to me in 2 days with estimates of how long it will take your teams to make the application

[2 days later]

Javascript man: I'll need 30 days to work out how long it will take to program it

XForms man: I've already done it.



XForms 1.1 → XForms 2.0

XForms 2.0 (first public working draft) will be published today or tomorrow.

Biggest changes: XPath 2.0, AVTs

This means: everything in the display is under control of your data. Even @class.

Another change: accept data in other formats than XML

Data Opacity


An obvious data format widely in use on the web is JSON.

There are several mappings defined in both directions between XML and JSON, but largely because JSON can only represent a subset of what XML can represent, many of the mappings are cumbersome, and unnatural.

So what we are doing is making a mapping that makes the XPath selectors as natural as possible.

JSON in XForms

During the design phase we went through several iterations

Key realisation: since the aim is only to address existing JSON stores, it is not necessary to be able to convert every possible XML representation into an equivalent JSON representation, only the reverse.

This reduces the task considerably, since it means several features of XML do not have to be addressed, such as namespaces, attributes, and mixed content.


Some of the requirements for a mapping from JSON to XML for XForms included:

Opaque data

Ideally, an XForm processing JSON data shouldn't have to know which data format has been used; so that, for instance, data such as

{"company":"", "locations":[{"city": "Amsterdam"},{"city": "London"}]}

with the right mapping could be selected with XPath selectors like


In this way data could be loaded using content negotiation, and will work whether the data comes in as XML or JSON.


Implementation of the mapping is relatively trivial:

At the point where an implementation normally receives a document of type application/xml (or similar), either during initial instance initialisation from an external resource, or as the return value of a submission, if the media type of the resource is application/json, the resource can be parsed, and transformed to an equivalent XML instance.

The media type can be recorded as an attribute of the root element, so that it can be reused if the instance is to be resubmitted as JSON.

Other formats

Clearly this method can be extended to other datatypes such as VCARD and iCalendar. For instance an iCalendar value such as


can be transformed to



XForms has proved its value as an application language.

XForms 2.0 continues its path to more generality.

XForms resources

A tutorial:

For an overview of all features, elements and attributes of XForms 1.1, see the XForms 1.1 Quick Reference.

It's not easy reading, but the final arbiter in questions of doubt is the XForms 1.1 Specification.

Living XForms 2.0 Draft:

The implementation used for the examples in this talk is XSLTForms.