Declarative Programming

The author Steven Pemberton, CWI, Amsterdam


WG 2.1 and me

WG 2.1 has always been presence in the background for me.

I was an Algol Bulletin subscriber from 1973.

My second Real Job was at Manchester University working with Charles Lindsey on his Algol68 Compiler for the MU5.

I submitted corrections (that were adopted) to the A68 revised report.

Went to Amsterdam and worked with Lambert Meertens on ABC.

Developed Views system with Lambert.

Co-designed CSS, HTML, XHTML, XForms

1957 vs 2015

The First Computer in Norwich

How do they compare?

The Elliot ran for about a decade, 24 hours a day.

It would take the Raspberry Pi Zero about 5 minutes to duplicate that amount of computing.

The Raspberry Pi is about one million times faster.

It is also one millionth the price.

A factor of a million million.

In fact a million million times improvement is about what you would expect from Moore's Law over 58 years.

Except: the Raspberry Pi is two million times smaller as well, so it is much better than even that.


In the 50's, computers were so expensive that nearly no one bought them, nearly everyone leased them.

To rent time on a computer then would cost you of the order of $1000 per hour: several times the annual salary of a programmer!

When you leased a computer in those days, you would get programmers for free to go with it.

Compared to the cost of a computer, a programmer was almost free.

The design of programming languages

The first programming languages were designed in the 50s:

Cobol, Fortran, Lisp, Algol.

They were designed with that economic relationship of computer and programmer in mind.

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 tell the computer exactly what to do, in its terms, not what you want to achieve in yours.

Back to now

It happened slowly, almost unnoticed, but nowadays we have the exact opposite position:

Compared to the cost of a programmer, a computer is almost free.

I call this Moore's Switch.

Moore's Switch

Moore's Switch illustrated
Relative costs of computers and programmers, 1957-now

But, most programmers are still using languages that are direct descendants of the languages designed in the 1950's!

We are still telling the computers what to do.

Declarative programming

A new way of programming: describes what you want to achieve, but not how to achieve it..

Let me give some examples

The first declarative definition

Declarative approaches describe the solution space.

A classic example is when you learn in school that

The square root of a number n is the number r such that r × r = n

This doesn't tell you how to calculate the square root; but no problem, because we have machines to do that for us.

Procedural code

function f a: {
    x ← a
    x' ← (a + 1) ÷ 2
    epsilon ← 1.19209290e-07
    while abs(x − x') > epsilon × x: {
        x ← x'
        x' ← ((a ÷ x') + x') ÷ 2
    return x'

This is why documentation is so essential in procedural programming.

What does 'Declarative programming' mean?

A Procedural Clock

A clock in C, 1000+ lines

1000 lines, almost all of it administrative. Only 2 or 3 lines have anything to do with time.

And this was the smallest example I could find. The largest was more than 4000 lines.

A Declarative Clock

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 Running Declarative Clock

The Views System


XForms is a declarative system for defining applications. BBC Sport App

It is a W3C standard, implementations from NL, FR, BE, UK, DE, US, and in worldwide use.

Example: 150 person years becomes 10!

A certain company makes one-off 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)

Example: Insurance Industry

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.

Example: Insurance Industry

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.

[Two days later]

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

Example: Insurance Industry

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.

[Two days later]

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

XFormser: I've already programmed it!

Example: NHS

The British National Health Service started a project for a health records system.

Example: NHS

The British National Health Service started a project for a health records system.

One person then created a system using XForms.


XForms 1.0 was designed for online Forms.

After some experience it was realised that the design had followed HTML too slavishly, and with some slight generalisation, it could be more useful.

So was born XForms 1.1, a Turing-complete declarative programming language.

Implementations from Belgium, France, Germany, NL, UK, USA.

XForms 2.0 is in preparation.

Form and content


"The term form refers to the work's style, techniques and media used, and how the elements of design are implemented.

Content, on the other hand, refers to a work's essence, or what is being depicted."

This is a nearly-perfect description of XForms.

XForms applications have two parts:

  1. the model, describing the data, and its relationships;
  2. the user interface, describing the content, and connecting to the values in the model.


XForms is all about state. (Which means it meshes well with REST - Representational State Transfer).

Initially the system is in a state of stasis.

When a value changes, by whatever means, the system updates related values to bring it back to stasis.

This is like spreadsheets, but much more general.

The result is: programming is much easier, since the system does all the administrative work for you.


I've got a position in the world as x and y coordinates, and I want to display the map tile of that location at a certain zoom.

My data:

x, y, zoom

Openstreetmap has a REST interface for getting such a thing:<zoom>/<x>/<y>.png

Map interface

However, the Openstreetmap coordinate system changes at each level of zoom.

As you zoom out, there are in each axis half as many tiles, so there are ¼ as many tiles. And the interface indexes tiles, not locations.

So to get a tile:

  1. You have to know how big a tile is
  2. you have to calculate the correct index using this plus the zoom.

Declarative Example

The data: x, y, zoom

scale = 226 - zoom
tilex = floor(x/scale)
tiley = floor(y/scale)
url = concat("", zoom, "/", tilex, "/", tiley, ".png")

That is really all that is needed (modulo syntax, which looks like this:)

<bind ref="tilex" calculate="floor(../x div ../scale)"/>

That's the form. Now the content:

<input ref="zoom" label="zoom"/>
<input ref="x" label="x"/>
<input ref="y" label="y"/>
<output ref="url" mediatype="image/*"/>

and the tile will be updated each time any of the values change.

Live tile with zoom






XForms, an overview

Separation of data from UI, similar to separation of style from content with CSS, with similar advantages.

Instances of data + properties.

<instance src="data.xml"/>
<bind ref="something" property="something"/>

Properties include:

As you saw above in the map example, whenever a value changes, the related values are automatically updated, in spread-sheet style.

Example properties: relevant and required

<bind ref="address/state"
      required="../country = 'USA'"/>

This means that state is only required for the USA. The field will always be visible.

<bind ref="address/state"
      relevant="../country = 'USA'"

This means that state will only be visible for the USA, but once visible will be required.

Controls in the UI then bind to data nodes, inheriting their properties.

<input ref="address/state" label="State"/>


Here is an example. If you select USA as country, the control for state appears.



Similarly, the billing address is only relevant if it is different from the delivery address:

<bind ref="address[@type='Billing']"



Events and Actions

Typically XForms works automatically.

You can hook into the processing model to respond in special ways.

Events announce changes in the state;
effect changes to the state.

E.g. xforms-ready announces that the system has initialised (and is at stasis). You could respond to this by recording today's date and time:

<action ev:event="xforms-ready">
   <setvalue ref="today" value="now()"/>

Events and Actions

Other events announce when a value changes, or when it changes validity, relevance, etc.

Other useful actions include setting a value, and inserting and deleting elements and attributes from data.

In fact the only way to get a vanilla button to do anything is to listen for the activation event, and then respond with an action.

<trigger label="Restart">
   <action ev:event="DOMActivate">
      <setvalue ref="score" value="0"/>


The user-facing part is done with controls.

Controls are declarative too: they are designed to be device and modality independent, and describe what they do, but now how they do it, nor how they look.

Example control

For instance, the select1 control selects a value from a list of values. It can be implemented visually as a menu, or as radio buttons, and it can be implemented in other modalities as necessary.

<select1 ref="colour">

For instance these three are just different visual representations of this control:


Controls adapt

<input ref="x"/>

can adapt according to the type of x. For instance, string, integer, boolean, date:


Dynamic Controls

Controls can also get their labels and content from data.

<select1 ref="colour">
   <label ref="messages/colour"/>
   <itemset ref="colours/colour">
      <label ref="."/>
      <value ref="@code"/>

using data like

   <data xmlns="">
         <colour code="red">red</colour>
         <colour code="yellow">yellow</colour>
         <colour code="lime">green</colour>


Here it is in use:



XForms is an approach to programming by describing the problem instead of the solution.

It is work in progress; version 2 is in preparation.