Declarative vs. Procedural

The author Steven Pemberton, CWI, Amsterdam

Contents

  1. Contents
  2. About me
  3. A typical project meeting
  4. Discussing HTML
  5. Thirty years of Internet in Europe
  6. 1957: The first municipal computer (Norwich, UK)
  7. 2015: The Raspberry Pi Zero
  8. How do they compare?
  9. How do they compare?
  10. Compare
  11. Compare
  12. Moore's Law
  13. Moore's Law: not dead yet
  14. 1957
  15. Programming in the 50's
  16. The design of programming languages
  17. 1968
  18. Back to now
  19. Moore's Switch
  20. Declarative programming
  21. The first declarative definition
  22. Procedural code
  23. What does 'Declarative programming' mean?
  24. A Procedural Clock
  25. A Declarative Clock
  26. A Running Declarative Clock
  27. XForms
  28. Example: 150 person years becomes 10!
  29. Example: Insurance Industry
  30. Example: Insurance Industry
  31. Example: Insurance Industry
  32. Example: NHS
  33. Example: NHS
  34. XForms
  35. Form and content
  36. State
  37. Example
  38. Map interface
  39. Declarative Example
  40. Live tile with zoom
  41. Map
  42. Map
  43. XForms, an overview
  44. Example properties: relevant and required
  45. Relevance
  46. Relevance
  47. Relevance
  48. Events and Actions
  49. Events and Actions
  50. Content
  51. Example control
  52. Dynamic Controls
  53. Multilingual
  54. Conclusion

Declarative Amsterdam

Declarative

Amsterdam

Rosencrantz and Guildenstern Are Dead

Every exit is an entrance somewhere else. -- Tom Stoppard

Thirty years of Internet in Europe

Just over thirty years ago, the open internet started in Europe.

I would contend that we still consider the internet as something new.

But on that day thirty years ago, public computer was also only just over thirty years old.

1957: The first municipal computer (Norwich, UK)

The First Computer in Norwich

Just one of 21 cabinets making up the computer.

2015: The Raspberry Pi Zero

Raspberry Pi in Norwich

The first computer so cheap that they gave it away on the cover of a magazine

How do they compare?

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

How long do you think it would take the Raspberry Pi Zero to duplicate that amount of computing?

How do they compare?

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

How long do you think it would take the Raspberry Pi Zero to duplicate that amount of computing?

The Raspberry Pi is about one million times faster...

Compare

The Raspberry Pi is not only one million times faster. It is also one millionth the price.

A factor of a million million.

A terabyte is a million million bytes: nowadays we talk in terms of very large numbers.

Want to guess how long a million million seconds is?

Compare

The Raspberry Pi is not only one million times faster. It is also one millionth the price.

A factor of a million million.

A terabyte is a million million bytes: nowadays we talk in terms of very large numbers.

Want to guess how long a million million seconds is?

30,000 years...

A really big number...

Moore's Law

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.

Moore's Law: not dead yet

42 years of processors

1957

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.

Programming in the 50's

TypingIn the 50's the computer's time was expensive. A programmer would:

Why? Because it was much cheaper to let 3 people check it, than to let the computer discover the errors.

The design of programming languages

The first programming languages were designed in the 50s:

Cobol, Fortran, Algol, Lisp.

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.

1968

A short ten years later, there was already talk of a software crisis.

Programs being written that weren't functional, and/or weren't delivered on time.

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, we are still programming in programming 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: declarative programming.

This 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 telling the 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 × 60)
   mhand = line(mlength) rotated (m × 60)
   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

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

It is a W3C standard, 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

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. This month is its 10th anniversary.

XForms 1.1

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

XForms 2.0 is in preparation.

Form and content

Wikipedia:

"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.

State

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.

Example

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:

http://openstreetmap.org/<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("http://tile.openstreetmap.org/", 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

Source

Map

Source

Map

Source

Conclusion

For historical reasons, present programming languages are at the wrong level of abstraction: they don't describe the problem, but only one particular solution.

Declarative programming allows programmers to be ten times more productive: what you now write in a week, would take a morning; what now takes a month would take a couple of days.

Once project managers realise they can save 90% on programming costs, they will switch to declarative programming.

I believe that eventually everyone will program declaratively: fewer errors, more time, more productivity.