Friday, 24 January 2014

"Earth falls toward a black hole and everyone dies"

In my post "The Programmer as Essayist", I quoted the famous computer scientist Donald Knuth on literate programming. I now want to give an example from my own work. It's a spreadsheet that generates science-fiction plots. Try it! Clicking on that link will open Excel if you're in Windows, and you can then run the spreadsheet immediately or save it for later. To run it, scroll to the top. The pale yellow column headed Story contains the output. To change it, click on cell B4, below the label saying Recalculate. The cell will then display as a menu containing one option, also called Recalculate. Select this option (even though it's already selected). That will activate a chain of random-number generators, and you'll get a new story in column A.

I wrote this spreadsheet using literate programming. This is where it came from.

In the anthology The Year's Best Science Fiction No. 5 (edited by Harry Harrison and Brian Aldiss, Sphere, 1972), there are stories. That's usual for anthologies. There is also a story generator. That's less usual. The story generator is called "The Science Fiction Horror Movie Pocket Computer" and is written by Gahan Wilson.

Or perhaps I should say drawn, because the SFHMPC is a flowchart. It consists of boxes containing phrases such as "Earth", "is struck by a giant comet and", "destroyed", "is attacked by", "bug(s)", "which (who)", "look upon us only as a source of nourishment", "and are", "radioactive", "and", "can be killed by", "the atomic bomb (The End)". The boxes are connected by lines. Start at the top, follow it round, choose an exit at each box, and you end up with a cheesy SF plot that Hollywood would be proud of.

Wilson's SFHMPC went through several incarnations in my hands before I got it working in Excel. The first was in felt-pen and card. I used to teach Artificial Intelligence at Oxford, and I also used to publicise AI at Freshers' Fair. There's not actually much AI in the SFHMPC, but it's fun and it demonstrates that computers can do interesting things with data other than numbers. So I drew a huge copy of it on a monster piece of card bought from the Broad Canvas art shop, and used it as one of my Freshers' Fair props, together with such tantalisers as the Dennett and Hofstadter "A Conversation with Einstein's Brain", a wodge of output from Eliza, and a photocopy of John Varley's short story "Overdrawn at the Memory Bank".

A bit later, I implemented the SFHMPC in Prolog. That's because Prolog was my main language for teaching AI, and I wanted an amusing demo to show how Prolog can represent networks.

Then I put that up on my Web site, as a PHP script running SWI-Prolog. Go here to try it, to see the Prolog source, or to read about how to call Prolog from PHP.

Then even later, as part of my spreadsheet research, I implemented the SFHMPC in Excel. Now, this is an unusual spreadsheet. That's because the natural way of generating a story from the flowchart would be by recursion. Like this:
To generate a story, go to the box at the top of the flowchart, then generate the rest of the story.
To generate the rest of the story, output the contents of the box you're at, then choose one of its exits, go to the corresponding box, and generate the rest of the story. Stop if there are no exits.
That, indeed, is how the Prolog version works.But you can't do recursion in Excel, unless you descend into Visual Basic, so how did I make the spreadsheet version work? That's a topic for another posting. What I want to say in this one is that I wrote it in Excelsior, a language I've implemented that compiles into Excel.

And you can write Excelsior programs in two modes. One is similar to the way you'd write in a language such as Pascal or Java. Everything is assumed to be source code unless you precede it by comment delimiters, in which case it's comment. When you feed this into Excelsior, it generates a spreadsheet.

The other mode is for literate programming. Everything is assumed to be comment unless you indent it, in which case it's code. When you ask Excelsior to compile this, it generates two pieces of output. One is a spreadsheet. The other, from Excelsior's documentation engine, is a nicely formatted Web page. Here's the documentation engine's output for the SFHMPC.

In this Web page, commentary is left alone. Code is rendered in a monospace font, on a lightly coloured background to make it stand out. I wrote the code and commentary for the SFHMPC as a mathematical essay, thinking of the code as inserts in the same way that equations would be in a normal maths essay. I introduced the data structures in an order which, I hope, makes it easy to understand how the code works. I included some simple examples. And that is literate programming.


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