"Even tried multi" error explained

If you were around during the early days of the web, you surely remember the message
Error 404 Not found - file doesn't exist or is read protected [even tried multi]
But what is "multi"? And why does the web server act like it's doing you a big favor trying it? Is it trying multiple times, maybe?

These days, 404 messages are still common, but the "multi" message is fairly rare. The message was once so common, though, it spawned a large number of parodies.

I decided to get to the bottom of this, and find out what this "multi" is. It didn't take long to discover that this message comes from the CERN httpd server, written by Tim Berners-Lee and others back in the 1990's.

A bit of poking around found the source code that generates the infamous multi message. It turns out that "multi" is short for multi-format documents, and was a cool new feature back in 1993. It is a technique for the server to have multiple formats of a file and provides the best one your browser can handle. For instance, suppose you try to fetch an image /cat. Back in the olden days, browsers weren't so good with images, so your browser might only display gifs and not jpegs. Your browser asks for /cat and says that it can handle gifs. Now for the multi magic. The server can have multiple files, say /cat.gif, /cat.jpg, and /cat.png. It looks at what your browser can accept, looks at what it has, and returns the best choice, in this case /cat.gif. Likewise, a file could have versions in text and PDF, and the server would return the best type for your browser. Web servers still support content negotiation, but they generally don't mention it in 404 messages any more.

So there you have it, the definitive answer to what "even tried multi" means, and a bit of web history.

Arc continuation puzzle

In case anyone thinks continuations are straightforward, I've ported Eugene Kohlbecker's classic Scheme puzzle to Arc. What does (mondo-bizarro) print?

(def mondo-bizarro ()
  (let k (catch throw)
    (pr 1)
    (catch (k throw))
    (pr 2)
    (catch (k throw))
    (pr 3))
  nil)

Reference: ACM SIGPLAN Lisp Pointers, Volume 1, Issue 2 (June-July 1987) pp 22-28.

Lots more documentation

I've added lots more documentation on the Arc language: filesystem manipulation, anaphoric operations, iteration, threading, time operations, queues, and trees.

I/O in the Arc language

The Arc language provides many I/O operations. I have written an overview of them: I/O in the Arc language. This covers file I/O, string I/O, and the many operations to perform reads and writes. The operations strike me as very non-orthogonal; it's unpredictable which ones take ports, have defaults, or specify an eof value. I try to bring some organization to them.

Internals of places and setforms

The documentation continues with Internals of places and setforms. This continues where the assignment and places documentation left off. If you want to know how to create your own places and generalized variables in Arc, you're in luck.

Arc documentation table of contents

I've created a table of contents summarizing my documentation on the Arc language. (By request, it's now on the sidebar too.)

Arc assignment and places documentation

As part of my Arc language documentation series. I've created a page on assignment and places in Arc