[Ezra]

Basically what's going on is this, but with lists:

3 + 5 = X?

X = 8

3 + X = 8?

X = 5

5 + X = 8?

X = 3

In most programming languages, you can't infer a parameter by specifying a result.

(Though that's not strictly what's going on here, either)

You could further say something like X + Y = 8 and get

X = 0, Y = 8

X = 1, Y = 7

etc. for some definitions.

Which is, as the OP was getting at, sometimes incredibly useful.

[moe]

Ah thanks for clarifying. Now it makes sense. Are there any real world applications for this, for us mortal web developers?

[Ezra]

If you're asking "are there potentially awesome applications of this?" then the answer is probably "yes".

If you're asking "is there anything in the wild that I would know of and uses this?" then the answer is probably "no".

Most of the prolog projects I've ever seen are pretty inbred; implementing compilers or interpreters, hooks for another language-type things. Though it must be said that I'm not deeply embedded in the prolog community.

[gommm]

In my 4th year university project we worked on an expert system for a card game in Prolog... For doing an expert system based AI, prolog is pretty good...