Thanks for your answer, I think it emphasizes that my example matches the exclaimed conditions
* small * useful * demonstrate Haskell's power * preferably something that might be a bit tricky in another language
It's easy to encode this in some object oriented language with generics, ^^^^ [some dozen lines of java]
The haskell solution may be much shorter, but it is far from impossible to encode such things in a plain-and-boring mainstream language like java. The promotional value of this (and similar) examples shrinks down to "haskell code is much shorter". Wich is true, and important, but may not be enough to consider learning some crazy programming language.
Okay, nobody sayed its impossible. GHC compiles Haskell programs to some kind of C. But first: Shorter code has less chances for bugs and is easier to maintain... More important: Correct me, if I'm wrong, but as far as I understand java, it is still impossible in your solution to evaluate the equivalent of head $ mirror $ rel [ (i,i) | i <- [0..] ] in finite time, that is, your MirrorRel is not lazy in the elements. You have to build this also by hand and your code becomes even longer and more complex.
(Java developers who don't understand Java's advanced features like generics and anonymous classes may not be able to write or understand the above written Java solution; but do you expect them to understand Haskell?)
Add 1: This statement contradicts your "easyness" claim!? Add 2: In contrast, the Haskell solution does'nt uses "advanced Haskell features" (whatever this might be), it consists of 6 lines of plain Haskell 98 only. Regards, -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---