[Haskell-cafe] Haskell showcase in 5 minutes

Hello Cafe, I will be (re)presenting Haskell in a "Batlle Language" event Wednesday evening: A fun and interactive contest where various programming language champions try to attract as much followers as possible in 5 minutes. Having successfully experimented the power of live coding in a recent Haskell introduction for the Paris Scala User Group, I would like to do the same but given the time frame I need a simpler example than the music synthesizer program. So I would like to tap in the collective wisdom looking for some concise, eye-opening, mind-shaking and if possible fun example of what one can achieve in Haskell. Things that sprung to my mind are rather dull: prime factors, fibonacci numbers. Thanks in advance, Arnaud