Simon Peyton-Jones
But, just to remind you all: I'm particularly interested in
concrete examples (pref running code) of programs that are * small * useful * demonstrate Haskell's power * preferably something that might be a bit tricky in another language
I have something that I think nearly fits the bill. Unfortunately, I don't think it quite works because it's a bit specialised. However, I think it suggests a possible area to look, which I'll mention at the end. It's a theorem prover for intuitionistic propositional logic: http://www.polyomino.f2s.com/david/haskell/gentzen.html It's much shorter in Haskell than it would be in other languages. (It's even shorter than the ML that I based it on, because of some shortcuts I can take using lazy evaluation.) Strengths of Haskell that it demonstrates are: * How easy it is to define datatypes (eg trees), and manipulate them using pattern matching, with constructors, Eq, Show coming for free. * How lazy evaluation reduces code length by letting you write code that looks like it would do too much, and then lazy evaluate it (in the "proof" function) * The ability to extend the syntax with new symbolic operators * Use of higher order functions to simplify code (the (+++) operator) The problem is that I think Gentzen systems are a bit obscure. But I think you could probably show most of the same strengths of Haskell in something similar: game search, eg alpha-beta algorithm. Another advantage of doing game search would be that you'd get to show off persistent data structures (so that when you make a move in lookahead, you don't need to make a copy of the game state, because when you update the game state the old state still persists).