Getting started with Haskell

To add on others' answers - there is one useful that will help you when coding (for example when solving project Euler problems): Hoogle. You can use either the command line interface or the web interface.

Command Line

After you installed the Haskell platform be sure to cabal install hoogle

Hoogle usage example:

You have a function f x = 3 * x + 1 and you want to apply it on (5 :: Int), then apply it on the result and on that result and so on and get an infinite list of those values. You suspect there might already exist a function to assist you (not specifically for your f though).

That function would be of type (a -> a) -> a -> [a] if it takes f 5 or a -> (a -> a) -> [a] if it takes 5 f (we assume the function is for general types and not just Ints)

$ hoogle "a -> (a -> a) -> [a]"
Prelude iterate :: (a -> a) -> a -> [a]

yep, the function you need already exists and it's called iterate. you use it by iterate func 5!

Web interface

The result for the same example can be found here.

I'd suggest joining the #haskell irc channel and asking questions there. That's how I learned Haskell. If you go through Real World Haskell as suggested above, real time answers to your questions will help greatly. Lots of smart people on #haskell write Haskell for fun and for profit, so you'll get lots of good input. Try it!

If you only have experience with imperative/OO languages, I suggest using a more conventional functional language as a stepping stone. Haskell is really different and you have to understand a lot of different concepts to get anywhere. I suggest tackling a ML-style language (like e.g. F#) first.