Haskell is the first functional language I ever used, I spent quite a lot of time trying to learn how to be productive in it, unfortunately I am still far from being more than a Haskell beginner but I must say, every hour spent trying to figure out functors, monad, and recursive patterns payed off in terms of programming knowledge.
If you know Haskell, you probably have heard of the Glasgow/Glorious Haskell Compiler, or GHC. Not only GHC is a great pieace of software, it also comes with one of the best REPL you can lay your hand on: GHCI.
Being a VIM user, my flow is:
The play with my code part is very important while learning a new language, and Haskell offers a lot of features that make this experience pleasant.
Recently I wrote an article on decharm.com listing a bunch of resources I found of particular interest for Haskell beginners.
FP Complete is one of the newest resources I stumbled upon, and in my opinion is the best one to start with.
I discovered FP Complete while searching for tutorials, School Of Haskell is a collection of tutorials, spanning from basic to advanced stuff.
But FP Complete strong point is their IDE, which comes in different versions, from the community edition (which is free) to the Professional edition. The community edition is all you need to start learning Haskell.
Not only you can run all the code examples from their tutorial in the FP Complete IDE, you can also change them, playing with the code. When I say playing I really mean it: you can write functions and the IDE will dispaly their signature in the bottom panel, along with suggestions.
If you write something (bad) like:
len' [] = 0
len' (x:xs) = 1 + len' xs
You'll then get the following output in the Message panel:
Found: len' [] = 0
len' (x : xs) = 1 + len' xs
Why not: len' xs = foldr (\ x -> (+) 1) 0 xs
Isn't it delightful? Having this kind of suggestion gives you a good chance to learn faster good practices and tecniques obscure to imperative programmers.
Speaking of type signature, have you ever heard of Hoogle?
On Hoogle You can search package/modules by type signature, FP Complete IDE integrates
hoogle search in the bottom panel saving you other precious time, and information about identifiers, accessible with Ctrl+i.
Using this IDE really keeps the Haskell spark strong with me, not having to switch frantical between editor/browser/docs keeps me focused on the problem, and having suggestions prevents me from getting lost, plus I can use all my precious VIM bindings, they work perfectly (even stuff like ci').
Well done.