[Haskell-cafe] What features should an (fictitious) IDE for Haskell have?

Greetings, Imagine that you can decide what functionality and features an Integrated Development Environment for Haskell should get. You can make this IDE with your wishlist similar in power as other IDEs for Java for example. Now list as many features as you can, what would make the life of a Haskell developer as comfortable as possible!? I am dying to know how this list of features at the end (of this thread) will look like.