Interactive shell

People may recall that I was looking for an interactive Haskell shell at which I could type things and experiment, as you'd expect to get with any Lisp environment. At people's suggestion I installed GHCi and got it working, but I seem quite limited in what I can type at it. For instance, it balks at my various attempts to define a new algebraic type along the lines of "data Foo = Bar | Baz", for use in future intercourse. Is there some magical incantation I'm missing, or does it really not let me do that, and if not then does anything else? Thanks. -- Mark