[Haskell-beginners] Basic "data" question

[Costello, Roger L.]

Hi Folks,

I am trying to define a data type for this:

    A Lambda term is one of these:
        - Variable
        - Application of a Lambda term to a Lambda term
        - Abstraction of a Variable in a Lambda term

Here's an example of a Lambda term:

term = Abstraction (Variable X) (Application (Variable X) (Variable Y))

Is there a way to define Lambda term?

Here's an attempt at defining it:

data Var = V | W | X | Y | Z

data Lambda = Variable Var | Application Lambda Lambda | Abstraction (Variable Var) Lambda

But that yields the following error:

test.hs:5:71: Not in scope: type constructor or class 'Variable'

Any suggestions would be much appreciated.

/Roger