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Today's Topics:

  1.  Basic "data" question (Costello, Roger L.)


----------------------------------------------------------------------

Message: 1
Date: Thu, 24 Nov 2011 10:43:58 +0000
From: "Costello, Roger L." <costello@mitre.org>
Subject: [Haskell-beginners] Basic "data" question
To: "beginners@haskell.org" <beginners@haskell.org>
Message-ID:
       <B5FEE00B53CF054AA8439027E8FE1775181911E9@IMCMBX04.MITRE.ORG>
Content-Type: text/plain; charset="us-ascii"

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



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