2011/11/24

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Today's Topics:

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

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Message: 1 Date: Thu, 24 Nov 2011 10:43:58 +0000 From: "Costello, Roger L." Subject: [Haskell-beginners] Basic "data" question To: "beginners@haskell.org" Message-ID: 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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value constructor followed by types. And (Variable Var) belong to Lambda type. Could define like this: data Lambda var= Variable var | Application (Lambda var) (Lambda var) var be any types