[Haskell-beginners] Beginners Digest, Vol 41, Issue 33

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2011/11/24 <beginners-request at haskell.org>

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> Today's Topics:
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>   1.  Basic "data" question (Costello, Roger L.)
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>
> ----------------------------------------------------------------------
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> Message: 1
> Date: Thu, 24 Nov 2011 10:43:58 +0000
> From: "Costello, Roger L." <costello at mitre.org>
> Subject: [Haskell-beginners] Basic "data" question
> To: "beginners at haskell.org" <beginners at haskell.org>
> Message-ID:
>        <B5FEE00B53CF054AA8439027E8FE1775181911E9 at IMCMBX04.MITRE.ORG>
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> 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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> End of Beginners Digest, Vol 41, Issue 33
> *****************************************
>
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
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