[Haskell-beginners] Basic "data" question

[Ozgur Akgun]

Hi Roger,

On 24 November 2011 10:43, Costello, Roger L. <costello at mitre.org> wrote:

> data Var = V | W | X | Y | Z
>
> data Lambda = Variable Var | Application Lambda Lambda | Abstraction
> (Variable Var) Lambda
>

Variable is a constructor of the data type Lambda. Abstraction is another.
You use Variable as if it was a data type while defining Abstraction.

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

You can use the above declaration to successfully create your example
lambda terms. However, it doesn't stop you from constructing things like:

Abstraction (Application .. ..) (Variable ..)

If you want to go to that land, you either need smart constructors
(simpler) or GADTs (more hairy, unfortunately).

HTH,
Ozgur
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