[Haskell-cafe] Re: Wikipedia on first-class object

[Ryan Ingram]

On 12/30/07, Cristian Baboi <cristi at ot.onrc.ro> wrote:
>
> Thank you.
>
> data Something = This | S Something
>
> ppp :: Something -> String
>
> ppp This = ""
> ppp (S x) = 'S':(ppp x)
>
>
> How can I prevent one to pass  'let x = S x in x' to ppp ?


 {-# LANGUAGE GADTs, EmptyDataDecls #-}
data Z
data S a

data Something a where
    This :: Something Z
    S :: Something a -> Something (S a)

ppp :: Something a -> String
ppp This =  ""
ppp (S x) = 'S' : ppp x
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