[Haskell-cafe] A question about data declaration

[Erik Hesselink]

You could use a GADT:

{-# LANGUAGE GADTs #-}

data T a where
  C1 :: Int -> T Int
  C2 :: Char -> T Char
  C3 :: T Char -> T Char

This will allow you to put a C3 in a C3. If you want to prevent that,
just invent some other index, something like:

{-# LANGUAGE GADTs, EmptyDataDecls #-}

data Yes
data No

data T a where
  C1 :: Int -> T No
  C2 :: Char -> T Yes
  C3 :: T Yes -> T No

Not sure if this is a *better* way though. Your initial solution is
also ok, I guess.

Regards,

Erik

On Thu, Mar 21, 2013 at 1:48 PM, C K Kashyap <ckkashyap at gmail.com> wrote:
> Hi,
>
> I have a situation where I need to define a data type T such that
>
> data T = C1 Int | C2 Char | C3 T
>
> However, I want to enforce a constraint that C3 only allows (C2 Char) and
> not (C1 Int). That is
>
> x = C3 (C1 10) -- should not compile - but my above definition will let it
> compile
>
>
> I was thinking of this -
>
> data C1 = C1 Int
> data C2 = C2 Char
> data T = TC1 C1 | TC1 C2 | TC3 C2
>
> Is there a better way to do it?
>
> Regards,
> Kashyap
>
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