Thanks Eric and Brent,

Even with GADT, it appears that I'd need extra data definitions. I'll go without GADT then ...

Brent, my use case is not particularly complicated. I am trying to model the pdf spec - which says that pdf contains Objects that could of of types Number, String, Name, Array and Dictionary - while array is list of objects, the Disctionary is a list of tuples (Name, Object) not (Object, Object) - hence my situation.

Regards,
Kashyap


On Thu, Mar 21, 2013 at 8:58 PM, Brent Yorgey <byorgey@seas.upenn.edu> wrote:
On Thu, Mar 21, 2013 at 06:18:46PM +0530, C K Kashyap 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

If C3 should only be able to hold a C2 Char, then why have it hold a T
at all?  i.e. why not

  data T = C1 Int | C2 Char | C3 Char

but I suppose your real problem is probably more complicated, in which
case I would recommend using a GADT as others have suggested.

-Brent

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