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
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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