Template Haskell doesn't support GADTs directly but
we can use equality constraints to do the job. Here's
an example of the dump from splicing such a declaration:

    [d| data T a b
            where
              T1 :: Int -> T Int Char
              T2 :: a -> T a a
              T3 :: a -> T [a] a
              T4 :: a -> b -> T b [a] |]
  ======>
    T5217.hs:(6,3)-(9,53)
    data T a[aQW] b[aQX]
        = (b[aQX] ~ Char, a[aQW] ~ Int) => T1 Int |
          b[aQX] ~ a[aQW] => T2 a[aQW] |
          a[aQW] ~ [b[aQX]] => T3 b[aQX] |
          forall a[aQY]. b[aQX] ~ [a[aQY]] => T4 a[aQY] a[aQW]