You could constrain d in the type signature of SC. {-# LANGUAGE GADTs #-} data S e where SC :: Show d => d -> e -> S e instance Show e => Show (S e) where show (SC x y) = show x ++ show y On Fri, Jun 19, 2015 at 12:16 AM, Leza Morais Lutonda < leza.ml@fecrd.cujae.edu.cu> wrote:
Hi All,
I have the following data type:
data S e where
SC :: d -> e -> S e
And I want to declare a Show instance for it in the following way:
instance Show e => Show (S e) where
show (SC x y) = show x ++ show y
But, of course it don't typechecks because: could not deduce `Show d` arising from a use of `show`. Is there a way to constraint the `d` type in the `SC` constructor definition to have the same constraints of the `e` type? Something like:
SC :: SameConstraints d e => d -> e -> S e ???
Thanks!
-- Leza Morais Lutonda, Lemol-C http://lemol.github.io
50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
-- Cell: 1.630.740.8204