On 19 June 2015 at 22:09, Leza Morais Lutonda
On 06/19/2015 03:02 AM, MigMit wrote:
Typo.
{-# LANGUAGE ConstraintKinds, GADTs #-} data SC c e where SC :: c d => d -> e -> SC c e
Yes, I have activated the GADTs extension too and the data definition itself typechecks but the Show instance do not:
instance Show e => Show (S c e) where show (SC x y) = show x ⧺ show y
Because: Could not deduce (Show d) arising from a use of ‘show’
This works for me if I also enable FlexibleInstances and restrict it to: instance (Show e) => Show (S Show e) where ... You could probably also use Edward Kmett's constraints package [1] to generalise this to any c which is a sub-class of Show. [1]: http://hackage.haskell.org/package/constraints -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com