On Fri, Jun 19, 2015 at 2:38 PM, Ivan Lazar Miljenovic wrote:
On 19 June 2015 at 22:09, Leza Morais Lutonda wrote:
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 ...
Note this won't work for a Num instance mentioned earlier because the existentially quantified d types in two SC values are not provably the same type. In other words, you can't write instance Num e => Show (S Num e) where SC x1 y1 + SC x2 y2 = SC (x1 + x2) (y1 + y2) because x1 and x2 can have different types. Regards, Sean