Wrong argument kind when using GHC Generics

Question

I've been following the GHC.Generics tutorial to make a simple generic typeclass for providing default values for arbitrary types. However when I try to load my file (relevant snippet, which still produces the error)

{-# LANGUAGE DefaultSignatures, DeriveGeneric, TypeOperators, FlexibleContexts #-}
import GHC.Generics

class Default a where
    def :: a
    default def :: (Generic a, GDefault (Rep a)) => a
    def = to gdef

class GDefault f where
    gdef :: f a

instance (Default a, Default b) => GDefault (a :+: b) where
    gdef (L1 x) = gdef x
    gdef (R1 x) = gdef x

I get the following error:

Generic.hs:12:46:
    The first argument of ‘:+:’ should have kind ‘* -> *’,
      but ‘a’ has kind ‘*’
    In the instance declaration for ‘GDefault (a :+: b)’

What am I doing wrong?

Answer 1

Don't you mean...?

instance (GDefault a, GDefault b) => GDefault (a :+: b) where ...
  --      ^           ^