Derives a generic show, for example

data MyList a = MyNil | MyCons { hd :: a, tl :: MyList a } deriving Generic

myListValue :: MyList Integer
myListValue = MyCons 1 (MyCons 2 (MyCons 3 MyNil))

instance Show a => Show (MyList a) where
  show = gshow

The code here was adapted from `generic-deriving` https://github.com/dreixel/generic-deriving/blob/master/src/Generics/Deriving/Show.hs