Add Generic instance for Maybe (#9)

Author: LiamGoodacre

src/Data/Generic/Rep.purs

@@ -12,6 +12,8 @@ module Data.Generic.Rep
   , Field(..)
   ) where
 
+import Data.Maybe (Maybe(..))
+
 -- | A representation for types with no constructors.
 data NoConstructors
 
@@ -43,3 +45,13 @@ newtype Field (field :: Symbol) a = Field a
 class Generic a rep | a -> rep where
   to :: rep -> a
   from :: a -> rep
+
+instance genericMaybe
+  :: Generic (Maybe a) (Sum (Constructor "Nothing" NoArguments)
+                            (Constructor "Just" (Argument a))) where
+  to (Inl _) = Nothing
+  to (Inr (Constructor (Argument a))) = Just a
+
+  from Nothing = Inl (Constructor NoArguments)
+  from (Just a) = Inr (Constructor (Argument a))
+