Provide ToJSON/FromJSON instances for NExpr

This is not the same as the JSON format used by Nix, but is intended for other
uses where one wishes to write analysis programs over expression trees, where
hnix is only being used to convert the source code to its JSON representation.

src/Nix/Expr/Types.hs

@@ -26,6 +26,7 @@ module Nix.Expr.Types where
 import           Codec.Serialise (Serialise)
 import qualified Codec.Serialise as Ser
 import           Control.DeepSeq
+import           Data.Aeson
 import           Data.Binary (Binary)
 import qualified Data.Binary as Bin
 import           Data.Data
@@ -330,6 +331,32 @@ instance Binary NUnaryOp
 instance Binary NBinaryOp
 instance Binary a => Binary (NExprF a)
 
+instance (ToJSON v, ToJSON a) => ToJSON (Antiquoted v a)
+instance ToJSON a => ToJSON (NString a)
+instance ToJSON a => ToJSON (Binding a)
+instance ToJSON Pos where
+    toJSON x = toJSON (unPos x)
+instance ToJSON SourcePos
+instance ToJSON a => ToJSON (NKeyName a)
+instance ToJSON a => ToJSON (Params a)
+instance ToJSON NAtom
+instance ToJSON NUnaryOp
+instance ToJSON NBinaryOp
+instance ToJSON a => ToJSON (NExprF a)
+
+instance (FromJSON v, FromJSON a) => FromJSON (Antiquoted v a)
+instance FromJSON a => FromJSON (NString a)
+instance FromJSON a => FromJSON (Binding a)
+instance FromJSON Pos where
+    parseJSON = fmap mkPos . parseJSON
+instance FromJSON SourcePos
+instance FromJSON a => FromJSON (NKeyName a)
+instance FromJSON a => FromJSON (Params a)
+instance FromJSON NAtom
+instance FromJSON NUnaryOp
+instance FromJSON NBinaryOp
+instance FromJSON a => FromJSON (NExprF a)
+
 stripPositionInfo :: NExpr -> NExpr
 stripPositionInfo = transport phi
   where