Adds JSON instances for HexString

hexstring.cabal

@@ -31,6 +31,7 @@ library
                      , text
                      , bytestring
                      , base16-bytestring
+                     , aeson
 
 test-suite test-suite
   type:                exitcode-stdio-1.0

src/Data/HexString.hs

@@ -6,10 +6,13 @@ module Data.HexString ( HexString
                       , toBytes
                       , toText ) where
 
+import           Control.Applicative    (pure)
+
+import           Data.Aeson
 import           Data.Word              (Word8)
 
-import qualified Data.ByteString.Base16 as BS16 (decode, encode)
 import qualified Data.ByteString        as BS
+import qualified Data.ByteString.Base16 as BS16 (decode, encode)
 import qualified Data.ByteString.Lazy   as BSL
 
 import qualified Data.Text              as T
@@ -23,6 +26,12 @@ data HexString =
   HexString BS.ByteString
   deriving ( Show, Eq, Ord )
 
+instance FromJSON HexString where
+  parseJSON = withText "HexString" $ pure . hexString . TE.encodeUtf8
+
+instance ToJSON HexString where
+  toJSON = String . toText
+
 -- | Smart constructor which validates that all the text are actually
 --   hexadecimal characters.
 hexString :: BS.ByteString -> HexString