Use nats package to provided Natural instances with old bases

Data/Aeson/Types/Instances.hs

@@ -86,6 +86,7 @@ import Data.Word (Word8, Word16, Word32, Word64)
 import Data.Version (Version, showVersion, parseVersion)
 import Text.ParserCombinators.ReadP (readP_to_S)
 import Foreign.Storable (Storable)
+import Numeric.Natural (Natural)
 import Prelude hiding (foldr)
 import qualified Data.Aeson.Encode.Builder as E
 import qualified Data.Aeson.Parser.Time as Time
@@ -111,7 +112,6 @@ import qualified Data.Vector.Storable as VS
 import qualified Data.Vector.Unboxed as VU
 
 #if MIN_VERSION_base(4,8,0)
-import Numeric.Natural
 #else
 import Control.Applicative ((<$>), (<*>), pure)
 import Data.Monoid (mempty)
@@ -353,7 +353,6 @@ instance FromJSON Integer where
     parseJSON = withScientific "Integral" $ pure . truncate
     {-# INLINE parseJSON #-}
 
-#if MIN_VERSION_base(4,8,0)
 instance ToJSON Natural where
     toJSON = toJSON . toInteger
     {-# INLINE toJSON #-}
@@ -366,7 +365,6 @@ instance FromJSON Natural where
       if Scientific.coefficient s < 0
         then fail $ "Expected a Natural number but got the negative number: " <> show s
         else pure $ truncate s
-#endif
 
 instance ToJSON Int8 where
     toJSON = Number . fromIntegral

aeson.cabal

@@ -107,9 +107,13 @@ library
     vector >= 0.7.1
 
   if !impl(ghc >= 8.0)
-    -- `Data.Semigroup` is available in base only since GHC 8.0
+    -- `Data.Semigroup` is available in base only since GHC 8.0 / base 4.9
     build-depends: semigroups >= 0.16.1
 
+  if !impl(ghc >= 7.10)
+    -- `Numeric.Natural` is available in base only since GHC 7.10 / base 4.8
+    build-depends: nats >=1 && <1.1
+
   if flag(old-locale)
     build-depends: time < 1.5, old-locale
   else
@@ -167,6 +171,9 @@ test-suite tests
   else
     build-depends: time >= 1.5
 
+  if !impl(ghc >= 7.10)
+    build-depends: nats >=1 && <1.1
+
 source-repository head
   type:     git
   location: git://github.com/bos/aeson.git

tests/Instances.hs

@@ -21,6 +21,10 @@ import Functions
 
 import Test.QuickCheck.Instances ()
 
+#if !MIN_VERSION_base(4,8,0) && !MIN_VERSION_QuickCheck(2,8,3)
+import Numeric.Natural
+#endif
+
 -- "System" types.
 
 instance Arbitrary DotNetTime where
@@ -145,3 +149,8 @@ instance Arbitrary Version where
 makeVersion :: [Int] -> Version
 makeVersion b = Version b []
 #endif
+
+#if !MIN_VERSION_base(4,8,0) && !MIN_VERSION_QuickCheck(2,8,3)
+instance Arbitrary Natural where
+  arbitrary = fromInteger . abs <$> arbitrary
+#endif

tests/Properties.hs

@@ -12,6 +12,7 @@ import Data.Int (Int8)
 import Data.Time (Day, LocalTime, NominalDiffTime, TimeOfDay, UTCTime,
                   ZonedTime)
 import Data.Version (Version)
+import Numeric.Natural (Natural)
 import Encoders
 import Instances ()
 import Test.Framework (Test, testGroup)
@@ -139,6 +140,7 @@ tests = testGroup "properties" [
     , testProperty "ZonedTime" $ roundTripEq (undefined :: ZonedTime)
     , testProperty "NominalDiffTime" $ roundTripEq (undefined :: NominalDiffTime)
     , testProperty "Version" $ roundTripEq (undefined :: Version)
+    , testProperty "Natural" $ roundTripEq (undefined :: Natural)
     , testGroup "ghcGenerics" [
         testProperty "OneConstructor" $ roundTripEq OneConstructor
       , testProperty "Product2" $ roundTripEq (undefined :: Product2 Int Bool)