Fix Natural parsing

Author: cblp

CONTRIBUTING.md

@@ -8,7 +8,7 @@ Changes that affect users of aeson should be mentioned in `changelog.md`. It sho
   * When it should be used
   * What it supersedes
 
-Add this entry under an `# Upcoming` header above existing releases.
+Add this entry under an `## Upcoming` header above existing releases.
 
 The exact format of entries is not important as we'll go through everything before a release is made.
 

Data/Aeson/Types/FromJSON.hs

@@ -188,7 +188,7 @@ parseIntegralFromScientific :: forall a. Integral a => String -> Scientific -> P
 parseIntegralFromScientific expected s =
     case Scientific.floatingOrInteger s :: Either Double a of
         Right x -> pure x
-        Left _  -> fail $ "Floating number specified for " ++ expected ++ ": " ++ show s
+        Left _  -> fail $ "expected " ++ expected ++ ", encountered floating number " ++ show s
 {-# INLINE parseIntegralFromScientific #-}
 
 parseIntegral :: Integral a => String -> Value -> Parser a
@@ -1273,16 +1273,20 @@ instance FromJSONKey Integer where
     fromJSONKey = FromJSONKeyTextParser $ parseIntegralText "Integer"
 
 instance FromJSON Natural where
-    parseJSON = withScientific "Natural" $ \s ->
-      if Scientific.coefficient s < 0
-        then fail $ "Expected a Natural number but got the negative number: " <> show s
-        else pure $ truncate s
+    parseJSON value = do
+        integer :: Integer <- parseIntegral "Natural" value
+        if integer < 0 then
+            fail $ "expected Natural, encountered negative number " <> show integer
+        else
+            pure $ fromIntegral integer
 
 instance FromJSONKey Natural where
-    fromJSONKey = FromJSONKeyTextParser $ \t -> parseScientificText t >>= \s ->
-      if Scientific.coefficient s < 0
-        then fail $ "Expected a Natural number but got the negative number: " <> show s
-        else pure $ truncate s
+    fromJSONKey = FromJSONKeyTextParser $ \text -> do
+        integer :: Integer <- parseIntegralText "Natural" text
+        if integer < 0 then
+            fail $ "expected Natural, encountered negative number " <> show integer
+        else
+            pure $ fromIntegral integer
 
 instance FromJSON Int8 where
     parseJSON = parseBoundedIntegral "Int8"

changelog.md

@@ -3,6 +3,9 @@ For the latest version of this document, please see [https://github.com/bos/aeso
 ## Upcoming
 
 * Add `FromJSON` and `ToJSON` instances for `DiffTime`, thanks to Víctor López Juan
+* Fix handling of fractions when parsing Natural.
+  Change text in error messages for Integral types to make them follow the common pattern.
+  PVP level is "Non-breaking change".
 
 ## 1.2.1.0
 

tests/ErrorMessages.hs

@@ -12,6 +12,7 @@ import Prelude.Compat
 import Data.Aeson (FromJSON(..), eitherDecode)
 import Data.Proxy (Proxy(..))
 import Instances ()
+import Numeric.Natural (Natural)
 import Test.Framework (Test)
 import Test.Framework.Providers.HUnit (testCase)
 import Test.HUnit (Assertion, assertFailure, assertEqual)
@@ -22,6 +23,8 @@ tests :: [Test]
 tests =
     [
       testCase "Int" int
+    , testCase "Integer" integer
+    , testCase "Natural" natural
     , testCase "String" string
     , testCase "HashMap" hashMap
     ]
@@ -34,6 +37,17 @@ int = do
   t "{}" $ expected "Int" "Object"
   t "null" $ expected "Int" "Null"
 
+integer :: Assertion
+integer = do
+  let t = test (Proxy :: Proxy Integer)
+  t "44.44" $ expected "Integer" "floating number 44.44"
+
+natural :: Assertion
+natural = do
+  let t = test (Proxy :: Proxy Natural)
+  t "44.44" $ expected "Natural" "floating number 44.44"
+  t "-50" $ expected "Natural" "negative number -50"
+
 string :: Assertion
 string = do
   let t = test (Proxy :: Proxy String)