Catch 0 denominators when parsing Ratio

Author: bergmark

Data/Aeson/Types/FromJSON.hs

@@ -1258,9 +1258,12 @@ instance FromJSONKey Float where
         _           -> Scientific.toRealFloat <$> parseScientificText t
 
 instance (FromJSON a, Integral a) => FromJSON (Ratio a) where
-    parseJSON = withObject "Rational" $ \obj ->
-                  (%) <$> obj .: "numerator"
-                      <*> obj .: "denominator"
+    parseJSON = withObject "Rational" $ \obj -> do
+        numerator <- obj .: "numerator"
+        denominator <- obj .: "denominator"
+        if denominator == 0
+        then fail "Ratio denominator was 0"
+        else pure $ numerator % denominator
     {-# INLINE parseJSON #-}
 
 -- | /WARNING:/ Only parse fixed-precision numbers from trusted input

tests/UnitTests.hs

@@ -100,6 +100,7 @@ tests = testGroup "unit" [
   , testGroup "SingleMaybeField" singleMaybeField
   , testCase "withEmbeddedJSON" withEmbeddedJSONTest
   , testCase "SingleFieldCon" singleFieldCon
+  , testCase "Ratio with denominator 0" ratioDenominator0
   ]
 
 roundTripCamel :: String -> Assertion
@@ -546,6 +547,12 @@ singleFieldCon :: Assertion
 singleFieldCon =
   assertEqual "fromJSON" (Right (SingleFieldCon 0)) (eitherDecode "0")
 
+ratioDenominator0 :: Assertion
+ratioDenominator0 =
+  assertEqual "Ratio with denominator 0"
+    (Left "Error in $: Ratio denominator was 0")
+    (eitherDecode "{ \"numerator\": 1, \"denominator\": 0 }" :: Either String Rational)
+
 deriveJSON defaultOptions{omitNothingFields=True} ''MyRecord
 
 deriveToJSON  defaultOptions ''Foo