Improving the resiliency of BigInt decoding.

It will now decode ints as well.

Note that this is just us being more tolerant of recieving something
like `1234`. If the number sent is actually in the "big" integer range
(>2^32) then it won't work and it's the sender's fault. The JSON spec
doesn't support numbers that big. They'd have to be sent as strings.

src/Foreign/Generic/Class.purs

@@ -207,9 +207,14 @@ instance eitherDecode :: (Decode a, Decode b) => Decode (Either a b) where
       (readProp "Right" value >>= (map Right <<< decode))
 
 instance bigIntDecode :: Decode BigInt where
-  decode value = do
-    str <- readString value
-    except $ note (pure (ForeignError ("Expected BigInt"))) $ BigInt.fromString str
+  decode json = decodeAsString json <|> decodeAsDigits json
+    where
+      decodeAsString value = do
+        str <- readString value
+        except $ note (pure (ForeignError ("Expected BigInt"))) $ BigInt.fromString str
+      decodeAsDigits value = do
+        int <- readInt value
+        pure $ BigInt.fromInt int
 
 -- | The `Encode` class is used to generate encoding functions
 -- | of the form `a -> Foreign` using `generics-rep` deriving.

test/Main.purs

@@ -149,7 +149,10 @@ main = do
               , actual: runExcept (decodeJSON "null")
               }
   testRoundTrip [ Left 5, Right "Test" ]
-  testRoundTrip (BigInt.pow (BigInt.fromInt 2) (BigInt.fromInt 60)) -- 2^60. Anything over 2^32 will confuse JavaScript.
+  testRoundTrip (BigInt.pow (BigInt.fromInt 2) (BigInt.fromInt 60)) -- 2^60. Anything over 2^32 would baffle JavaScript.
+  assertEqual { expected: Right (BigInt.fromInt 50)
+              , actual: runExcept (decodeJSON "50")
+              }
   testUnaryConstructorLiteral
   let opts = defaultOptions { fieldTransform = toUpper }
   testGenericRoundTrip opts (RecordTest { foo: 1, bar: "test", baz: 'a' })