Haskell Data.Decimal as Aeson type

Question

Is it possible to parse a Data.Decimal from JSON using the Aeson package?

Suppose I have the following JSON:

{
    "foo": 5.231,
    "bar": "smth"
}

And the following record type:

data test { foo :: Data.Decimal
          , bar :: String } deriving Generic

with

instance FromJSON test
instance ToJSON test

This would work, if it weren't for the Data.Decimal value "foo".

From what I understand I would need to manually create a FromJSON and ToJSON (for converting back to JSON) instance of Data.Decimal, since it doesn't derive from Generic. How can I do that?

Thanks in advance.

Answer 1

I know this is an old thread, but might help someone. Here is how I am converting Decimal to/from json (I assembled this code from a bunch of other code which I don't have the source for now):

instance A.ToJSON (DecimalRaw Integer) where
  -- maybe this is not the best, but it works
  toJSON d = A.toJSON $ show d

instance A.FromJSON (DecimalRaw Integer) where
  parseJSON = A.withText "Decimal" (go . (read :: String -> [(DecimalRaw Integer, String)]) . T.unpack)
    where
      go [(v, [])] = return v
      go (_ : xs) = go xs
      go _ = fail "Could not parse number"