Handling `id` in derived Aeson FromJSON instances with Aeson/JSON

Question

If I have JSON and I try to derive the FromJSON instances automatically with Generics, I run into problems with id existing in more than one place in the JSON.

Is there a way for me to override just the id part or do I have to write the whole instance in order to change those particular entries? The JSON actually has more fields, but I have left most of it out in this example. So it is actually rather tedious to write out the whole FromJSON instance.

JSON:

{
  "response": [
    {
      "id": 1,
      "brandId": 1,
      "productTypeId": 1,
      "identity": {
        "sku": "x",
        "barcode": "Ax"
      },
      "stock": {
        "stockTracked": false,
        "weight": {
          "magnitude": 0
        },
        "dimensions": {
          "length": 0,
          "height": 0,
          "width": 0,
          "volume": 0
        }
      },
      "financialDetails": {
        "taxable": false,
        "taxCode": {
          "id": 1,
          "code": "x"
        }
      },
... etc
  ]
}

CODE So far:

data Response = Response
    { response :: [Body]
    } deriving (Show,Generic)

data Body = Body
    { id                    :: Int
    , brandId               :: Int
    , productTypeId         :: Int
    , identity              :: Identity
    , productGroupId        :: Int
    , stock                 :: Stock
    , financialDetails      :: FinancialDetails
    } deriving (Show,Generic)                  

data Identity = Identity
    { sku       :: String
    , ean       :: String
    , barcode   :: String
    } deriving (Show,Generic)                  

data Stock = Stock
    { stockTracked  :: Bool
    , weight        :: Weight
    , dimensions    :: Dimensions
    } deriving (Show,Generic)                  

data Weight = Weight
    { magnitude  :: Int
    } deriving (Show,Generic)                  

data Dimensions = Dimensions
    { length :: Int
    , height :: Int
    , width  :: Int
    , volume :: Int
    } deriving (Show,Generic)                  

data FinancialDetails = FinancialDetails
     { taxable :: Bool
     , taxCode :: TaxCode
     } deriving (Show,Generic)                  

data TaxCode = TaxCode
     { id      :: Int
     , code    :: String 
     } deriving (Show,Generic)                  


instance FromJSON Response
instance FromJSON Body
instance FromJSON Identity
instance FromJSON Stock
instance FromJSON Weight
instance FromJSON Dimensions
instance FromJSON FinancialDetails

This gives the error:

[1 of 1] Compiling Main             ( reponse.hs, interpreted )

response.hs:73:8:
    Multiple declarations of `id'
    Declared at: response.hs:19:7
                 response.hs:73:8
Failed, modules loaded: none.

Ideally I would like to change the first id to body_id and the second to taxCode_id without having to write out the whole instance.

The upcoming GHC 8.0 will have a DuplicateRecordFields extension ghc.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields/… that will alleviate this problem. — danidiaz

zakyggaps zakyggaps · Accepted Answer · 2016-02-29T15:25:17

When deriving FromJSON instances you can pass an option to the genericParseJSON function. It's usually

data Foo = {- ... -} deriving (Show, Generic)

instance FromJSON Foo where
    parseJSON = genericParseJSON defaultOptions
    -- defaultOptions :: Options

while you can replace defaultOptions with an Option you constructed manually. The Option type have a field fieldLabelModifier that can preprocess the field name of your data type. You can define your data type as

data Body = Body
  { body_id :: Int
  ...

And write a helper function which maps "body_id" to "id" and anything else unchanged:

body_noprefix "body_id" = "id"
body_noprefix s = s

Then define the instance as

instance FromJSON Body where
  parseJSON = genericParseJSON (defaultOptions { fieldLabelModifier = body_noprefix })

Handling `id` in derived Aeson FromJSON instances with Aeson/JSON

3 Answers