How to deal with Haskell's reserved keywords in record fields?

Question

JSON in response of Github Gists Rest API contains Haskell's keyword type. But type couldn't be used as a record field.

Thus it couldn't be used in implementation of Aeson's Generic FromJSON/ToJSON instances.

import Data.Text (Text)

import GHC.Generics (Generic)

type URL = Text

data OwnerType = User deriving (Show)

data Owner = Owner {
      id :: Int,
      gravatar_id :: Text,
      login :: Text,
      avatar_url :: Text,
      events_url :: URL,
      followers_url :: URL,
      following_url :: URL,
      gists_url :: URL,
      html_url :: URL,
      organizations_url :: URL,
      received_events_url :: URL,
      repos_url :: URL,
      starred_url :: URL,
      subscriptions_url :: URL,
      url :: URL,
      -- type :: Text,
      site_admin :: Bool
  } deriving (Generic, Show)

instance ToJSON Owner
instance FromJSON Owner

Question: Is there a proper approach to deal with such kind of conflicts?

Answer 1

We can solve this by using TemplateHaskell. Instead of writing ToJSON and FromJON, we can use a specific mapping of the keys.

First of all, we have to construct a name for the field that is not type, for instance:

data Owner = Owner {
      id :: Int,
      gravatar_id :: Text,
      login :: Text,
      avatar_url :: Text,
      events_url :: URL,
      followers_url :: URL,
      following_url :: URL,
      gists_url :: URL,
      html_url :: URL,
      organizations_url :: URL,
      received_events_url :: URL,
      repos_url :: URL,
      starred_url :: URL,
      subscriptions_url :: URL,
      url :: URL,
      owner_type :: Text,
      site_admin :: Bool
  } deriving (Generic, Show)

Now we can use the deriveJSON :: Options -> Name -> Q [Dec] function that will construct a fromJSON and toJSON instance.

The key here is the Options parameter: it contains a fieldLabelModifier :: String -> String field that can rewrite the names of the fields to the keys in JSON. We can thus here generate a function that will rewrite it.

So we first construct a function ownerFieldRename :: String -> String:

ownerFieldRename :: String -> String
ownerFieldRename "owner_type" = "type"
ownerFieldRename name = name

So this function acts as an identity function, except for "owner_type", which is mapped on "type".

So now we can call the deriveJSON function with custom options like:

$(deriveJSON defaultOptions {fieldLabelModifier = ownerFieldRename} ''Owner)

Or in full:

RenameUtils.hs:

module RenameUtils where

ownerFieldRename :: String -> String
ownerFieldRename "owner_type" = "type"
ownerFieldRename name = name

MainFile.hs:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE DeriveGeneric #-}

import Data.Aeson.TH(deriveJSON, defaultOptions, Options(fieldLabelModifier))
import RenameUtils(ownerFieldRename)

import Data.Text (Text)

type URL = Text

data Owner = Owner {
      id :: Int,
      gravatar_id :: Text,
      login :: Text,
      avatar_url :: Text,
      events_url :: URL,
      followers_url :: URL,
      following_url :: URL,
      gists_url :: URL,
      html_url :: URL,
      organizations_url :: URL,
      received_events_url :: URL,
      repos_url :: URL,
      starred_url :: URL,
      subscriptions_url :: URL,
      url :: URL,
      owner_type :: Text,
      site_admin :: Bool
  } deriving (Show)

$(deriveJSON defaultOptions {fieldLabelModifier = ownerFieldRename} ''Owner)

Now we obtain a JSON object with type as key:

Prelude Main Data.Aeson> encode (Owner 1 "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" True)
"{\"id\":1,\"gravatar_id\":\"\",\"login\":\"\",\"avatar_url\":\"\",\"events_url\":\"\",\"followers_url\":\"\",\"following_url\":\"\",\"gists_url\":\"\",\"html_url\":\"\",\"organizations_url\":\"\",\"received_events_url\":\"\",\"repos_url\":\"\",\"starred_url\":\"\",\"subscriptions_url\":\"\",\"url\":\"\",\"type\":\"\",\"site_admin\":true}"

For a simple fieldLabelModifier function we do not need to write a specific function (that we have to define in a specific module), we can also use an lambda expression here:

MainFile.hs:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE DeriveGeneric #-}

import Data.Aeson.TH(deriveJSON, defaultOptions, Options(fieldLabelModifier))
import Data.Text (Text)

type URL = Text

data Owner = Owner {
      id :: Int,
      gravatar_id :: Text,
      login :: Text,
      avatar_url :: Text,
      events_url :: URL,
      followers_url :: URL,
      following_url :: URL,
      gists_url :: URL,
      html_url :: URL,
      organizations_url :: URL,
      received_events_url :: URL,
      repos_url :: URL,
      starred_url :: URL,
      subscriptions_url :: URL,
      url :: URL,
      owner_type :: Text,
      site_admin :: Bool
  } deriving (Show)

$(deriveJSON defaultOptions {fieldLabelModifier = \x -> if x == "owner_type" then "type" else x} ''Owner)