如何处理记录字段中Haskell的保留关键字? [英] How to deal with Haskell's reserved keywords in record fields?

问题描述

JSON 包含 Haskell的关键字 type.但是type不能用作记录字段.

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

因此，它不能用于实现 Aeson的通用 FromJSON/ToJSON实例.

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

问题: 是否有解决此类冲突的适当方法?

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

推荐答案

我们可以使用TemplateHaskell解决此问题.代替编写ToJSON和FromJON，我们可以使用键的特定映射.

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

首先，我们必须为不是 类型的字段构造一个名称，例如:

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)

现在，我们可以使用 deriveJSON :: Options -> Name -> Q [Dec] 函数，该函数将构造一个fromJSON和toJSON实例.

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

此处的关键是 Options 参数:它包含一个 fieldLabelModifier :: String -> String 字段，该字段可以将字段名称重写为JSON中的键.因此，我们可以在此处生成一个将其重写的函数.

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.

所以我们首先构造一个函数ownerFieldRename :: String -> String:

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

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

因此，除了映射到"type"上的"owner_type"之外，此函数还用作标识函数.

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

所以现在我们可以使用自定义选项调用deriveJSON函数:

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

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

或全部:

RenameUtils.hs :

RenameUtils.hs:

module RenameUtils where

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

MainFile.hs :

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)

现在，我们获得一个以type作为键的JSON对象:

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}"

对于简单的fieldLabelModifier函数，我们不需要编写特定的函数(必须在特定的模块中定义)，我们还可以在此处使用 lambda表达式:

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 :

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)

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