Data.Aeson.TH

Functions to mechanically derive ToJSON and FromJSON instances. Note that you need to enable the TemplateHaskell language extension in order to use this module.

An example shows how instances are generated for arbitrary data types. First we define a data type:

data D a = Nullary
         | Unary Int
         | Product String Char a
         | Record { testOne   :: Double
                  , testTwo   :: Bool
                  , testThree :: D a
                  } deriving Eq

Next we derive the necessary instances. Note that we make use of the feature to change record field names. In this case we drop the first 4 characters of every field name.

$(deriveJSON (drop 4) ''D)

This will result in the following (simplified) code to be spliced in your program:

import Control.Applicative
import Control.Monad
import Data.Aeson
import Data.Aeson.TH
import qualified Data.Map    as M
import qualified Data.Text   as T
import qualified Data.Vector as V

instance ToJSON a => ToJSON (D a) where
    toJSON =
      value ->
        case value of
          Nullary ->
              object [pack "Nullary" .= toJSON ([] :: [()])]
          Unary arg1 ->
              object [pack "Unary" .= toJSON arg1]
          Product arg1 arg2 arg3 ->
              object [ pack "Product"
                       .= toJSON [ toJSON arg1
                                 , toJSON arg2
                                 , toJSON arg3
                                 ]
                     ]
          Record arg1 arg2 arg3 ->
              object [ pack "Record"
                       .= object [ pack "One"   .= arg1
                                 , pack "Two"   .= arg2
                                 , pack "Three" .= arg3
                                 ]
                     ]

instance FromJSON a => FromJSON (D a) where
    parseJSON =
      value ->
        case value of
          Object obj ->
            case toList obj of
              [(conKey, conVal)] ->
                  case conKey of
                    _ | (conKey == pack "Nullary") ->
                          case conVal of
                            Array arr | null arr -> pure Nullary
                            _ -> mzero
                      | (conKey == pack "Unary") ->
                          case conVal of
                            arg -> Unary <$> parseJSON arg
                      | (conKey == pack "Product") ->
                          case conVal of
                            Array arr | length arr == 3 ->
                              Product <$> parseJSON (arr ! 0)
                                      <*> parseJSON (arr ! 1)
                                      <*> parseJSON (arr ! 2)
                            _ -> mzero
                      | (conKey == pack "Record") ->
                          case conVal of
                            Object obj ->
                              Record <$> (obj .: pack "One")
                                     <*> (obj .: pack "Two")
                                     <*> (obj .: pack "Three")
                            _ -> mzero
                     | otherwise -> mzero
              _ -> mzero
          _ -> mzero

Now we can use the newly created instances.

d :: D Int
d = Record { testOne = 3.14159
           , testTwo = True
           , testThree = Product "test" 'A' 123
           }

>>> fromJSON (toJSON d) == Success d
> True

Documentation

deriveJSON

Arguments

:: (String -> String)

Function to change field names.

-> Name

Name of the type for which to generate ToJSON and FromJSON instances.

-> Q [Dec]

Generates both ToJSON and FromJSON instance declarations for the given data type.

This is a convienience function which is equivalent to calling both deriveToJSON and deriveFromJSON.

deriveToJSON

Arguments

:: (String -> String)

Function to change field names.

-> Name

Name of the type for which to generate a ToJSON instance declaration.

-> Q [Dec]

Generates a ToJSON instance declaration for the given data type.

Example:

 data Foo = Foo Char Int
 $(deriveToJSON id ''Foo)

This will splice in the following code:

 instance ToJSON Foo where
      toJSON =
          value -> case value of
                      Foo arg1 arg2 -> toJSON [toJSON arg1, toJSON arg2]

deriveFromJSON

Arguments

:: (String -> String)

Function to change field names.

-> Name

Name of the type for which to generate a FromJSON instance declaration.

-> Q [Dec]

Generates a FromJSON instance declaration for the given data type.

Example:

 data Foo = Foo Char Int
 $(deriveFromJSON id ''Foo)

This will splice in the following code:

 instance FromJSON Foo where
     parseJSON =
         value -> case value of
                     Array arr | (length arr == 2) ->
                        Foo <$> parseJSON (arr ! 0)
                            <*> parseJSON (arr ! 1)
                     _ -> mzero

mkToJSON

Arguments

:: (String -> String)

Function to change field names.

-> Name

Name of the type to encode.

-> Q Exp

Generates a lambda expression which encodes the given data type as JSON.

Example:

 data Foo = Foo Int

 encodeFoo :: Foo -> Value
 encodeFoo = $(mkToJSON id ''Foo)

This will splice in the following code:

 value -> case value of Foo arg1 -> toJSON arg1

mkParseJSON

Arguments

:: (String -> String)

Function to change field names.

-> Name

Name of the encoded type.

-> Q Exp

Generates a lambda expression which parses the JSON encoding of the given data type.

Example:

 data Foo = Foo Int

 parseFoo :: Value -> Parser Foo
 parseFoo = $(mkParseJSON id ''Foo)

This will splice in the following code:

 \value -> case value of arg -> Foo <$> parseJSON arg