Data.Aeson

Efficiently deserialize a JSON value from a strict B.ByteString. If this fails due to incomplete or invalid input, Nothing is returned.

Efficiently deserialize a JSON value from a file. If this fails due to incomplete or invalid input, Nothing is returned.

The input file's content must consist solely of a JSON document, with no trailing data except for whitespace.

This function parses immediately, but defers conversion. See json for details.

Efficiently deserialize a JSON value from a strict ByteString. If this fails due to incomplete or invalid input, Nothing is returned.

Since 2.2.0.0 an alias for decodeStrict.

Efficiently deserialize a JSON value from a file. If this fails due to incomplete or invalid input, Nothing is returned.

Since 2.2.0.0 an alias for decodeFileStrict.

Like decodeStrict but returns an error message when decoding fails.

Like decodeFileStrict but returns an error message when decoding fails.

Like decodeStrict' but returns an error message when decoding fails.

Since 2.2.0.0 an alias for eitherDecodeStrict.

Like decodeFileStrict' but returns an error message when decoding fails.

Since 2.2.0.0 an alias for eitherDecodeFileStrict'.

Efficiently deserialize a JSON value from a strict Text. If this fails due to incomplete or invalid input, Nothing is returned.

Like decodeStrictText but returns an error message when decoding fails.

Exception thrown by throwDecode and variants.

Like decode but throws an AesonException when decoding fails.

throwDecode is in aeson since 2.1.2.0, but this variant is added later.

Like decodeStrict but throws an AesonException when decoding fails.

Like decodeStrictText but throws an AesonException when decoding fails.

Like decode' but throws an AesonException when decoding fails.

Since 2.2.0.0 an alias for throwDecode.

Like decodeStrict' but throws an AesonException when decoding fails.

Since 2.2.0.0 an alias for throwDecodeStrict.

Typeclass for types that can be used as the key of a map-like container (like Map or HashMap). For example, since Text has a ToJSONKey instance and Char has a ToJSON instance, we can encode a value of type Map Text Char:

>>> LBC8.putStrLn $ encode $ Map.fromList [("foo" :: Text, 'a')]{"foo":"a"}

Since Int also has a ToJSONKey instance, we can similarly write:

>>> LBC8.putStrLn $ encode $ Map.fromList [(5 :: Int, 'a')]{"5":"a"}

JSON documents only accept strings as object keys. For any type from base that has a natural textual representation, it can be expected that its ToJSONKey instance will choose that representation.

For data types that lack a natural textual representation, an alternative is provided. The map-like container is represented as a JSON array instead of a JSON object. Each value in the array is an array with exactly two values. The first is the key and the second is the value.

For example, values of type '[Text]' cannot be encoded to a string, so a Map with keys of type '[Text]' is encoded as follows:

>>> LBC8.putStrLn $ encode $ Map.fromList [(["foo","bar","baz" :: Text], 'a')][[["foo","bar","baz"],"a"]]

The default implementation of ToJSONKey chooses this method of encoding a key, using the ToJSON instance of the type.

To use your own data type as the key in a map, all that is needed is to write a ToJSONKey (and possibly a FromJSONKey) instance for it. If the type cannot be trivially converted to and from Text, it is recommended that ToJSONKeyValue is used. Since the default implementations of the typeclass methods can build this from a ToJSON instance, there is nothing that needs to be written:

data Foo = Foo { fooAge :: Int, fooName :: Text }
  deriving (Eq,Ord,Generic)
instance ToJSON Foo
instance ToJSONKey Foo

That's it. We can now write:

>>> let m = Map.fromList [(Foo 4 "bar",'a'),(Foo 6 "arg",'b')]>>> LBC8.putStrLn $ encode m[[{"fooName":"bar","fooAge":4},"a"],[{"fooName":"arg","fooAge":6},"b"]]

The next case to consider is if we have a type that is a newtype wrapper around Text. The recommended approach is to use generalized newtype deriving:

newtype RecordId = RecordId { getRecordId :: Text }
  deriving (Eq,Ord,ToJSONKey)

Then we may write:

>>> LBC8.putStrLn $ encode $ Map.fromList [(RecordId "abc",'a')]{"abc":"a"}

Simple sum types are a final case worth considering. Suppose we have:

data Color = Red | Green | Blue
  deriving (Show,Read,Eq,Ord)

It is possible to get the ToJSONKey instance for free as we did with Foo. However, in this case, we have a natural way to go to and from Text that does not require any escape sequences. So ToJSONKeyText can be used instead of ToJSONKeyValue to encode maps as objects instead of arrays of pairs. This instance may be implemented using generics as follows:

instance ToJSONKey Color where
  toJSONKey = genericToJSONKey defaultJSONKeyOptions

Low-level implementations

The Show instance can be used to help write ToJSONKey:

instance ToJSONKey Color where
  toJSONKey = ToJSONKeyText f g
    where f = Text.pack . show
          g = text . Text.pack . show
          -- text function is from Data.Aeson.Encoding

The situation of needing to turning function a -> Text into a ToJSONKeyFunction is common enough that a special combinator is provided for it. The above instance can be rewritten as:

instance ToJSONKey Color where
  toJSONKey = toJSONKeyText (Text.pack . show)

The performance of the above instance can be improved by not using String as an intermediate step when converting to Text. One option for improving performance would be to use template haskell machinery from the text-show package. However, even with the approach, the Encoding (a wrapper around a bytestring builder) is generated by encoding the Text to a ByteString, an intermediate step that could be avoided. The fastest possible implementation would be:

-- Assuming that OverloadedStrings is enabled
instance ToJSONKey Color where
  toJSONKey = ToJSONKeyText f g
    where f x = case x of {Red -> "Red";Green ->"Green";Blue -> "Blue"}
          g x = case x of {Red -> text "Red";Green -> text "Green";Blue -> text "Blue"}
          -- text function is from Data.Aeson.Encoding

This works because GHC can lift the encoded values out of the case statements, which means that they are only evaluated once. This approach should only be used when there is a serious need to maximize performance.

Read the docs for ToJSONKey first. This class is a conversion in the opposite direction. If you have a newtype wrapper around Text, the recommended way to define instances is with generalized newtype deriving:

newtype SomeId = SomeId { getSomeId :: Text }
  deriving (Eq,Ord,Hashable,FromJSONKey)

If you have a sum of nullary constructors, you may use the generic implementation:

data Color = Red | Green | Blue
  deriving Generic

instance FromJSONKey Color where
  fromJSONKey = genericFromJSONKey defaultJSONKeyOptions

This type is related to ToJSONKeyFunction. If FromJSONKeyValue is used in the FromJSONKey instance, then ToJSONKeyValue should be used in the ToJSONKey instance. The other three data constructors for this type all correspond to ToJSONKeyText. Strictly speaking, FromJSONKeyTextParser is more powerful than FromJSONKeyText, which is in turn more powerful than FromJSONKeyCoerce. For performance reasons, these exist as three options instead of one.

fromJSONKey for Generic types. These types must be sums of nullary constructors, whose names will be used as keys for JSON objects.

See also genericToJSONKey.

Example

data Color = Red | Green | Blue
  deriving Generic

instance FromJSONKey Color where
  fromJSONKey = genericFromJSONKey defaultJSONKeyOptions

Lifting of the FromJSON class to unary type constructors.

Instead of manually writing your FromJSON1 instance, there are two options to do it automatically:

• Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time. The generated instance is optimized for your type so it will probably be more efficient than the following option.

• The compiler can provide a default generic implementation for liftParseJSON.

To use the second, simply add a deriving Generic1 clause to your datatype and declare a FromJSON1 instance for your datatype without giving a definition for liftParseJSON.

For example:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

data Pair a b = Pair { pairFst :: a, pairSnd :: b } deriving Generic1

instance FromJSON a => FromJSON1 (Pair a)

or

deriving via Generically1 (Pair a) instance FromJSON1 (Pair a)

If the default implementation doesn't give exactly the results you want, you can customize the generic decoding with only a tiny amount of effort, using genericLiftParseJSON with your preferred Options:

customOptions = defaultOptions
                { fieldLabelModifier = map toUpper
                }

instance FromJSON a => FromJSON1 (Pair a) where
    liftParseJSON = genericLiftParseJSON customOptions

Lift the standard parseJSON function through the type constructor.

Lifting of the FromJSON class to binary type constructors.

Instead of manually writing your FromJSON2 instance, Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time.

Lift the standard parseJSON function through the type constructor.

Lifting of the ToJSON class to unary type constructors.

Instead of manually writing your ToJSON1 instance, there are two options to do it automatically:

• Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time. The generated instance is optimized for your type so it will probably be more efficient than the following option.

• The compiler can provide a default generic implementation for toJSON1.

To use the second, simply add a deriving Generic1 clause to your datatype and declare a ToJSON1 instance for your datatype without giving definitions for liftToJSON or liftToEncoding.

For example:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

data Pair a b = Pair { pairFst :: a, pairSnd :: b } deriving Generic1

instance ToJSON a => ToJSON1 (Pair a)

If the default implementation doesn't give exactly the results you want, you can customize the generic encoding with only a tiny amount of effort, using genericLiftToJSON and genericLiftToEncoding with your preferred Options:

customOptions = defaultOptions
                { fieldLabelModifier = map toUpper
                }

instance ToJSON a => ToJSON1 (Pair a) where
    liftToJSON     = genericLiftToJSON customOptions
    liftToEncoding = genericLiftToEncoding customOptions

See also ToJSON.

Lift the standard toJSON function through the type constructor.

Lift the standard toEncoding function through the type constructor.

Lifting of the ToJSON class to binary type constructors.

Instead of manually writing your ToJSON2 instance, Data.Aeson.TH provides Template Haskell functions which will derive an instance at compile time.

The compiler cannot provide a default generic implementation for liftToJSON2, unlike toJSON and liftToJSON.