| Copyright | (c) 2011-2016 Bryan O'Sullivan (c) 2011 MailRank Inc. |
|---|---|
| License | BSD3 |
| Maintainer | Bryan O'Sullivan <bos@serpentine.com> |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | None |
| Language | Haskell2010 |
Data.Aeson
Description
Types and functions for working efficiently with JSON data.
(A note on naming: in Greek mythology, Aeson was the father of Jason.)
Synopsis
- decode :: FromJSON a => ByteString -> Maybe a
- decode' :: FromJSON a => ByteString -> Maybe a
- eitherDecode :: FromJSON a => ByteString -> Either String a
- eitherDecode' :: FromJSON a => ByteString -> Either String a
- encode :: ToJSON a => a -> ByteString
- encodeFile :: ToJSON a => FilePath -> a -> IO ()
- decodeStrict :: FromJSON a => ByteString -> Maybe a
- decodeFileStrict :: FromJSON a => FilePath -> IO (Maybe a)
- decodeStrict' :: FromJSON a => ByteString -> Maybe a
- decodeFileStrict' :: FromJSON a => FilePath -> IO (Maybe a)
- eitherDecodeStrict :: FromJSON a => ByteString -> Either String a
- eitherDecodeFileStrict :: FromJSON a => FilePath -> IO (Either String a)
- eitherDecodeStrict' :: FromJSON a => ByteString -> Either String a
- eitherDecodeFileStrict' :: FromJSON a => FilePath -> IO (Either String a)
- data Value
- type Encoding = Encoding' Value
- fromEncoding :: Encoding' tag -> Builder
- type Array = Vector Value
- type Object = KeyMap Value
- data Key
- newtype DotNetTime = DotNetTime {}
- class FromJSON a where
- data Result a
- fromJSON :: FromJSON a => Value -> Result a
- class ToJSON a where
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- toJSONList :: [a] -> Value
- toEncodingList :: [a] -> Encoding
- class KeyValue kv where
- (<?>) :: Parser a -> JSONPathElement -> Parser a
- type JSONPath = [JSONPathElement]
- class ToJSONKey a where
- toJSONKey :: ToJSONKeyFunction a
- toJSONKeyList :: ToJSONKeyFunction [a]
- data ToJSONKeyFunction a
- = ToJSONKeyText !(a -> Key) !(a -> Encoding' Key)
- | ToJSONKeyValue !(a -> Value) !(a -> Encoding)
- class FromJSONKey a where
- data FromJSONKeyFunction a where
- FromJSONKeyCoerce :: Coercible Text a => FromJSONKeyFunction a
- FromJSONKeyText :: !(Text -> a) -> FromJSONKeyFunction a
- FromJSONKeyTextParser :: !(Text -> Parser a) -> FromJSONKeyFunction a
- FromJSONKeyValue :: !(Value -> Parser a) -> FromJSONKeyFunction a
- class GetConName f => GToJSONKey f
- genericToJSONKey :: (Generic a, GToJSONKey (Rep a)) => JSONKeyOptions -> ToJSONKeyFunction a
- class (ConstructorNames f, SumFromString f) => GFromJSONKey f
- genericFromJSONKey :: forall a. (Generic a, GFromJSONKey (Rep a)) => JSONKeyOptions -> FromJSONKeyFunction a
- class FromJSON1 f where
- parseJSON1 :: (FromJSON1 f, FromJSON a) => Value -> Parser (f a)
- class FromJSON2 f where
- parseJSON2 :: (FromJSON2 f, FromJSON a, FromJSON b) => Value -> Parser (f a b)
- class ToJSON1 f where
- liftToJSON :: (a -> Value) -> ([a] -> Value) -> f a -> Value
- liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [f a] -> Value
- liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> f a -> Encoding
- liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [f a] -> Encoding
- toJSON1 :: (ToJSON1 f, ToJSON a) => f a -> Value
- toEncoding1 :: (ToJSON1 f, ToJSON a) => f a -> Encoding
- class ToJSON2 f where
- liftToJSON2 :: (a -> Value) -> ([a] -> Value) -> (b -> Value) -> ([b] -> Value) -> f a b -> Value
- liftToJSONList2 :: (a -> Value) -> ([a] -> Value) -> (b -> Value) -> ([b] -> Value) -> [f a b] -> Value
- liftToEncoding2 :: (a -> Encoding) -> ([a] -> Encoding) -> (b -> Encoding) -> ([b] -> Encoding) -> f a b -> Encoding
- liftToEncodingList2 :: (a -> Encoding) -> ([a] -> Encoding) -> (b -> Encoding) -> ([b] -> Encoding) -> [f a b] -> Encoding
- toJSON2 :: (ToJSON2 f, ToJSON a, ToJSON b) => f a b -> Value
- toEncoding2 :: (ToJSON2 f, ToJSON a, ToJSON b) => f a b -> Encoding
- class GFromJSON arity f
- data FromArgs arity a
- type GToJSON = GToJSON' Value
- type GToEncoding = GToJSON' Encoding
- class GToJSON' enc arity f
- data ToArgs res arity a
- data Zero
- data One
- genericToJSON :: (Generic a, GToJSON' Value Zero (Rep a)) => Options -> a -> Value
- genericLiftToJSON :: (Generic1 f, GToJSON' Value One (Rep1 f)) => Options -> (a -> Value) -> ([a] -> Value) -> f a -> Value
- genericToEncoding :: (Generic a, GToJSON' Encoding Zero (Rep a)) => Options -> a -> Encoding
- genericLiftToEncoding :: (Generic1 f, GToJSON' Encoding One (Rep1 f)) => Options -> (a -> Encoding) -> ([a] -> Encoding) -> f a -> Encoding
- genericParseJSON :: (Generic a, GFromJSON Zero (Rep a)) => Options -> Value -> Parser a
- genericLiftParseJSON :: (Generic1 f, GFromJSON One (Rep1 f)) => Options -> (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (f a)
- data Options
- defaultOptions :: Options
- fieldLabelModifier :: Options -> String -> String
- constructorTagModifier :: Options -> String -> String
- allNullaryToStringTag :: Options -> Bool
- omitNothingFields :: Options -> Bool
- sumEncoding :: Options -> SumEncoding
- unwrapUnaryRecords :: Options -> Bool
- tagSingleConstructors :: Options -> Bool
- rejectUnknownFields :: Options -> Bool
- data SumEncoding
- camelTo2 :: Char -> String -> String
- defaultTaggedObject :: SumEncoding
- data JSONKeyOptions
- keyModifier :: JSONKeyOptions -> String -> String
- defaultJSONKeyOptions :: JSONKeyOptions
- withObject :: String -> (Object -> Parser a) -> Value -> Parser a
- withText :: String -> (Text -> Parser a) -> Value -> Parser a
- withArray :: String -> (Array -> Parser a) -> Value -> Parser a
- withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser a
- withBool :: String -> (Bool -> Parser a) -> Value -> Parser a
- withEmbeddedJSON :: String -> (Value -> Parser a) -> Value -> Parser a
- data Series
- pairs :: Series -> Encoding
- foldable :: (Foldable t, ToJSON a) => t a -> Encoding
- (.:) :: FromJSON a => Object -> Key -> Parser a
- (.:?) :: FromJSON a => Object -> Key -> Parser (Maybe a)
- (.:!) :: FromJSON a => Object -> Key -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- object :: [Pair] -> Value
- json :: Parser Value
- json' :: Parser Value
- parseIndexedJSON :: (Value -> Parser a) -> Int -> Value -> Parser a
How to use this library
This section contains basic information on the different ways to work with data using this library. These range from simple but inflexible, to complex but flexible.
The most common way to use the library is to define a data type,
corresponding to some JSON data you want to work with, and then
write either a FromJSON instance, a to ToJSON instance, or both
for that type.
For example, given this JSON data:
{ "name": "Joe", "age": 12 }we create a matching data type:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Person = Person {
name :: Text
, age :: Int
} deriving (Generic, Show)The LANGUAGE pragma and Generic instance let us write empty
FromJSON and ToJSON instances for which the compiler will
generate sensible default implementations.
instanceToJSONPerson where -- No need to provide atoJSONimplementation. -- For efficiency, we write a simpletoEncodingimplementation, as -- the default version usestoJSON.toEncoding=genericToEncodingdefaultOptionsinstanceFromJSONPerson -- No need to provide aparseJSONimplementation.
We can now encode a value like so:
>>> encode (Person {name = "Joe", age = 12})
"{\"name\":\"Joe\",\"age\":12}"Writing instances by hand
When necessary, we can write ToJSON and FromJSON instances by
hand. This is valuable when the JSON-on-the-wire and Haskell data
are different or otherwise need some more carefully managed
translation. Let's revisit our JSON data:
{ "name": "Joe", "age": 12 }We once again create a matching data type, without bothering to add
a Generic instance this time:
data Person = Person {
name :: Text
, age :: Int
} deriving ShowTo decode data, we need to define a FromJSON instance:
{-# LANGUAGE OverloadedStrings #-}
instance FromJSON Person where
parseJSON = withObject "Person" $ \v -> Person
<$> v .: "name"
<*> v .: "age"We can now parse the JSON data like so:
>>> decode "{\"name\":\"Joe\",\"age\":12}" :: Maybe Person
Just (Person {name = "Joe", age = 12})To encode data, we need to define a ToJSON instance. Let's begin
with an instance written entirely by hand.
instance ToJSON Person where
-- this generates a Value
toJSON (Person name age) =
object ["name" .= name, "age" .= age]
-- this encodes directly to a bytestring Builder
toEncoding (Person name age) =
pairs ("name" .= name <> "age" .= age)
We can now encode a value like so:
>>> encode (Person {name = "Joe", age = 12})
"{\"name\":\"Joe\",\"age\":12}"There are predefined FromJSON and ToJSON instances for many
types. Here's an example using lists and Ints:
>>> decode "[1,2,3]" :: Maybe [Int] Just [1,2,3]
And here's an example using the Map type to get a map of
Ints.
>>> decode "{\"foo\":1,\"bar\":2}" :: Maybe (Map String Int)
Just (fromList [("bar",2),("foo",1)])Working with the AST
Sometimes you want to work with JSON data directly, without first
converting it to a custom data type. This can be useful if you want
to e.g. convert JSON data to YAML data, without knowing what the
contents of the original JSON data was. The Value type, which is
an instance of FromJSON, is used to represent an arbitrary JSON
AST (abstract syntax tree). Example usage:
>>> decode "{\"foo\": 123}" :: Maybe Value
Just (Object (fromList [("foo",Number 123)]))>>> decode "{\"foo\": [\"abc\",\"def\"]}" :: Maybe Value
Just (Object (fromList [("foo",Array (fromList [String "abc",String "def"]))]))Once you have a Value you can write functions to traverse it and
make arbitrary transformations.
Decoding to a Haskell value
We can decode to any instance of FromJSON:
λ> decode "[1,2,3]" :: Maybe [Int] Just [1,2,3]
Alternatively, there are instances for standard data types, so you
can use them directly. For example, use the Map type to
get a map of Ints.
λ> import Data.Map
λ> decode "{\"foo\":1,\"bar\":2}" :: Maybe (Map String Int)
Just (fromList [("bar",2),("foo",1)])Decoding a mixed-type object
The above approach with maps of course will not work for mixed-type objects that don't follow a strict schema, but there are a couple of approaches available for these.
The Object type contains JSON objects:
λ> decode "{\"name\":\"Dave\",\"age\":2}" :: Maybe Object
Just (fromList [("name",String "Dave"),("age",Number 2)])You can extract values from it with a parser using parse,
parseEither or, in this example, parseMaybe:
λ> do result <- decode "{\"name\":\"Dave\",\"age\":2}"
flip parseMaybe result $ \obj -> do
age <- obj .: "age"
name <- obj .: "name"
return (name ++ ": " ++ show (age*2))
Just "Dave: 4"Considering that any type that implements FromJSON can be used
here, this is quite a powerful way to parse JSON. See the
documentation in FromJSON for how to implement this class for
your own data types.
The downside is that you have to write the parser yourself; the upside is that you have complete control over the way the JSON is parsed.
Encoding and decoding
Decoding is a two-step process.
- When decoding a value, the process is reversed: the bytes are
converted to a
Value, then theFromJSONclass is used to convert to the desired type.
There are two ways to encode a value.
- Convert to a
ValueusingtoJSON, then possibly further encode. This was the only method available in aeson 0.9 and earlier. - Directly encode (to what will become a
ByteString) usingtoEncoding. This is much more efficient (about 3x faster, and less memory intensive besides), but is only available in aeson 0.10 and newer.
For convenience, the encode and decode functions combine both
steps.
Direct encoding
In older versions of this library, encoding a Haskell value
involved converting to an intermediate Value, then encoding that.
A "direct" encoder converts straight from a source Haskell value
to a ByteString without constructing an intermediate Value.
This approach is faster than toJSON, and allocates less memory.
The toEncoding method makes it possible to implement direct
encoding with low memory overhead.
To complicate matters, the default implementation of toEncoding
uses toJSON. Why? The toEncoding method was added to this
library much more recently than toJSON. Using toJSON ensures
that packages written against older versions of this library will
compile and produce correct output, but they will not see any
speedup from direct encoding.
To write a minimal implementation of direct encoding, your type
must implement GHC's Generic class, and your code should look
like this:
toEncoding=genericToEncodingdefaultOptions
What if you have more elaborate encoding needs? For example, perhaps you need to change the names of object keys, omit parts of a value.
To encode to a JSON "object", use the pairs function.
toEncoding(Person name age) =pairs("name".=name<>"age".=age)
Any container type that implements Foldable can be encoded to a
JSON "array" using foldable.
> import Data.Sequence as Seq > encode (Seq.fromList [1,2,3]) "[1,2,3]"
decode :: FromJSON a => ByteString -> Maybe a Source #
Efficiently deserialize a JSON value from a lazy ByteString.
If this fails due to incomplete or invalid input, Nothing is
returned.
The input 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.
decode' :: FromJSON a => ByteString -> Maybe a Source #
Efficiently deserialize a JSON value from a lazy ByteString.
If this fails due to incomplete or invalid input, Nothing is
returned.
The input must consist solely of a JSON document, with no trailing data except for whitespace.
This function parses and performs conversion immediately. See
json' for details.
eitherDecode :: FromJSON a => ByteString -> Either String a Source #
Like decode but returns an error message when decoding fails.
eitherDecode' :: FromJSON a => ByteString -> Either String a Source #
Like decode' but returns an error message when decoding fails.
encode :: ToJSON a => a -> ByteString Source #
Efficiently serialize a JSON value as a lazy ByteString.
This is implemented in terms of the ToJSON class's toEncoding method.
encodeFile :: ToJSON a => FilePath -> a -> IO () Source #
Efficiently serialize a JSON value as a lazy ByteString and write it to a file.
Variants for strict bytestrings
decodeStrict :: FromJSON a => ByteString -> Maybe a Source #
Efficiently deserialize a JSON value from a strict ByteString.
If this fails due to incomplete or invalid input, Nothing is
returned.
The input 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.
decodeFileStrict :: FromJSON a => FilePath -> IO (Maybe a) Source #
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.
decodeStrict' :: FromJSON a => ByteString -> Maybe a Source #
Efficiently deserialize a JSON value from a strict ByteString.
If this fails due to incomplete or invalid input, Nothing is
returned.
The input must consist solely of a JSON document, with no trailing data except for whitespace.
This function parses and performs conversion immediately. See
json' for details.
decodeFileStrict' :: FromJSON a => FilePath -> IO (Maybe a) Source #
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 and performs conversion immediately. See
json' for details.
eitherDecodeStrict :: FromJSON a => ByteString -> Either String a Source #
Like decodeStrict but returns an error message when decoding fails.
eitherDecodeFileStrict :: FromJSON a => FilePath -> IO (Either String a) Source #
Like decodeFileStrict but returns an error message when decoding fails.
eitherDecodeStrict' :: FromJSON a => ByteString -> Either String a Source #
Like decodeStrict' but returns an error message when decoding fails.
eitherDecodeFileStrict' :: FromJSON a => FilePath -> IO (Either String a) Source #
Like decodeFileStrict' but returns an error message when decoding fails.
Core JSON types
A JSON value represented as a Haskell value.
Instances
| Eq Value Source # | |
| Data Value Source # | |
Defined in Data.Aeson.Types.Internal Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Value -> c Value # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c Value # dataTypeOf :: Value -> DataType # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c Value) # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c Value) # gmapT :: (forall b. Data b => b -> b) -> Value -> Value # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Value -> r # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Value -> r # gmapQ :: (forall d. Data d => d -> u) -> Value -> [u] # gmapQi :: Int -> (forall d. Data d => d -> u) -> Value -> u # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Value -> m Value # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Value -> m Value # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Value -> m Value # | |
| Ord Value Source # | The ordering is total, consistent with Since: 1.5.2.0 |
| Read Value Source # | |
| Show Value Source # | Since version 1.5.6.0 version object values are printed in lexicographic key order
|
| IsString Value Source # | |
Defined in Data.Aeson.Types.Internal Methods fromString :: String -> Value # | |
| Generic Value Source # | |
| Function Value Source # | Since: 2.0.3.0 |
| Arbitrary Value Source # | Since: 2.0.3.0 |
| CoArbitrary Value Source # | Since: 2.0.3.0 |
Defined in Data.Aeson.Types.Internal Methods coarbitrary :: Value -> Gen b -> Gen b # | |
| NFData Value Source # | |
Defined in Data.Aeson.Types.Internal | |
| Hashable Value Source # | |
Defined in Data.Aeson.Types.Internal | |
| FromJSON Value Source # | |
| KeyValue Pair Source # | |
| KeyValue Object Source # | Constructs a singleton |
| ToJSON Value Source # | |
| Lift Value Source # | Since: 0.11.0.0 |
| GToJSON' Value arity (U1 :: Type -> Type) Source # | |
| GToJSON' Value arity (V1 :: Type -> Type) Source # | |
| GToJSON' Encoding arity (U1 :: Type -> Type) Source # | |
| ToJSON1 f => GToJSON' Value One (Rec1 f) Source # | |
| ToJSON1 f => GToJSON' Encoding One (Rec1 f) Source # | |
| (WriteProduct arity a, WriteProduct arity b, ProductSize a, ProductSize b) => GToJSON' Value arity (a :*: b) Source # | |
| ToJSON a => GToJSON' Value arity (K1 i a :: Type -> Type) Source # | |
| (EncodeProduct arity a, EncodeProduct arity b) => GToJSON' Encoding arity (a :*: b) Source # | |
| ToJSON a => GToJSON' Encoding arity (K1 i a :: Type -> Type) Source # | |
| (ToJSON1 f, GToJSON' Value One g) => GToJSON' Value One (f :.: g) Source # | |
| (ToJSON1 f, GToJSON' Encoding One g) => GToJSON' Encoding One (f :.: g) Source # | |
| type Rep Value Source # | |
Defined in Data.Aeson.Types.Internal type Rep Value = D1 ('MetaData "Value" "Data.Aeson.Types.Internal" "aeson-2.0.3.0-Ad6fk1RlxnEEsrfKUuj9Hu" 'False) ((C1 ('MetaCons "Object" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Object)) :+: (C1 ('MetaCons "Array" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Array)) :+: C1 ('MetaCons "String" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Text)))) :+: (C1 ('MetaCons "Number" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Scientific)) :+: (C1 ('MetaCons "Bool" 'PrefixI 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 Bool)) :+: C1 ('MetaCons "Null" 'PrefixI 'False) (U1 :: Type -> Type)))) | |
fromEncoding :: Encoding' tag -> Builder Source #
Acquire the underlying bytestring builder.
Instances
Convenience types
newtype DotNetTime Source #
A newtype wrapper for UTCTime that uses the same non-standard
serialization format as Microsoft .NET, whose
System.DateTime
type is by default serialized to JSON as in the following example:
/Date(1302547608878)/
The number represents milliseconds since the Unix epoch.
Constructors
| DotNetTime | |
Fields
| |
Instances
Type conversion
class FromJSON a where Source #
A type that can be converted from JSON, with the possibility of failure.
In many cases, you can get the compiler to generate parsing code for you (see below). To begin, let's cover writing an instance by hand.
There are various reasons a conversion could fail. For example, an
Object could be missing a required key, an Array could be of
the wrong size, or a value could be of an incompatible type.
The basic ways to signal a failed conversion are as follows:
failyields a custom error message: it is the recommended way of reporting a failure;empty(ormzero) is uninformative: use it when the error is meant to be caught by some(;<|>)typeMismatchcan be used to report a failure when the encountered value is not of the expected JSON type;unexpectedis an appropriate alternative when more than one type may be expected, or to keep the expected type implicit.
prependFailure (or modifyFailure) add more information to a parser's
error messages.
An example type and instance using typeMismatch and prependFailure:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSONCoord whereparseJSON(Objectv) = Coord<$>v.:"x"<*>v.:"y" -- We do not expect a non-Objectvalue here. -- We could useemptyto fail, buttypeMismatch-- gives a much more informative error message.parseJSONinvalid =prependFailure"parsing Coord failed, " (typeMismatch"Object" invalid)
For this common case of only being concerned with a single
type of JSON value, the functions withObject, withScientific, etc.
are provided. Their use is to be preferred when possible, since
they are more terse. Using withObject, we can rewrite the above instance
(assuming the same language extension and data type) as:
instanceFromJSONCoord whereparseJSON=withObject"Coord" $ \v -> Coord<$>v.:"x"<*>v.:"y"
Instead of manually writing your FromJSON 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
parseJSON.
To use the second, simply add a deriving clause to your
datatype and declare a GenericFromJSON instance for your datatype without giving
a definition for parseJSON.
For example, the previous example can be simplified to just:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord = Coord { x :: Double, y :: Double } deriving Generic
instance FromJSON Coord
The default implementation will be equivalent to
parseJSON = ; if you need different
options, you can customize the generic decoding by defining:genericParseJSON defaultOptions
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceFromJSONCoord whereparseJSON=genericParseJSONcustomOptions
Minimal complete definition
Nothing
Instances
The result of running a Parser.
Instances
fromJSON :: FromJSON a => Value -> Result a Source #
Convert a value from JSON, failing if the types do not match.
A type that can be converted to JSON.
Instances in general must specify toJSON and should (but don't need
to) specify toEncoding.
An example type and instance:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceToJSONCoord wheretoJSON(Coord x y) =object["x".=x, "y".=y]toEncoding(Coord x y) =pairs("x".=x<>"y".=y)
Instead of manually writing your ToJSON 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
toJSON.
To use the second, simply add a deriving clause to your
datatype and declare a GenericToJSON instance. If you require nothing other than
defaultOptions, it is sufficient to write (and this is the only
alternative where the default toJSON implementation is sufficient):
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord = Coord { x :: Double, y :: Double } deriving Generic
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
If on the other hand you wish to customize the generic decoding, you have to implement both methods:
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceToJSONCoord wheretoJSON=genericToJSONcustomOptionstoEncoding=genericToEncodingcustomOptions
Previous versions of this library only had the toJSON method. Adding
toEncoding had two reasons:
- toEncoding is more efficient for the common case that the output of
toJSONis directly serialized to aByteString. Further, expressing either method in terms of the other would be non-optimal. - The choice of defaults allows a smooth transition for existing users:
Existing instances that do not define
toEncodingstill compile and have the correct semantics. This is ensured by making the default implementation oftoEncodingusetoJSON. This produces correct results, but since it performs an intermediate conversion to aValue, it will be less efficient than directly emitting anEncoding. (this also means that specifying nothing more thaninstance ToJSON Coordwould be sufficient as a generically decoding instance, but there probably exists no good reason to not specifytoEncodingin new instances.)
Minimal complete definition
Nothing
Methods
Convert a Haskell value to a JSON-friendly intermediate type.
toEncoding :: a -> Encoding Source #
Encode a Haskell value as JSON.
The default implementation of this method creates an
intermediate Value using toJSON. This provides
source-level compatibility for people upgrading from older
versions of this library, but obviously offers no performance
advantage.
To benefit from direct encoding, you must provide an
implementation for this method. The easiest way to do so is by
having your types implement Generic using the DeriveGeneric
extension, and then have GHC generate a method body as follows.
instanceToJSONCoord wheretoEncoding=genericToEncodingdefaultOptions
toJSONList :: [a] -> Value Source #
toEncodingList :: [a] -> Encoding Source #
Instances
class KeyValue kv where Source #
A key-value pair for encoding a JSON object.
(<?>) :: Parser a -> JSONPathElement -> Parser a Source #
Add JSON Path context to a parser
When parsing a complex structure, it helps to annotate (sub)parsers with context, so that if an error occurs, you can find its location.
withObject "Person" $ \o ->
Person
<$> o .: "name" <?> Key "name"
<*> o .: "age" <?> Key "age"(Standard methods like (.:) already do this.)
With such annotations, if an error occurs, you will get a JSON Path location of that error.
Since 0.10
type JSONPath = [JSONPathElement] Source #
Keys for maps
class ToJSONKey a where Source #
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 FooThat'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:
instanceToJSONKeyColor wheretoJSONKey=genericToJSONKeydefaultJSONKeyOptions
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.EncodingThe 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.EncodingThis 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.
Minimal complete definition
Nothing
Methods
toJSONKey :: ToJSONKeyFunction a Source #
Strategy for rendering the key for a map-like container.
default toJSONKey :: ToJSON a => ToJSONKeyFunction a Source #
toJSONKeyList :: ToJSONKeyFunction [a] Source #
This is similar in spirit to the showsList method of Show.
It makes it possible to give String keys special treatment
without using OverlappingInstances. End users should always
be able to use the default implementation of this method.
default toJSONKeyList :: ToJSON a => ToJSONKeyFunction [a] Source #
Instances
data ToJSONKeyFunction a Source #
Constructors
| ToJSONKeyText !(a -> Key) !(a -> Encoding' Key) | key is encoded to string, produces object |
| ToJSONKeyValue !(a -> Value) !(a -> Encoding) | key is encoded to value, produces array |
Instances
| Contravariant ToJSONKeyFunction Source # | |
Defined in Data.Aeson.Types.ToJSON Methods contramap :: (a -> b) -> ToJSONKeyFunction b -> ToJSONKeyFunction a # (>$) :: b -> ToJSONKeyFunction b -> ToJSONKeyFunction a # | |
class FromJSONKey a where Source #
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 instanceFromJSONKeyColor wherefromJSONKey=genericFromJSONKeydefaultJSONKeyOptions
Minimal complete definition
Nothing
Methods
fromJSONKey :: FromJSONKeyFunction a Source #
Strategy for parsing the key of a map-like container.
default fromJSONKey :: FromJSON a => FromJSONKeyFunction a Source #
fromJSONKeyList :: FromJSONKeyFunction [a] Source #
This is similar in spirit to the readList method of Read.
It makes it possible to give String keys special treatment
without using OverlappingInstances. End users should always
be able to use the default implementation of this method.
default fromJSONKeyList :: FromJSON a => FromJSONKeyFunction [a] Source #
Instances
data FromJSONKeyFunction a where Source #
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.
Constructors
| FromJSONKeyCoerce | |
| FromJSONKeyText | |
Fields
| |
| FromJSONKeyTextParser | |
Fields
| |
| FromJSONKeyValue | |
Fields
| |
Instances
| Functor FromJSONKeyFunction Source # | Only law abiding up to interpretation |
Defined in Data.Aeson.Types.FromJSON Methods fmap :: (a -> b) -> FromJSONKeyFunction a -> FromJSONKeyFunction b # (<$) :: a -> FromJSONKeyFunction b -> FromJSONKeyFunction a # | |
Generic keys
class GetConName f => GToJSONKey f Source #
Instances
| GetConName f => GToJSONKey (f :: k -> Type) Source # | |
Defined in Data.Aeson.Types.ToJSON | |
genericToJSONKey :: (Generic a, GToJSONKey (Rep a)) => JSONKeyOptions -> ToJSONKeyFunction a Source #
class (ConstructorNames f, SumFromString f) => GFromJSONKey f Source #
Instances
| (ConstructorNames f, SumFromString f) => GFromJSONKey f Source # | |
Defined in Data.Aeson.Types.FromJSON | |
genericFromJSONKey :: forall a. (Generic a, GFromJSONKey (Rep a)) => JSONKeyOptions -> FromJSONKeyFunction a Source #
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 derivingGenericinstanceFromJSONKeyColor wherefromJSONKey=genericFromJSONKeydefaultJSONKeyOptions
Liftings to unary and binary type constructors
class FromJSON1 f where Source #
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 clause to your
datatype and declare a Generic1FromJSON1 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)
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=maptoUpper} instanceFromJSONa =>FromJSON1(Pair a) whereliftParseJSON=genericLiftParseJSONcustomOptions
Minimal complete definition
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Methods
liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (f a) Source #
default liftParseJSON :: (Generic1 f, GFromJSON One (Rep1 f)) => (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (f a) Source #
liftParseJSONList :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser [f a] Source #
Instances
parseJSON1 :: (FromJSON1 f, FromJSON a) => Value -> Parser (f a) Source #
Lift the standard parseJSON function through the type constructor.
class FromJSON2 f where Source #
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.
Minimal complete definition
Methods
liftParseJSON2 :: (Value -> Parser a) -> (Value -> Parser [a]) -> (Value -> Parser b) -> (Value -> Parser [b]) -> Value -> Parser (f a b) Source #
liftParseJSONList2 :: (Value -> Parser a) -> (Value -> Parser [a]) -> (Value -> Parser b) -> (Value -> Parser [b]) -> Value -> Parser [f a b] Source #