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 |
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.
instanceToJSON
Person where -- No need to provide atoJSON
implementation. -- For efficiency, we write a simpletoEncoding
implementation, as -- the default version usestoJSON
.toEncoding
=genericToEncoding
defaultOptions
instanceFromJSON
Person -- No need to provide aparseJSON
implementation.
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 Show
To 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 aValue
toJSON
(Person name age) =object
["name".=
name, "age".=
age] -- this encodes directly to a bytestring BuildertoEncoding
(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 Int
s:
>>> decode "[1,2,3]" :: Maybe [Int] Just [1,2,3]
And here's an example using the Map
type to get a map of
Int
s.
>>> 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 Int
s.
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 theFromJSON
class is used to convert to the desired type.
There are two ways to encode a value.
- Convert to a
Value
usingtoJSON
, 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
=genericToEncoding
defaultOptions
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 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 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 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.
DotNetTime | |
|
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:
fail
yields 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(
;<|>
)typeMismatch
can be used to report a failure when the encountered value is not of the expected JSON type;unexpected
is 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 writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSON
Coord whereparseJSON
(Object
v) = Coord<$>
v.:
"x"<*>
v.:
"y" -- We do not expect a non-Object
value here. -- We could useempty
to fail, buttypeMismatch
-- gives a much more informative error message.parseJSON
invalid =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:
instanceFromJSON
Coord 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 Generic
FromJSON
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 } derivingGeneric
instanceFromJSON
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
=map
toUpper
} instanceFromJSON
Coord whereparseJSON
=genericParseJSON
customOptions
Nothing
Instances
The result of running a Parser
.
Instances
Monad Result Source # | |
Functor Result Source # | |
MonadFail Result Source # | |
Defined in Data.Aeson.Types.Internal | |
Applicative Result Source # | |
Foldable Result Source # | |
Defined in Data.Aeson.Types.Internal fold :: Monoid m => Result m -> m # foldMap :: Monoid m => (a -> m) -> Result a -> m # foldMap' :: Monoid m => (a -> m) -> Result a -> m # foldr :: (a -> b -> b) -> b -> Result a -> b # foldr' :: (a -> b -> b) -> b -> Result a -> b # foldl :: (b -> a -> b) -> b -> Result a -> b # foldl' :: (b -> a -> b) -> b -> Result a -> b # foldr1 :: (a -> a -> a) -> Result a -> a # foldl1 :: (a -> a -> a) -> Result a -> a # elem :: Eq a => a -> Result a -> Bool # maximum :: Ord a => Result a -> a # minimum :: Ord a => Result a -> a # | |
Traversable Result Source # | |
Alternative Result Source # | |
MonadPlus Result Source # | |
Eq a => Eq (Result a) Source # | |
Show a => Show (Result a) Source # | |
Semigroup (Result a) Source # | |
Monoid (Result a) Source # | |
NFData a => NFData (Result a) Source # | |
Defined in Data.Aeson.Types.Internal |
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 writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceToJSON
Coord 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 Generic
ToJSON
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 } derivingGeneric
instanceToJSON
Coord wheretoEncoding
=genericToEncoding
defaultOptions
If on the other hand you wish to customize the generic decoding, you have to implement both methods:
customOptions =defaultOptions
{fieldLabelModifier
=map
toUpper
} instanceToJSON
Coord wheretoJSON
=genericToJSON
customOptionstoEncoding
=genericToEncoding
customOptions
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
toJSON
is 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
toEncoding
still compile and have the correct semantics. This is ensured by making the default implementation oftoEncoding
usetoJSON
. 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 Coord
would be sufficient as a generically decoding instance, but there probably exists no good reason to not specifytoEncoding
in new instances.)
Nothing
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.
instanceToJSON
Coord wheretoEncoding
=genericToEncoding
defaultOptions
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 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:
instanceToJSONKey
Color wheretoJSONKey
=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.
Nothing
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 #
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 contramap :: (a -> b) -> ToJSONKeyFunction b -> ToJSONKeyFunction a # (>$) :: b -> ToJSONKeyFunction b -> ToJSONKeyFunction a # |
class FromJSONKey a where Source #
Read the docs for ToJSONKey
first. This class is a conversion
in the opposite direction. If you have a newtype wrapper around Text
,
the recommended way to define instances is with generalized newtype deriving:
newtype SomeId = SomeId { getSomeId :: Text } deriving (Eq,Ord,Hashable,FromJSONKey)
If you have a sum of nullary constructors, you may use the generic implementation:
data Color = Red | Green | Blue deriving Generic instanceFromJSONKey
Color wherefromJSONKey
=genericFromJSONKey
defaultJSONKeyOptions
Nothing
fromJSONKey :: FromJSONKeyFunction a Source #
Strategy for parsing the key of a map-like container.
default fromJSONKey :: FromJSON a => FromJSONKeyFunction a Source #
fromJSONKeyList :: FromJSONKeyFunction [a] Source #
This is similar in spirit to the readList
method of Read
.
It makes it possible to give String
keys special treatment
without using OverlappingInstances
. End users should always
be able to use the default implementation of this method.
default fromJSONKeyList :: FromJSON a => FromJSONKeyFunction [a] Source #
Instances
data FromJSONKeyFunction a where Source #
This type is related to ToJSONKeyFunction
. If FromJSONKeyValue
is used in the
FromJSONKey
instance, then ToJSONKeyValue
should be used in the ToJSONKey
instance. The other three data constructors for this type all correspond to
ToJSONKeyText
. Strictly speaking, FromJSONKeyTextParser
is more powerful than
FromJSONKeyText
, which is in turn more powerful than FromJSONKeyCoerce
.
For performance reasons, these exist as three options instead of one.
FromJSONKeyCoerce | |
FromJSONKeyText | |
| |
FromJSONKeyTextParser | |
| |
FromJSONKeyValue | |
|
Instances
Functor FromJSONKeyFunction Source # | Only law abiding up to interpretation |
Defined in Data.Aeson.Types.FromJSON fmap :: (a -> b) -> FromJSONKeyFunction a -> FromJSONKeyFunction b # (<$) :: 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 derivingGeneric
instanceFromJSONKey
Color wherefromJSONKey
=genericFromJSONKey
defaultJSONKeyOptions
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 Generic1
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 } derivingGeneric1
instanceFromJSON
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
=map
toUpper
} instanceFromJSON
a =>FromJSON1
(Pair a) whereliftParseJSON
=genericLiftParseJSON
customOptions
Nothing
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.
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 #