| 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.Types
Contents
Description
Types for working with JSON data.
- data Value
- type Encoding = Encoding' Value
- unsafeToEncoding :: Builder -> Encoding' a
- fromEncoding :: Encoding' tag -> Builder
- data Series
- type Array = Vector Value
- emptyArray :: Value
- type Pair = (Text, Value)
- type Object = HashMap Text Value
- emptyObject :: Value
- newtype DotNetTime = DotNetTime {}
- typeMismatch :: String -> Value -> Parser a
- data Parser a
- data Result a
- class FromJSON a where
- fromJSON :: FromJSON a => Value -> Result a
- parse :: (a -> Parser b) -> a -> Result b
- parseEither :: (a -> Parser b) -> a -> Either String b
- parseMaybe :: (a -> Parser b) -> a -> Maybe b
- class ToJSON a where
- class KeyValue kv where
- modifyFailure :: (String -> String) -> Parser a -> Parser a
- parserThrowError :: JSONPath -> String -> Parser a
- parserCatchError :: Parser a -> (JSONPath -> String -> Parser a) -> Parser a
- class ToJSONKey a where
- data ToJSONKeyFunction a
- = ToJSONKeyText !(a -> Text) !(a -> Encoding' Text)
- | ToJSONKeyValue !(a -> Value) !(a -> Encoding)
- toJSONKeyText :: (a -> Text) -> ToJSONKeyFunction a
- contramapToJSONKeyFunction :: (b -> a) -> ToJSONKeyFunction a -> ToJSONKeyFunction b
- class FromJSONKey a where
- data FromJSONKeyFunction a
- = FromJSONKeyCoerce !(CoerceText a)
- | FromJSONKeyText !(Text -> a)
- | FromJSONKeyTextParser !(Text -> Parser a)
- | FromJSONKeyValue !(Value -> Parser a)
- fromJSONKeyCoerce :: Coercible Text a => FromJSONKeyFunction a
- coerceFromJSONKeyFunction :: Coercible a b => FromJSONKeyFunction a -> FromJSONKeyFunction b
- mapFromJSONKeyFunction :: (a -> b) -> FromJSONKeyFunction a -> FromJSONKeyFunction b
- 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
- toJSON1 :: (ToJSON1 f, ToJSON a) => f a -> Value
- toEncoding1 :: (ToJSON1 f, ToJSON a) => f a -> Encoding
- class ToJSON2 f where
- 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 where
- data FromArgs arity a where
- type GToJSON = GToJSON Value
- type GToEncoding = GToJSON Encoding
- data ToArgs res arity a where
- 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)
- withObject :: String -> (Object -> Parser a) -> Value -> Parser a
- withText :: String -> (Text -> Parser a) -> Value -> Parser a
- withArray :: String -> (Array -> Parser a) -> Value -> Parser a
- withNumber :: String -> (Number -> Parser a) -> Value -> Parser a
- withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser a
- withBool :: String -> (Bool -> Parser a) -> Value -> Parser a
- pairs :: Series -> Encoding
- foldable :: (Foldable t, ToJSON a) => t a -> Encoding
- (.:) :: FromJSON a => Object -> Text -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.:!) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- object :: [Pair] -> Value
- parseField :: FromJSON a => Object -> Text -> Parser a
- parseFieldMaybe :: FromJSON a => Object -> Text -> Parser (Maybe a)
- parseFieldMaybe' :: FromJSON a => Object -> Text -> Parser (Maybe a)
- explicitParseField :: (Value -> Parser a) -> Object -> Text -> Parser a
- explicitParseFieldMaybe :: (Value -> Parser a) -> Object -> Text -> Parser (Maybe a)
- explicitParseFieldMaybe' :: (Value -> Parser a) -> Object -> Text -> Parser (Maybe a)
- listEncoding :: (a -> Encoding) -> [a] -> Encoding
- listValue :: (a -> Value) -> [a] -> Value
- listParser :: (Value -> Parser a) -> Value -> Parser [a]
- data Options
- data SumEncoding
- camelTo :: Char -> String -> String
- camelTo2 :: Char -> String -> String
- defaultOptions :: Options
- defaultTaggedObject :: SumEncoding
Core JSON types
A JSON value represented as a Haskell value.
unsafeToEncoding :: Builder -> Encoding' a Source #
Make Encoding from Builder.
Use with care! You have to make sure that the passed Builder is a valid JSON Encoding!
fromEncoding :: Encoding' tag -> Builder Source #
Acquire the underlying bytestring builder.
A series of values that, when encoded, should be separated by
commas. Since 0.11.0.0, the .= operator is overloaded to create
either (Text, Value) or Series. You can use Series when
encoding directly to a bytestring builder as in the following
example:
toEncoding (Person name age) = pairs ("name" .= name <> "age" .= age)emptyArray :: Value Source #
The empty array.
emptyObject :: Value Source #
The empty object.
Convenience types and functions
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
| |
Arguments
| :: String | The name of the type you are trying to parse. |
| -> Value | The actual value encountered. |
| -> Parser a |
Fail parsing due to a type mismatch, with a descriptive message.
Example usage:
instance FromJSON Coord where parseJSON (Objectv) = {- type matches, life is good -} parseJSON wat =typeMismatch"Coord" wat
Type conversion
A JSON parser.
The result of running a Parser.
Instances
| Monad Result Source # | |
| Functor Result Source # | |
| MonadFail Result Source # | |
| Applicative Result Source # | |
| Foldable Result Source # | |
| 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 # | |
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:
emptyandmzerowork, but are terse and uninformative;failyields a custom error message;typeMismatchproduces an informative message for cases when the value encountered is not of the expected type.
An example type and instance using typeMismatch:
-- 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 usemzeroto fail, buttypeMismatch-- gives a much more informative error message.parseJSONinvalid =typeMismatch"Coord" invalid
For this common case of only being concerned with a single
type of JSON value, the functions withObject, withNumber, 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
Methods
parseJSON :: Value -> Parser a Source #
parseJSON :: (Generic a, GFromJSON Zero (Rep a)) => Value -> Parser a Source #
parseJSONList :: Value -> Parser [a] Source #
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 to 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.)
Methods
Convert a Haskell value to a JSON-friendly intermediate type.
toJSON :: (Generic a, GToJSON Value Zero (Rep a)) => a -> Value Source #
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.
Minimal complete definition
modifyFailure :: (String -> String) -> Parser a -> Parser a Source #
If the inner Parser failed, modify the failure message using the
provided function. This allows you to create more descriptive error messages.
For example:
parseJSON (Object o) = modifyFailure
("Parsing of the Foo value failed: " ++)
(Foo <$> o .: "someField")Since 0.6.2.0
parserThrowError :: JSONPath -> String -> Parser a Source #
Throw a parser error with an additional path.
Since: 1.2.1.0
parserCatchError :: Parser a -> (JSONPath -> String -> Parser a) -> Parser a Source #
A handler function to handle previous errors and return to normal execution.
Since: 1.2.1.0
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, in
this example, ToJSONKeyText will be used instead of ToJSONKeyValue.
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.
Methods
toJSONKey :: ToJSONKeyFunction a Source #
Strategy for rendering the key for a map-like container.
toJSONKey :: ToJSON a => ToJSONKeyFunction a Source #
Strategy for rendering the key for a map-like container.
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.
toJSONKeyList :: ToJSON a => ToJSONKeyFunction [a] Source #
Instances
data ToJSONKeyFunction a Source #
Constructors
| ToJSONKeyText !(a -> Text) !(a -> Encoding' Text) | key is encoded to string, produces object |
| ToJSONKeyValue !(a -> Value) !(a -> Encoding) | key is encoded to value, produces array |
toJSONKeyText :: (a -> Text) -> ToJSONKeyFunction a Source #
Helper for creating textual keys.
instanceToJSONKeyMyKey wheretoJSONKey=toJSONKeyTextmyKeyToText where myKeyToText = Text.pack . show -- or showt from text-show
contramapToJSONKeyFunction :: (b -> a) -> ToJSONKeyFunction a -> ToJSONKeyFunction b Source #
Contravariant map, as ToJSONKeyFunction is a contravariant functor.
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)Methods
fromJSONKey :: FromJSONKeyFunction a Source #
Strategy for parsing the key of a map-like container.
fromJSONKey :: FromJSON a => FromJSONKeyFunction a Source #
Strategy for parsing the key of a map-like container.
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.
fromJSONKeyList :: FromJSON a => FromJSONKeyFunction [a] Source #
Instances
data FromJSONKeyFunction a 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 !(CoerceText a) | uses |
| FromJSONKeyText !(Text -> a) | conversion from |
| FromJSONKeyTextParser !(Text -> Parser a) | conversion from |
| FromJSONKeyValue !(Value -> Parser a) | conversion for non-textual keys |
Instances
| Functor FromJSONKeyFunction Source # | Only law abiding up to interpretation |
fromJSONKeyCoerce :: Coercible Text a => FromJSONKeyFunction a Source #
Construct FromJSONKeyFunction for types coercible from Text. This
conversion is still unsafe, as Hashable and Eq instances of a should be
compatible with Text i.e. hash values should be equal for wrapped values as well.
This property will always be maintained if the Hashable and Eq instances
are derived with generalized newtype deriving.
compatible with Text i.e. hash values be equal for wrapped values as well.
On pre GHC 7.8 this is unconstrainted function.
coerceFromJSONKeyFunction :: Coercible a b => FromJSONKeyFunction a -> FromJSONKeyFunction b Source #
Semantically the same as coerceFromJSONKeyFunction = fmap coerce = coerce.
See note on fromJSONKeyCoerce.
mapFromJSONKeyFunction :: (a -> b) -> FromJSONKeyFunction a -> FromJSONKeyFunction b Source #
Same as fmap. Provided for the consistency with ToJSONKeyFunction.
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
Methods
liftParseJSON :: (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (f a) Source #
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 #
Instances
parseJSON2 :: (FromJSON2 f, FromJSON a, FromJSON b) => Value -> Parser (f a b) Source #
Lift the standard parseJSON function through the type constructor.
class ToJSON1 f where Source #
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 clause to your
datatype and declare a Generic1ToJSON1 instance for your datatype without giving
definitions for liftToJSON or liftToEncoding.
For example:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Pair = 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=maptoUpper} instanceToJSONa =>ToJSON1(Pair a) whereliftToJSON=genericLiftToJSONcustomOptionsliftToEncoding=genericLiftToEncodingcustomOptions
See also ToJSON.
Methods
liftToJSON :: (a -> Value) -> ([a] -> Value) -> f a -> Value Source #
liftToJSON :: (Generic1 f, GToJSON Value One (Rep1 f)) => (a -> Value) -> ([a] -> Value) -> f a -> Value Source #
liftToJSONList :: (a -> Value) -> ([a] -> Value) -> [f a] -> Value Source #
liftToEncoding :: (a -> Encoding) -> ([a] -> Encoding) -> f a -> Encoding Source #
liftToEncoding :: (Generic1 f, GToJSON Encoding One (Rep1 f)) => (a -> Encoding) -> ([a] -> Encoding) -> f a -> Encoding Source #
liftToEncodingList :: (a -> Encoding) -> ([a] -> Encoding) -> [f a] -> Encoding Source #
Instances
toJSON1 :: (ToJSON1 f, ToJSON a) => f a -> Value Source #
Lift the standard toJSON function through the type constructor.
toEncoding1 :: (ToJSON1 f, ToJSON a) => f a -> Encoding Source #
Lift the standard toEncoding function through the type constructor.
class ToJSON2 f where Source #
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.
Minimal complete definition
Methods
liftToJSON2 :: (a -> Value) -> ([a] -> Value) -> (b -> Value) -> ([b] -> Value) -> f a b -> Value Source #
liftToJSONList2 :: (a -> Value) -> ([a] -> Value) -> (b -> Value) -> ([b] -> Value) -> [f a b] -> Value Source #
liftToEncoding2 :: (a -> Encoding) -> ([a] -> Encoding) -> (b -> Encoding) -> ([b] -> Encoding) -> f a b -> Encoding Source #
liftToEncodingList2 :: (a -> Encoding) -> ([a] -> Encoding) -> (b -> Encoding) -> ([b] -> Encoding) -> [f a b] -> Encoding Source #
Instances
toJSON2 :: (ToJSON2 f, ToJSON a, ToJSON b) => f a b -> Value Source #
Lift the standard toJSON function through the type constructor.
toEncoding2 :: (ToJSON2 f, ToJSON a, ToJSON b) => f a b -> Encoding Source #
Lift the standard toEncoding function through the type constructor.
Generic JSON classes
class GFromJSON arity f where Source #
Class of generic representation types that can be converted from JSON.
Minimal complete definition
Methods
gParseJSON :: Options -> FromArgs arity a -> Value -> Parser (f a) Source #
This method (applied to defaultOptions) is used as the
default generic implementation of parseJSON (if the arity is Zero)
or liftParseJSON (if the arity is One).
Instances
| GFromJSON arity U1 Source # | |
| GFromJSON One Par1 Source # | |
| FromJSON1 f => GFromJSON One (Rec1 f) Source # | |
| (AllNullary ((:+:) a b) allNullary, ParseSum * arity ((:+:) a b) allNullary) => GFromJSON arity ((:+:) a b) Source # | |
| (FromProduct arity a, FromProduct arity b, ProductSize a, ProductSize b) => GFromJSON arity ((:*:) a b) Source # | |
| ConsFromJSON arity a => GFromJSON arity (C1 c a) Source # | |
| (ConsFromJSON arity a, AllNullary (C1 c a) allNullary, ParseSum * arity (C1 c a) allNullary) => GFromJSON arity (D1 d (C1 c a)) Source # | |
| FromJSON a => GFromJSON arity (K1 i a) Source # | |
| (FromJSON1 f, GFromJSON One g) => GFromJSON One ((:.:) f g) Source # | |
| GFromJSON arity a => GFromJSON arity (M1 i c a) Source # | |
type GToEncoding = GToJSON Encoding Source #
A type-level indicator that ToJSON1 or FromJSON1 is being derived generically.
genericToJSON :: (Generic a, GToJSON Value Zero (Rep a)) => Options -> a -> Value Source #
A configurable generic JSON creator. This function applied to
defaultOptions is used as the default for toJSON when the type
is an instance of Generic.
genericLiftToJSON :: (Generic1 f, GToJSON Value One (Rep1 f)) => Options -> (a -> Value) -> ([a] -> Value) -> f a -> Value Source #
A configurable generic JSON creator. This function applied to
defaultOptions is used as the default for liftToJSON when the type
is an instance of Generic1.
genericToEncoding :: (Generic a, GToJSON Encoding Zero (Rep a)) => Options -> a -> Encoding Source #
A configurable generic JSON encoder. This function applied to
defaultOptions is used as the default for toEncoding when the type
is an instance of Generic.
genericLiftToEncoding :: (Generic1 f, GToJSON Encoding One (Rep1 f)) => Options -> (a -> Encoding) -> ([a] -> Encoding) -> f a -> Encoding Source #
A configurable generic JSON encoder. This function applied to
defaultOptions is used as the default for liftToEncoding when the type
is an instance of Generic1.
genericParseJSON :: (Generic a, GFromJSON Zero (Rep a)) => Options -> Value -> Parser a Source #
A configurable generic JSON decoder. This function applied to
defaultOptions is used as the default for parseJSON when the
type is an instance of Generic.
genericLiftParseJSON :: (Generic1 f, GFromJSON One (Rep1 f)) => Options -> (Value -> Parser a) -> (Value -> Parser [a]) -> Value -> Parser (f a) Source #
A configurable generic JSON decoder. This function applied to
defaultOptions is used as the default for liftParseJSON when the
type is an instance of Generic1.
Inspecting Values
ValueswithObject :: String -> (Object -> Parser a) -> Value -> Parser a Source #
withObject expected f valuef to the Object when value
is an Object and fails using typeMismatch expected
withText :: String -> (Text -> Parser a) -> Value -> Parser a Source #
withText expected f valuef to the Text when value is a
String and fails using typeMismatch expected
withArray :: String -> (Array -> Parser a) -> Value -> Parser a Source #
withArray expected f valuef to the Array when value is
an Array and fails using typeMismatch expected
withNumber :: String -> (Number -> Parser a) -> Value -> Parser a Source #
Deprecated: Use withScientific instead
withNumber expected f valuef to the Number when value
is a Number and fails using typeMismatch expected
withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser a Source #
withScientific expected f valuef to the Scientific number
when value is a Number and fails using typeMismatch expected
withBool :: String -> (Bool -> Parser a) -> Value -> Parser a Source #
withBool expected f valuef to the Bool when value is a
Bool and fails using typeMismatch expected
(.:) :: FromJSON a => Object -> Text -> Parser a Source #
Retrieve the value associated with the given key of an Object.
The result is empty if the key is not present or the value cannot
be converted to the desired type.
This accessor is appropriate if the key and value must be present
in an object for it to be valid. If the key and value are
optional, use .:? instead.
(.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a) Source #
Retrieve the value associated with the given key of an Object. The
result is Nothing if the key is not present or if its value is Null,
or empty if the value cannot be converted to the desired type.
This accessor is most useful if the key and value can be absent
from an object without affecting its validity. If the key and
value are mandatory, use .: instead.
(.!=) :: Parser (Maybe a) -> a -> Parser a Source #
Helper for use in combination with .:? to provide default
values for optional JSON object fields.
This combinator is most useful if the key and value can be absent
from an object without affecting its validity and we know a default
value to assign in that case. If the key and value are mandatory,
use .: instead.
Example usage:
v1 <- o.:?"opt_field_with_dfl" .!= "default_val" v2 <- o.:"mandatory_field" v3 <- o.:?"opt_field2"
parseFieldMaybe :: FromJSON a => Object -> Text -> Parser (Maybe a) Source #
Function variant of .:?.
parseFieldMaybe' :: FromJSON a => Object -> Text -> Parser (Maybe a) Source #
Function variant of .:!.
explicitParseField :: (Value -> Parser a) -> Object -> Text -> Parser a Source #
Variant of .: with explicit parser function.
E.g. explicitParseField parseJSON1 :: (FromJSON1 f, FromJSON a) -> Object -> Text -> Parser (f a)
explicitParseFieldMaybe :: (Value -> Parser a) -> Object -> Text -> Parser (Maybe a) Source #
Variant of .:? with explicit parser function.
explicitParseFieldMaybe' :: (Value -> Parser a) -> Object -> Text -> Parser (Maybe a) Source #
Variant of .:! with explicit parser function.
listEncoding :: (a -> Encoding) -> [a] -> Encoding Source #
Helper function to use with liftToEncoding.
Useful when writing own ToJSON1 instances.
newtype F a = F [a] -- This instance encodesStringas an array of chars instanceToJSON1F whereliftToJSONtj _ (F xs) =liftToJSONtj (listValuetj) xsliftToEncodingte _ (F xs) =liftToEncodingte (listEncodingte) xs instanceFromJSON1F whereliftParseJSONp _ v = F <$>liftParseJSONp (listParserp) v
listValue :: (a -> Value) -> [a] -> Value Source #
Helper function to use with liftToJSON, see listEncoding.
listParser :: (Value -> Parser a) -> Value -> Parser [a] Source #
Helper function to use with liftParseJSON. See listEncoding.
Generic and TH encoding configuration
Options that specify how to encode/decode your datatype to/from JSON.
data SumEncoding Source #
Specifies how to encode constructors of a sum datatype.
Constructors
| TaggedObject | A constructor will be encoded to an object with a field
|
Fields | |
| UntaggedValue | Constructor names won't be encoded. Instead only the contents of the constructor will be encoded as if the type had a single constructor. JSON encodings have to be disjoint for decoding to work properly. When decoding, constructors are tried in the order of definition. If some encodings overlap, the first one defined will succeed. Note: Nullary constructors are encoded as strings (using
Note: Only the last error is kept when decoding, so in the case of malformed JSON, only an error for the last constructor will be reported. |
| ObjectWithSingleField | A constructor will be encoded to an object with a single
field named after the constructor tag (modified by the
|
| TwoElemArray | A constructor will be encoded to a 2-element array where the
first element is the tag of the constructor (modified by the
|
Instances
camelTo :: Char -> String -> String Source #
Deprecated: Use camelTo2 for better results
Converts from CamelCase to another lower case, interspersing
the character between all capital letters and their previous
entries, except those capital letters that appear together,
like API.
For use by Aeson template haskell calls.
camelTo '_' 'CamelCaseAPI' == "camel_case_api"
camelTo2 :: Char -> String -> String Source #
Better version of camelTo. Example where it works better:
camelTo '_' 'CamelAPICase' == "camel_apicase" camelTo2 '_' 'CamelAPICase' == "camel_api_case"
defaultOptions :: Options Source #
Default encoding Options:
Options{fieldLabelModifier= id ,constructorTagModifier= id ,allNullaryToStringTag= True ,omitNothingFields= False ,sumEncoding=defaultTaggedObject,unwrapUnaryRecords= False ,tagSingleConstructors= False }
defaultTaggedObject :: SumEncoding Source #
Default TaggedObject SumEncoding options:
defaultTaggedObject =TaggedObject{tagFieldName= "tag" ,contentsFieldName= "contents" }