| Portability | portable |
|---|---|
| Stability | experimental |
| Maintainer | Bryan O'Sullivan <bos@serpentine.com> |
| Safe Haskell | None |
Data.Aeson.Types
Contents
Description
Types for working with JSON data.
- data Value
- 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
- modifyFailure :: (String -> String) -> Parser a -> Parser a
- class GFromJSON f where
- gParseJSON :: Options -> Value -> Parser (f a)
- class GToJSON f where
- genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> Value
- genericParseJSON :: (Generic a, GFromJSON (Rep a)) => Options -> Value -> Parser 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
- (.=) :: ToJSON a => Text -> a -> Pair
- (.:) :: FromJSON a => Object -> Text -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- object :: [Pair] -> Value
- data Options = Options {}
- data SumEncoding
- camelTo :: Char -> String -> String
- defaultOptions :: Options
- defaultTaggedObject :: SumEncoding
Core JSON types
A JSON value represented as a Haskell value.
The empty array.
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.
Type conversion
A continuation-based parser type.
The result of running a Parser.
A type that can be converted from JSON, with the possibility of failure.
When writing an instance, use empty, mzero, or fail to make a
conversion fail, e.g. if an Object is missing a required key, or
the value is of the wrong type.
An example type and instance:
{-# LANGUAGE OverloadedStrings #-}
data Coord = Coord { x :: Double, y :: Double }
instance FromJSON Coord where
parseJSON (Object v) = Coord <$>
v .: "x" <*>
v .: "y"
-- A non-Object value is of the wrong type, so use mzero to fail.
parseJSON _ = mzero
Note the use of the OverloadedStrings language extension which enables
Text values to be written as string literals.
Instead of manually writing your FromJSON instance, there are three 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 will probably be more efficient than the following two options:
- Data.Aeson.Generic provides a generic
fromJSONfunction that parses to any type which is an instance ofData. - If your compiler has support for the
DeriveGenericandDefaultSignatureslanguage extensions,parseJSONwill have a default generic implementation.
To use this, 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
Note that, instead of using DefaultSignatures, it's also possible
to parameterize the generic decoding using genericParseJSON applied
to your encoding/decoding Options:
instance FromJSON Coord where
parseJSON = genericParseJSON defaultOptions
Instances
fromJSON :: FromJSON a => Value -> Result aSource
Convert a value from JSON, failing if the types do not match.
parseEither :: (a -> Parser b) -> a -> Either String bSource
A type that can be converted to JSON.
An example type and instance:
{-# LANGUAGE OverloadedStrings #-}
data Coord = Coord { x :: Double, y :: Double }
instance ToJSON Coord where
toJSON (Coord x y) = object ["x" .= x, "y" .= y]
Note the use of the OverloadedStrings language extension which enables
Text values to be written as string literals.
Instead of manually writing your ToJSON instance, there are three 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 will probably be more efficient than the following two options:
- Data.Aeson.Generic provides a generic
toJSONfunction that accepts any type which is an instance ofData. - If your compiler has support for the
DeriveGenericandDefaultSignatureslanguage extensions (GHC 7.2 and newer),toJSONwill have a default generic implementation.
To use the latter option, simply add a deriving clause to your
datatype and declare a GenericToJSON instance for your datatype without giving a
definition for toJSON.
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 ToJSON Coord
Note that, instead of using DefaultSignatures, it's also possible
to parameterize the generic encoding using genericToJSON applied
to your encoding/decoding Options:
instance ToJSON Coord where
toJSON = genericToJSON defaultOptions
Instances
modifyFailure :: (String -> String) -> Parser a -> Parser aSource
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
Generic JSON classes
Class of generic representation types (Rep) that can be converted from JSON.
Methods
gParseJSON :: Options -> Value -> Parser (f a)Source
This method (applied to defaultOptions) is used as the
default generic implementation of parseJSON.
Class of generic representation types (Rep) that can be converted to JSON.
genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> ValueSource
A configurable generic JSON encoder. This function applied to
defaultOptions is used as the default for toJSON when the type
is an instance of Generic.
genericParseJSON :: (Generic a, GFromJSON (Rep a)) => Options -> Value -> Parser aSource
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.
Inspecting Values
ValueswithObject :: String -> (Object -> Parser a) -> Value -> Parser aSource
withObject expected f value applies f to the Object when value is an Object
and fails using typeMismatch expected
withText :: String -> (Text -> Parser a) -> Value -> Parser aSource
withText expected f value applies f to the Text when value is a String
and fails using typeMismatch expected
withArray :: String -> (Array -> Parser a) -> Value -> Parser aSource
withArray expected f value applies f to the Array when value is an Array
and fails using typeMismatch expected
withNumber :: String -> (Number -> Parser a) -> Value -> Parser aSource
Deprecated: Use withScientific instead
withNumber expected f value applies f to the Number when value is a Number.
and fails using typeMismatch expected
withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser aSource
withScientific expected f value applies f to the Scientific number when value is a Number.
and fails using typeMismatch expected
withBool :: String -> (Bool -> Parser a) -> Value -> Parser aSource
withBool expected f value applies f to the Bool when value is a Bool
and fails using typeMismatch expected
Constructors and accessors
(.:) :: FromJSON a => Object -> Text -> Parser aSource
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 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 aSource
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"
Generic and TH encoding configuration
Options that specify how to encode/decode your datatype to/from JSON.
Constructors
| Options | |
Fields
| |
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 | |
| 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
|
camelTo :: Char -> String -> StringSource
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"
defaultOptions :: OptionsSource
Default encoding Options:
Options{fieldLabelModifier= id ,constructorTagModifier= id ,allNullaryToStringTag= True ,omitNothingFields= False ,sumEncoding=defaultTaggedObject}
defaultTaggedObject :: SumEncodingSource
Default TaggedObject SumEncoding options:
defaultTaggedObject =TaggedObject{tagFieldName= "tag" ,contentsFieldName= "contents" }