aeson-0.3.2.5: Fast JSON parsing and generation

Portabilityportable
Stabilityexperimental
MaintainerBryan O'Sullivan <bos@mailrank.com>

Data.Aeson.Types

Contents

Description

Types for working with JSON data.

Synopsis

Core JSON types

data Value Source

A JSON value represented as a Haskell value.

type Array = Vector ValueSource

A JSON "array" (sequence).

emptyArray :: ValueSource

The empty array.

type Pair = (Text, Value)Source

A key/value pair for an Object.

type Object = Map Text ValueSource

A JSON "object" (key/value map).

emptyObject :: ValueSource

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 

typeMismatchSource

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

data Parser a Source

A continuation-based parser type.

data Result a Source

The result of running a Parser.

Constructors

Error String 
Success a 

class FromJSON a whereSource

A type that can be converted from JSON, with the possibility of failure.

When writing an instance, use 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:

data 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

fromJSON :: FromJSON a => Value -> Result aSource

Convert a value from JSON, failing if the types do not match.

parse :: (a -> Parser b) -> a -> Result bSource

Run a Parser.

parseEither :: (a -> Parser b) -> a -> Either String bSource

Run a Parser with an Either result type.

parseMaybe :: (a -> Parser b) -> a -> Maybe bSource

Run a Parser with a Maybe result type.

class ToJSON a whereSource

A type that can be converted to JSON.

An example type and instance:

data Coord { x :: Double, y :: Double }

instance ToJSON Coord where
   toJSON (Coord x y) = object ["x" .= x, "y" .= y]

Methods

toJSON :: a -> ValueSource

Constructors and accessors

(.=) :: ToJSON a => Text -> a -> PairSource

Construct a Pair from a key and a value.

(.:) :: 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.

object :: [Pair] -> ValueSource

Create a Value from a list of name/value Pairs. If duplicate keys arise, earlier keys and their associated values win.