aeson-0.5.0.0: Fast JSON parsing and encoding

Portabilityportable
Stabilityexperimental
MaintainerBryan O'Sullivan <bos@serpentine.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 = HashMap 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 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 { 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 fromJSON function that parses to any type which is an instance of Data.
  • If your compiler has support for the DeriveGeneric and DefaultSignatures language extensions, parseJSON will have a default generic implementation.

To use this, simply add a deriving Generic clause to your datatype and declare a 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 { x :: Double, y :: Double } deriving Generic

instance FromJSON Coord

Instances

FromJSON Bool 
FromJSON Char 
FromJSON Double 
FromJSON Float 
FromJSON Int 
FromJSON Int8 
FromJSON Int16 
FromJSON Int32 
FromJSON Int64 
FromJSON Integer 
FromJSON Word 
FromJSON Word8 
FromJSON Word16 
FromJSON Word32 
FromJSON Word64 
FromJSON () 
FromJSON ByteString 
FromJSON Number 
FromJSON Text 
FromJSON Text 
FromJSON ByteString 
FromJSON IntSet 
FromJSON UTCTime 
FromJSON Value 
FromJSON DotNetTime 
FromJSON [Char] 
FromJSON a => FromJSON [a] 
FromJSON (Ratio Integer) 
FromJSON a => FromJSON (Maybe a) 
FromJSON a => FromJSON (Dual a) 
FromJSON a => FromJSON (First a) 
FromJSON a => FromJSON (Last a) 
FromJSON a => FromJSON (IntMap a) 
(Ord a, FromJSON a) => FromJSON (Set a) 
(Eq a, Hashable a, FromJSON a) => FromJSON (HashSet a) 
FromJSON a => FromJSON (Vector a) 
(Vector Vector a, FromJSON a) => FromJSON (Vector a) 
(Storable a, FromJSON a) => FromJSON (Vector a) 
(Prim a, FromJSON a) => FromJSON (Vector a) 
(FromJSON a, FromJSON b) => FromJSON (Either a b) 
(FromJSON a, FromJSON b) => FromJSON (a, b) 
FromJSON v => FromJSON (Map String v) 
FromJSON v => FromJSON (Map ByteString v) 
FromJSON v => FromJSON (Map Text v) 
FromJSON v => FromJSON (Map Text v) 
FromJSON v => FromJSON (Map ByteString v) 
FromJSON v => FromJSON (HashMap String v) 
FromJSON v => FromJSON (HashMap ByteString v) 
FromJSON v => FromJSON (HashMap Text v) 
FromJSON v => FromJSON (HashMap Text v) 
FromJSON v => FromJSON (HashMap ByteString v) 
(FromJSON a, FromJSON b, FromJSON c) => FromJSON (a, b, c) 
(FromJSON a, FromJSON b, FromJSON c, FromJSON d) => FromJSON (a, b, c, d) 

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:

{-# LANGUAGE OverloadedStrings #-}

data 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 toJSON function that accepts any type which is an instance of Data.
  • If your compiler has support for the DeriveGeneric and DefaultSignatures language extensions, toJSON will have a default generic implementation.

To use the latter option, simply add a deriving Generic clause to your datatype and declare a ToJSON 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 { x :: Double, y :: Double } deriving Generic

instance ToJSON Coord

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.

(.!=) :: 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"

object :: [Pair] -> ValueSource

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