Data.Aeson.Types

Types for working with JSON data.

Core JSON types

data Value

A JSON value represented as a Haskell value.

Constructors

Object !Object

Array !Array

String !Text

Number !Scientific

Bool !Bool

Null

Instances

Eq Value

Data Value

Read Value

Show Value

IsString Value

NFData Value

Hashable Value

Lift Value

data Encoding

An encoding of a JSON value.

Instances

Eq Encoding

Ord Encoding

Show Encoding

Monoid Encoding

Semigroup Encoding

fromEncoding :: Encoding -> Builder

Acquire the underlying bytestring builder.

data Series

A series of values that, when encoded, should be separated by commas.

Instances

Monoid Series

Semigroup Series

type Array = Vector Value

A JSON "array" (sequence).

emptyArray :: Value

The empty array.

type Pair = (Text, Value)

A key/value pair for an Object.

type Object = HashMap Text Value

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

emptyObject :: Value

The empty object.

Convenience types and functions

newtype DotNetTime

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

fromDotNetTime :: UTCTime

Acquire the underlying value.

Instances

Eq DotNetTime

Ord DotNetTime

Read DotNetTime

Show DotNetTime

FormatTime DotNetTime

typeMismatch

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 (Object v) = {- type matches, life is good -}
  parseJSON wat        = typeMismatch "Coord" wat

Type conversion

data Parser a

A JSON parser.

Instances

Monad Parser

Functor Parser

Applicative Parser

Alternative Parser

MonadPlus Parser

MonadFail Parser

Monoid (Parser a)

Semigroup (Parser a)

data Result a

The result of running a Parser.

Constructors

Error String

Success a

Instances

Monad Result

Functor Result

Applicative Result

Foldable Result

Traversable Result

Alternative Result

MonadPlus Result

MonadFail Result

Eq a => Eq (Result a)

Show a => Show (Result a)

Monoid (Result a)

NFData a => NFData (Result a)

Semigroup (Result a)

class FromJSON a where

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:

• empty and mzero work, but are terse and uninformative
• fail yields a custom error message
• typeMismatch produces an informative message for cases when the value encountered is not of the expected type

An example type and instance:

-- Allow ourselves to write Text literals.
{-# LANGUAGE OverloadedStrings #-}

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

instance FromJSON Coord where
  parseJSON (Object v) = Coord    <$>
                         v .: "x" <*>
                         v .: "y"

  -- We do not expect a non-Object value here.
  -- We could use mzero to fail, but typeMismatch
  -- gives a much more informative error message.
  parseJSON invalid    = typeMismatch "Coord" invalid

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 will probably be more efficient than the following two options:
• The compiler can provide a default generic implementation for parseJSON.

To use the second, 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 = Coord { x :: Double, y :: Double } deriving Generic

instance FromJSON Coord

If DefaultSignatures doesn't give exactly the results you want, you can customize the generic decoding with only a tiny amount of effort, using genericParseJSON with your preferred Options:

instance FromJSON Coord where
    parseJSON = genericParseJSON defaultOptions

Minimal complete definition

Nothing

Methods

parseJSON :: Value -> Parser a

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

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

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

Run a Parser.

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

Run a Parser with an Either result type. If the parse fails, the Left payload will contain an error message.

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

Run a Parser with a Maybe result type.

class ToJSON a where

A type that can be converted to JSON.

An example type and instance:

-- Allow ourselves to write Text literals.
{-# LANGUAGE OverloadedStrings #-}

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

instance ToJSON Coord where
  toJSON (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 will probably be more efficient than the following two options:
• The compiler can provide a default generic implementation for toJSON.

To use the second, simply add a deriving Generic clause to your datatype and declare a ToJSON instance for your datatype without giving definitions for toJSON or toEncoding.

For example, the previous example can be simplified to a more minimal instance:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

data Coord = Coord { x :: Double, y :: Double } deriving Generic

instance ToJSON Coord where
    toEncoding = genericToEncoding defaultOptions

Why do we provide an implementation for toEncoding here? The toEncoding function is a relatively new addition to this class. To allow users of older versions of this library to upgrade without having to edit all of their instances or encounter surprising incompatibilities, the default implementation of toEncoding uses toJSON. This produces correct results, but since it performs an intermediate conversion to a Value, it will be less efficient than directly emitting an Encoding. Our one-liner definition of toEncoding above bypasses the intermediate Value.

If DefaultSignatures doesn't give exactly the results you want, you can customize the generic encoding with only a tiny amount of effort, using genericToJSON and genericToEncoding with your preferred Options:

instance ToJSON Coord where
    toJSON     = genericToJSON defaultOptions
    toEncoding = genericToEncoding defaultOptions

Minimal complete definition

Nothing

Methods

toJSON :: a -> Value

Convert a Haskell value to a JSON-friendly intermediate type.

toEncoding :: a -> Encoding

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.

instance ToJSON Coord where
    toEncoding = genericToEncoding defaultOptions

class KeyValue kv where

A key-value pair for encoding a JSON object.

Methods

(.=) :: ToJSON v => Text -> v -> kv infixr 8

modifyFailure :: (String -> String) -> Parser a -> Parser a

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 GFromJSON f where

Class of generic representation types (Rep) that can be converted from JSON.

Methods

gParseJSON :: Options -> Value -> Parser (f a)

This method (applied to defaultOptions) is used as the default generic implementation of parseJSON.

class GToJSON f where

Class of generic representation types (Rep) that can be converted to JSON.

Methods

gToJSON :: Options -> f a -> Value

This method (applied to defaultOptions) is used as the default generic implementation of toJSON.

class GToEncoding f where

Class of generic representation types (Rep) that can be converted to a JSON Encoding.

Methods

gToEncoding :: Options -> f a -> Encoding

This method (applied to defaultOptions) can be used as the default generic implementation of toEncoding.

genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> Value

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.

genericToEncoding :: (Generic a, GToEncoding (Rep a)) => Options -> a -> Encoding

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.

genericParseJSON :: (Generic a, GFromJSON (Rep a)) => Options -> Value -> Parser a

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

withObject :: String -> (Object -> Parser a) -> Value -> Parser a

withObject expected f value applies f to the Object when value is an Object and fails using typeMismatch expected otherwise.

withText :: String -> (Text -> Parser a) -> Value -> Parser a

withText expected f value applies f to the Text when value is a String and fails using typeMismatch expected otherwise.

withArray :: String -> (Array -> Parser a) -> Value -> Parser a

withArray expected f value applies f to the Array when value is an Array and fails using typeMismatch expected otherwise.

withNumber :: String -> (Number -> Parser a) -> Value -> Parser a

Deprecated: Use withScientific instead

withNumber expected f value applies f to the Number when value is a Number. and fails using typeMismatch expected otherwise.

withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser a

withScientific expected f value applies f to the Scientific number when value is a Number. and fails using typeMismatch expected otherwise.

withBool :: String -> (Bool -> Parser a) -> Value -> Parser a

withBool expected f value applies f to the Bool when value is a Bool and fails using typeMismatch expected otherwise.

pairs :: Series -> Encoding

Encode a series of key/value pairs, separated by commas.

foldable :: (Foldable t, ToJSON a) => t a -> Encoding

Encode a Foldable as a JSON array.

(.:) :: FromJSON a => Object -> Text -> Parser a

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)

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.

(.:!) :: FromJSON a => Object -> Text -> Parser (Maybe a)

Like .:?, but the resulting parser will fail, if the key is present but is Null.

(.!=) :: Parser (Maybe a) -> a -> Parser a

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] -> Value

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

Generic and TH encoding configuration

data Options

Options that specify how to encode/decode your datatype to/from JSON.

Constructors

Options

Fields

fieldLabelModifier :: String -> String

Function applied to field labels. Handy for removing common record prefixes for example.

constructorTagModifier :: String -> String

Function applied to constructor tags which could be handy for lower-casing them for example.

allNullaryToStringTag :: Bool

If True the constructors of a datatype, with all nullary constructors, will be encoded to just a string with the constructor tag. If False the encoding will always follow the sumEncoding.

omitNothingFields :: Bool

If True record fields with a Nothing value will be omitted from the resulting object. If False the resulting object will include those fields mapping to null.

sumEncoding :: SumEncoding

Specifies how to encode constructors of a sum datatype.

unwrapUnaryRecords :: Bool

Hide the field name when a record constructor has only one field, like a newtype.

Instances

Show Options

data SumEncoding

Specifies how to encode constructors of a sum datatype.

Constructors

TaggedObject

A constructor will be encoded to an object with a field tagFieldName which specifies the constructor tag (modified by the constructorTagModifier). If the constructor is a record the encoded record fields will be unpacked into this object. So make sure that your record doesn't have a field with the same label as the tagFieldName. Otherwise the tag gets overwritten by the encoded value of that field! If the constructor is not a record the encoded constructor contents will be stored under the contentsFieldName field.

Fields

tagFieldName :: String

contentsFieldName :: String

ObjectWithSingleField

A constructor will be encoded to an object with a single field named after the constructor tag (modified by the constructorTagModifier) which maps to the encoded contents of the constructor.

TwoElemArray

A constructor will be encoded to a 2-element array where the first element is the tag of the constructor (modified by the constructorTagModifier) and the second element the encoded contents of the constructor.

Instances

Eq SumEncoding

Show SumEncoding

camelTo :: Char -> String -> String

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

Better version of camelTo. Example where it works better:

camelTo '_' 'CamelAPICase' == "camel_apicase"
camelTo2 '_' 'CamelAPICase' == "camel_api_case"

defaultOptions :: Options

Default encoding Options:

Options
{ fieldLabelModifier      = id
, constructorTagModifier  = id
, allNullaryToStringTag   = True
, omitNothingFields       = False
, sumEncoding             = defaultTaggedObject
, unwrapUnaryRecords      = False
}

defaultTaggedObject :: SumEncoding

Default TaggedObject SumEncoding options:

defaultTaggedObject = TaggedObject
                      { tagFieldName      = "tag"
                      , contentsFieldName = "contents"
                      }