aeson-0.6.1.0: Fast JSON parsing and encoding

Portability | portable
Stability | experimental
Maintainer | Bryan O'Sullivan <bos@serpentine.com>
Safe Haskell | None

Data.Aeson.Types

Contents

- Core JSON types
- Convenience types and functions
- Type conversion
- Inspecting ``Value`s`
- Constructors and accessors

Description

Types for working with JSON data.

Synopsis

- data Value
  - = Object !Object
  - | Array !Array
  - | String !Text
  - | Number !Number
  - | Bool !Bool
  - | Null
- type Array = Vector Value
- emptyArray :: Value
- type Pair = (Text, Value)
- type Object = HashMap Text Value
- emptyObject :: Value
- newtype DotNetTime = DotNetTime {
  - fromDotNetTime :: UTCTime
  }
- typeMismatch :: String -> Value -> Parser a
- data Parser a
- data Result a
  - = Error String
  - | Success a
- class FromJSON a where
  - parseJSON :: Value -> Parser a
- 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
  - toJSON :: a -> Value
- 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
- 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

# Core JSON types

data Value

A JSON value represented as a Haskell value.

Constructors

Object !Object |
Array !Array |
String !Text |
Number !Number |
Bool !Bool |
Null |

Instances

Eq Value |
Show Value |
Typeable Value |
IsString Value |
NFData Value |
Hashable Value |
FromJSON Value |
ToJSON Value |

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

Instances

Eq DotNetTime |
Ord DotNetTime |
Read DotNetTime |
Show DotNetTime |
Typeable DotNetTime |
FormatTime DotNetTime |
FromJSON DotNetTime |
ToJSON 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.

# Type conversion

data Parser a

A continuation-based parser type.

Instances

Monad Parser |
Functor Parser |
MonadPlus Parser |
Applicative Parser |
Alternative Parser |
Monoid (Parser a) |

data Result a

The result of running a `Parser`.

Constructors

Error String |
Success a |

Instances

Monad Result |
Functor Result |
Typeable1 Result |
MonadPlus Result |
Applicative Result |
Alternative Result |
Eq a => Eq (Result a) |
Show a => Show (Result a) |
Monoid (Result a) |
NFData a => NFData (Result a) |

class FromJSON a where

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

Methods

parseJSON :: Value -> Parser a

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 ByteString |
FromJSON Text |
FromJSON Number |
FromJSON Text |
FromJSON IntSet |
FromJSON ZonedTime |
FromJSON UTCTime |
FromJSON Value |
FromJSON DotNetTime |
FromJSON [Char] |
FromJSON a => FromJSON [a] |
FromJSON (Ratio Integer) |
FromJSON a => FromJSON (Maybe a) |
HasResolution a => FromJSON (Fixed 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 ByteString v) |
FromJSON v => FromJSON (Map Text v) |
FromJSON v => FromJSON (Map Text v) |
FromJSON v => FromJSON (HashMap String v) |
FromJSON v => FromJSON (HashMap ByteString v) |
FromJSON v => FromJSON (HashMap ByteString v) |
FromJSON v => FromJSON (HashMap Text v) |
FromJSON v => FromJSON (HashMap Text 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 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.

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:

    {-# 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 (GHC 7.2 and newer), `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 -> Value

Instances

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

# Inspecting ``Value`s`

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

`withObject 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

`withObject 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

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

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

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

# Constructors and accessors

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

Construct a `Pair` from a key and a value.

(.:) :: 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.

(.!=) :: 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 `Pair`s. If duplicate keys arise, earlier keys and their associated values win.