Permalink
Fetching contributors…
Cannot retrieve contributors at this time
456 lines (427 sloc) 14 KB
-- |
-- Module: Data.Aeson
-- Copyright: (c) 2011-2016 Bryan O'Sullivan
-- (c) 2011 MailRank, Inc.
-- License: BSD3
-- Maintainer: Bryan O'Sullivan <bos@serpentine.com>
-- Stability: experimental
-- Portability: portable
--
-- Types and functions for working efficiently with JSON data.
--
-- (A note on naming: in Greek mythology, Aeson was the father of Jason.)
module Data.Aeson
(
-- * How to use this library
-- $use
-- ** Writing instances by hand
-- $manual
-- ** Working with the AST
-- $ast
-- ** Decoding to a Haskell value
-- $haskell
-- ** Decoding a mixed-type object
-- $mixed
-- * Encoding and decoding
-- $encoding_and_decoding
-- ** Direct encoding
-- $encoding
decode
, decode'
, eitherDecode
, eitherDecode'
, encode
-- ** Variants for strict bytestrings
, decodeStrict
, decodeStrict'
, eitherDecodeStrict
, eitherDecodeStrict'
-- * Core JSON types
, Value(..)
, Encoding
, fromEncoding
, Array
, Object
-- * Convenience types
, DotNetTime(..)
-- * Type conversion
, FromJSON(..)
, Result(..)
, fromJSON
, ToJSON(..)
, KeyValue(..)
-- ** Keys for maps
, ToJSONKey(..)
, ToJSONKeyFunction(..)
, FromJSONKey(..)
, FromJSONKeyFunction(..)
-- ** Liftings to unary and binary type constructors
, FromJSON1(..)
, parseJSON1
, FromJSON2(..)
, parseJSON2
, ToJSON1(..)
, toJSON1
, toEncoding1
, ToJSON2(..)
, toJSON2
, toEncoding2
-- ** Generic JSON classes and options
, GFromJSON(..)
, FromArgs(..)
, GToJSON
, GToEncoding
, ToArgs(..)
, Zero
, One
, genericToJSON
, genericLiftToJSON
, genericToEncoding
, genericLiftToEncoding
, genericParseJSON
, genericLiftParseJSON
, defaultOptions
-- * Inspecting @'Value's@
, withObject
, withText
, withArray
, withNumber
, withScientific
, withBool
-- * Constructors and accessors
, Series
, pairs
, foldable
, (.:)
, (.:?)
, (.:!)
, (.!=)
, object
-- * Parsing
, json
, json'
) where
import Prelude ()
import Prelude.Compat
import Data.Aeson.Types.FromJSON (ifromJSON)
import Data.Aeson.Encoding (encodingToLazyByteString)
import Data.Aeson.Parser.Internal (decodeWith, decodeStrictWith, eitherDecodeWith, eitherDecodeStrictWith, jsonEOF, json, jsonEOF', json')
import Data.Aeson.Types
import Data.Aeson.Types.Internal (JSONPath, formatError)
import qualified Data.ByteString as B
import qualified Data.ByteString.Lazy as L
-- | Efficiently serialize a JSON value as a lazy 'L.ByteString'.
--
-- This is implemented in terms of the 'ToJSON' class's 'toEncoding' method.
encode :: (ToJSON a) => a -> L.ByteString
encode = encodingToLazyByteString . toEncoding
-- | Efficiently deserialize a JSON value from a lazy 'L.ByteString'.
-- If this fails due to incomplete or invalid input, 'Nothing' is
-- returned.
--
-- The input must consist solely of a JSON document, with no trailing
-- data except for whitespace.
--
-- This function parses immediately, but defers conversion. See
-- 'json' for details.
decode :: (FromJSON a) => L.ByteString -> Maybe a
decode = decodeWith jsonEOF fromJSON
{-# INLINE decode #-}
-- | Efficiently deserialize a JSON value from a strict 'B.ByteString'.
-- If this fails due to incomplete or invalid input, 'Nothing' is
-- returned.
--
-- The input must consist solely of a JSON document, with no trailing
-- data except for whitespace.
--
-- This function parses immediately, but defers conversion. See
-- 'json' for details.
decodeStrict :: (FromJSON a) => B.ByteString -> Maybe a
decodeStrict = decodeStrictWith jsonEOF fromJSON
{-# INLINE decodeStrict #-}
-- | Efficiently deserialize a JSON value from a lazy 'L.ByteString'.
-- If this fails due to incomplete or invalid input, 'Nothing' is
-- returned.
--
-- The input must consist solely of a JSON document, with no trailing
-- data except for whitespace.
--
-- This function parses and performs conversion immediately. See
-- 'json'' for details.
decode' :: (FromJSON a) => L.ByteString -> Maybe a
decode' = decodeWith jsonEOF' fromJSON
{-# INLINE decode' #-}
-- | Efficiently deserialize a JSON value from a strict 'B.ByteString'.
-- If this fails due to incomplete or invalid input, 'Nothing' is
-- returned.
--
-- The input must consist solely of a JSON document, with no trailing
-- data except for whitespace.
--
-- This function parses and performs conversion immediately. See
-- 'json'' for details.
decodeStrict' :: (FromJSON a) => B.ByteString -> Maybe a
decodeStrict' = decodeStrictWith jsonEOF' fromJSON
{-# INLINE decodeStrict' #-}
eitherFormatError :: Either (JSONPath, String) a -> Either String a
eitherFormatError = either (Left . uncurry formatError) Right
{-# INLINE eitherFormatError #-}
-- | Like 'decode' but returns an error message when decoding fails.
eitherDecode :: (FromJSON a) => L.ByteString -> Either String a
eitherDecode = eitherFormatError . eitherDecodeWith jsonEOF ifromJSON
{-# INLINE eitherDecode #-}
-- | Like 'decodeStrict' but returns an error message when decoding fails.
eitherDecodeStrict :: (FromJSON a) => B.ByteString -> Either String a
eitherDecodeStrict =
eitherFormatError . eitherDecodeStrictWith jsonEOF ifromJSON
{-# INLINE eitherDecodeStrict #-}
-- | Like 'decode'' but returns an error message when decoding fails.
eitherDecode' :: (FromJSON a) => L.ByteString -> Either String a
eitherDecode' = eitherFormatError . eitherDecodeWith jsonEOF' ifromJSON
{-# INLINE eitherDecode' #-}
-- | Like 'decodeStrict'' but returns an error message when decoding fails.
eitherDecodeStrict' :: (FromJSON a) => B.ByteString -> Either String a
eitherDecodeStrict' =
eitherFormatError . eitherDecodeStrictWith jsonEOF' ifromJSON
{-# INLINE eitherDecodeStrict' #-}
-- $use
--
-- This section contains basic information on the different ways to
-- work with data using this library. These range from simple but
-- inflexible, to complex but flexible.
--
-- The most common way to use the library is to define a data type,
-- corresponding to some JSON data you want to work with, and then
-- write either a 'FromJSON' instance, a to 'ToJSON' instance, or both
-- for that type.
--
-- For example, given this JSON data:
--
-- > { "name": "Joe", "age": 12 }
--
-- we create a matching data type:
--
-- > {-# LANGUAGE DeriveGeneric #-}
-- >
-- > import GHC.Generics
-- >
-- > data Person = Person {
-- > name :: Text
-- > , age :: Int
-- > } deriving (Generic, Show)
--
-- The @LANGUAGE@ pragma and 'Generic' instance let us write empty
-- 'FromJSON' and 'ToJSON' instances for which the compiler will
-- generate sensible default implementations.
--
-- @
-- instance 'ToJSON' Person where
-- \-- No need to provide a 'toJSON' implementation.
--
-- \-- For efficiency, we write a simple 'toEncoding' implementation, as
-- \-- the default version uses 'toJSON'.
-- 'toEncoding' = 'genericToEncoding' 'defaultOptions'
--
-- instance 'FromJSON' Person
-- \-- No need to provide a 'parseJSON' implementation.
-- @
--
-- We can now encode a value like so:
--
-- > >>> encode (Person {name = "Joe", age = 12})
-- > "{\"name\":\"Joe\",\"age\":12}"
-- $manual
--
-- When necessary, we can write 'ToJSON' and 'FromJSON' instances by
-- hand. This is valuable when the JSON-on-the-wire and Haskell data
-- are different or otherwise need some more carefully managed
-- translation. Let's revisit our JSON data:
--
-- > { "name": "Joe", "age": 12 }
--
-- We once again create a matching data type, without bothering to add
-- a 'Generic' instance this time:
--
-- > data Person = Person {
-- > name :: Text
-- > , age :: Int
-- > } deriving Show
--
-- To decode data, we need to define a 'FromJSON' instance:
--
-- > {-# LANGUAGE OverloadedStrings #-}
-- >
-- > instance FromJSON Person where
-- > parseJSON = withObject "Person" $ \v -> Person
-- > <$> v .: "name"
-- > <*> v .: "age"
--
-- We can now parse the JSON data like so:
--
-- > >>> decode "{\"name\":\"Joe\",\"age\":12}" :: Maybe Person
-- > Just (Person {name = "Joe", age = 12})
--
-- To encode data, we need to define a 'ToJSON' instance. Let's begin
-- with an instance written entirely by hand.
--
-- @
-- instance ToJSON Person where
-- \-- this generates a 'Value'
-- 'toJSON' (Person name age) =
-- 'object' [\"name\" '.=' name, \"age\" '.=' age]
--
-- \-- this encodes directly to a bytestring Builder
-- 'toEncoding' (Person name age) =
-- 'pairs' (\"name\" '.=' 'name' '<>' \"age\" '.=' age)
-- @
--
-- We can now encode a value like so:
--
-- > >>> encode (Person {name = "Joe", age = 12})
-- > "{\"name\":\"Joe\",\"age\":12}"
--
-- There are predefined 'FromJSON' and 'ToJSON' instances for many
-- types. Here's an example using lists and 'Int's:
--
-- > >>> decode "[1,2,3]" :: Maybe [Int]
-- > Just [1,2,3]
--
-- And here's an example using the 'Data.Map.Map' type to get a map of
-- 'Int's.
--
-- > >>> decode "{\"foo\":1,\"bar\":2}" :: Maybe (Map String Int)
-- > Just (fromList [("bar",2),("foo",1)])
-- While the notes below focus on decoding, you can apply almost the
-- same techniques to /encoding/ data. (The main difference is that
-- encoding always succeeds, but decoding has to handle the
-- possibility of failure, where an input doesn't match our
-- expectations.)
--
-- See the documentation of 'FromJSON' and 'ToJSON' for some examples
-- of how you can automatically derive instances in many common
-- circumstances.
-- $ast
--
-- Sometimes you want to work with JSON data directly, without first
-- converting it to a custom data type. This can be useful if you want
-- to e.g. convert JSON data to YAML data, without knowing what the
-- contents of the original JSON data was. The 'Value' type, which is
-- an instance of 'FromJSON', is used to represent an arbitrary JSON
-- AST (abstract syntax tree). Example usage:
--
-- > >>> decode "{\"foo\": 123}" :: Maybe Value
-- > Just (Object (fromList [("foo",Number 123)]))
--
-- > >>> decode "{\"foo\": [\"abc\",\"def\"]}" :: Maybe Value
-- > Just (Object (fromList [("foo",Array (fromList [String "abc",String "def"]))]))
--
-- Once you have a 'Value' you can write functions to traverse it and
-- make arbitrary transformations.
-- $haskell
--
-- We can decode to any instance of 'FromJSON':
--
-- > λ> decode "[1,2,3]" :: Maybe [Int]
-- > Just [1,2,3]
--
-- Alternatively, there are instances for standard data types, so you
-- can use them directly. For example, use the 'Data.Map.Map' type to
-- get a map of 'Int's.
--
-- > λ> import Data.Map
-- > λ> decode "{\"foo\":1,\"bar\":2}" :: Maybe (Map String Int)
-- > Just (fromList [("bar",2),("foo",1)])
-- $mixed
--
-- The above approach with maps of course will not work for mixed-type
-- objects that don't follow a strict schema, but there are a couple
-- of approaches available for these.
--
-- The 'Object' type contains JSON objects:
--
-- > λ> decode "{\"name\":\"Dave\",\"age\":2}" :: Maybe Object
-- > Just (fromList) [("name",String "Dave"),("age",Number 2)]
--
-- You can extract values from it with a parser using 'parse',
-- 'parseEither' or, in this example, 'parseMaybe':
--
-- > λ> do result <- decode "{\"name\":\"Dave\",\"age\":2}"
-- > flip parseMaybe result $ \obj -> do
-- > age <- obj .: "age"
-- > name <- obj .: "name"
-- > return (name ++ ": " ++ show (age*2))
-- >
-- > Just "Dave: 4"
--
-- Considering that any type that implements 'FromJSON' can be used
-- here, this is quite a powerful way to parse JSON. See the
-- documentation in 'FromJSON' for how to implement this class for
-- your own data types.
--
-- The downside is that you have to write the parser yourself; the
-- upside is that you have complete control over the way the JSON is
-- parsed.
-- $encoding_and_decoding
--
-- Decoding is a two-step process.
--
-- * When decoding a value, the process is reversed: the bytes are
-- converted to a 'Value', then the 'FromJSON' class is used to
-- convert to the desired type.
--
-- There are two ways to encode a value.
--
-- * Convert to a 'Value' using 'toJSON', then possibly further
-- encode. This was the only method available in aeson 0.9 and
-- earlier.
--
-- * Directly encode (to what will become a 'L.ByteString') using
-- 'toEncoding'. This is much more efficient (about 3x faster, and
-- less memory intensive besides), but is only available in aeson
-- 0.10 and newer.
--
-- For convenience, the 'encode' and 'decode' functions combine both
-- steps.
-- $encoding
--
-- In older versions of this library, encoding a Haskell value
-- involved converting to an intermediate 'Value', then encoding that.
--
-- A \"direct\" encoder converts straight from a source Haskell value
-- to a 'BL.ByteString' without constructing an intermediate 'Value'.
-- This approach is faster than 'toJSON', and allocates less memory.
-- The 'toEncoding' method makes it possible to implement direct
-- encoding with low memory overhead.
--
-- To complicate matters, the default implementation of 'toEncoding'
-- uses 'toJSON'. Why? The 'toEncoding' method was added to this
-- library much more recently than 'toJSON'. Using 'toJSON' ensures
-- that packages written against older versions of this library will
-- compile and produce correct output, but they will not see any
-- speedup from direct encoding.
--
-- To write a minimal implementation of direct encoding, your type
-- must implement GHC's 'Generic' class, and your code should look
-- like this:
--
-- @
-- 'toEncoding' = 'genericToEncoding' 'defaultOptions'
-- @
--
-- What if you have more elaborate encoding needs? For example,
-- perhaps you need to change the names of object keys, omit parts of
-- a value.
--
-- To encode to a JSON \"object\", use the 'pairs' function.
--
-- @
-- 'toEncoding' (Person name age) =
-- 'pairs' (\"name\" '.=' 'name' '<>' \"age\" '.=' age)
-- @
--
-- Any container type that implements 'Foldable' can be encoded to a
-- JSON \"array\" using 'foldable'.
--
-- > > import Data.Sequence as Seq
-- > > encode (Seq.fromList [1,2,3])
-- > "[1,2,3]"