Safe Haskell	None
Language	Haskell2010

Avers.TH

Synopsis

Documentation

deriveJSON

Arguments

:: Options	Encoding options.
-> Name	Name of the type for which to generate `ToJSON` and `FromJSON` instances.
-> Q [Dec]

Generates both ToJSON and FromJSON instance declarations for the given data type.

This is a convienience function which is equivalent to calling both deriveToJSON and deriveFromJSON.

deriveJSONOptions :: String -> Options Source

variantOptions :: String -> String -> String -> Options Source

defaultVariantOptions :: String -> Options Source

deriveEncoding :: Options -> Name -> Q [Dec] Source

deriveRecordEncoding :: Name -> String -> [(String, Q Exp)] -> Q [Dec] Source

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

instance FromJSON Coord @

Note that, instead of using DefaultSignatures, it's also possible to parameterize the generic decoding using genericParseJSON applied to your encoding/decoding Options:

instance FromJSON Coord where
    parseJSON = genericParseJSON defaultOptions

Minimal complete definition

Nothing

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	WARNING: Only parse Integers from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like `1e1000000000`.
FromJSON Word
FromJSON Word8
FromJSON Word16
FromJSON Word32
FromJSON Word64
FromJSON ()
FromJSON Scientific
FromJSON Number
FromJSON Text
FromJSON UTCTime
FromJSON Value
FromJSON DotNetTime
FromJSON Text
FromJSON IntSet
FromJSON Series
FromJSON Value
FromJSON Database
FromJSON User
FromJSON Admin
FromJSON Ping
FromJSON ShardSpace
FromJSON ResponseType
FromJSON ZonedTime
FromJSON Datum
FromJSON Operation
FromJSON ObjectId
FromJSON RevId
FromJSON ObjId
FromJSON Path
FromJSON Object
FromJSON Patch
FromJSON Snapshot
FromJSON Secret
FromJSON SecretId
FromJSON Release
FromJSON Blob
FromJSON BlobId
FromJSON Session
FromJSON SessionId
FromJSON [Char]
FromJSON a => FromJSON [a]
FromJSON (Ratio Integer)
FromJSON a => FromJSON (Maybe a)
HasResolution a => FromJSON (Fixed a)	WARNING: Only parse fixed-precision numbers from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like `1e1000000000`.
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)
FromJSON v => FromJSON (Tree v)
(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 (HashMap String v)
FromJSON v => FromJSON (HashMap Text v)
FromJSON v => FromJSON (HashMap Text v)
FromJSON v => FromJSON (Map String v)
FromJSON v => FromJSON (Map Text v)
FromJSON v => FromJSON (Map 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 a, FromJSON b, FromJSON c, FromJSON d, FromJSON e) => FromJSON (a, b, c, d, e)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f) => FromJSON (a, b, c, d, e, f)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g) => FromJSON (a, b, c, d, e, f, g)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h) => FromJSON (a, b, c, d, e, f, g, h)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i) => FromJSON (a, b, c, d, e, f, g, h, i)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j) => FromJSON (a, b, c, d, e, f, g, h, i, j)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k) => FromJSON (a, b, c, d, e, f, g, h, i, j, k)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n)
(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n, FromJSON o) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)

class ToJSON a where

A type that can be converted to JSON.

An example type and instance:

@{-# LANGUAGE OverloadedStrings #-}

data Coord = 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.