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

Like decode' but returns an error message when decoding fails.

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

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.

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.

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"

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.

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 ToJSON Coord @

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

instance ToJSON Coord where
    toJSON = genericToJSON defaultOptions

A JSON value represented as a Haskell value.

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

Construct a Pair from a key and a value.