| Safe Haskell | None |
|---|
IHaskell.IPython.Message.Writer
Description
This module contains the ToJSON instance for Message.
Documentation
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
toJSONfunction that accepts any type which is an instance ofData. - If your compiler has support for the
DeriveGenericandDefaultSignatureslanguage extensions (GHC 7.2 and newer),toJSONwill have a default generic implementation.
To use the latter option, simply add a deriving clause to your
datatype and declare a GenericToJSON 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
Instances