Safe Haskell | None |
---|---|
Language | Haskell2010 |
Documentation
:: Options | Encoding options. |
-> Name | Name of the type for which to generate |
-> 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
.
:: Options | Encoding options. |
-> Name | Name of the type for which to generate a |
-> Q [Dec] |
Generates a ToJSON
instance declaration for the given data type.
deriveJSONOptions :: String -> Options 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 ofData
. - If your compiler has support for the
DeriveGeneric
andDefaultSignatures
language extensions,parseJSON
will have a default generic implementation.
To use this, simply add a deriving
clause to your datatype and
declare a Generic
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
Nothing
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 ofData
. - If your compiler has support for the
DeriveGeneric
andDefaultSignatures
language extensions (GHC 7.2 and newer),toJSON
will have a default generic implementation.
To use the latter option, simply add a deriving
clause to your
datatype and declare a Generic
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
Nothing