| Copyright | (c) 2013-2015 Brendan Hay |
|---|---|
| License | Mozilla Public License, v. 2.0. |
| Maintainer | Brendan Hay <brendan.g.hay@gmail.com> |
| Stability | provisional |
| Portability | non-portable (GHC extensions) |
| Safe Haskell | None |
| Language | Haskell2010 |
Network.AWS.Data.JSON
Description
- class FromJSON a where
- parseJSONText :: FromText a => String -> Value -> Parser a
- eitherDecode' :: FromJSON a => ByteString -> Either String a
- withObject :: String -> (Object -> Parser a) -> Value -> Parser a
- (.:) :: FromJSON a => Object -> Text -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- eitherParseJSON :: FromJSON a => Object -> Either String a
- (.:>) :: FromJSON a => Object -> Text -> Either String a
- (.?>) :: FromJSON a => Object -> Text -> Either String (Maybe a)
- class ToJSON a where
- toJSONText :: ToText a => a -> Value
- data Value :: * = Object !Object
- object :: [Pair] -> Value
- (.=) :: ToJSON a => Text -> a -> Pair
FromJSON
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
fromJSONfunction that parses to any type which is an instance ofData. - If your compiler has support for the
DeriveGenericandDefaultSignatureslanguage extensions,parseJSONwill have a default generic implementation.
To use this, simply add a deriving clause to your datatype and
declare a GenericFromJSON 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
Instances
eitherDecode' :: FromJSON a => ByteString -> Either String a
Like decode' but returns an error message when decoding fails.
Parser a
withObject :: String -> (Object -> Parser a) -> Value -> Parser a
withObject expected f value applies f to the Object when value is an Object
and fails using typeMismatch expected
(.:) :: FromJSON a => Object -> Text -> Parser a
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.
(.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
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.
(.!=) :: Parser (Maybe a) -> a -> Parser a
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"
Either String a
ToJSON
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
Minimal complete definition
Nothing
Instances
toJSONText :: ToText a => a -> Value Source
data Value :: *
A JSON value represented as a Haskell value.