| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
RFC.JSON
Contents
- jsonOptions :: Options
- deriveJSON :: Options -> Name -> Q [Dec]
- class FromJSON a where
- class ToJSON a where
- eitherDecode :: FromJSON a => ByteString -> Either String a
- decodeEither :: FromJSON a => LazyByteString -> Either String a
- eitherDecode' :: FromJSON a => ByteString -> Either String a
- decodeEither' :: FromJSON a => LazyByteString -> Either String a
- decodeOrDie :: (FromJSON a, MonadIO m) => LazyByteString -> m a
- data DecodeError
- data Value :: *
- encode :: ToJSON a => a -> ByteString
- decode :: FromJSON a => ByteString -> Maybe a
- module Data.Aeson.Types
Documentation
Arguments
| :: 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 or data family instance constructor.
This is a convienience function which is equivalent to calling both
deriveToJSON and deriveFromJSON.
A type that can be converted from JSON, with the possibility of failure.
In many cases, you can get the compiler to generate parsing code for you (see below). To begin, let's cover writing an instance by hand.
There are various reasons a conversion could fail. For example, an
Object could be missing a required key, an Array could be of
the wrong size, or a value could be of an incompatible type.
The basic ways to signal a failed conversion are as follows:
emptyandmzerowork, but are terse and uninformative;failyields a custom error message;typeMismatchproduces an informative message for cases when the value encountered is not of the expected type.
An example type and instance using typeMismatch:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSONCoord whereparseJSON(Objectv) = Coord<$>v.:"x"<*>v.:"y" -- We do not expect a non-Objectvalue here. -- We could usemzeroto fail, buttypeMismatch-- gives a much more informative error message.parseJSONinvalid =typeMismatch"Coord" invalid
For this common case of only being concerned with a single
type of JSON value, the functions withObject, withNumber, etc.
are provided. Their use is to be preferred when possible, since
they are more terse. Using withObject, we can rewrite the above instance
(assuming the same language extension and data type) as:
instanceFromJSONCoord whereparseJSON=withObject"Coord" $ \v -> Coord<$>v.:"x"<*>v.:"y"
Instead of manually writing your FromJSON instance, there are two 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 it will probably be more efficient than the following option.
- The compiler can provide a default generic implementation for
parseJSON.
To use the second, 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
The default implementation will be equivalent to
parseJSON = ; If you need different
options, you can customize the generic decoding by defining:genericParseJSON defaultOptions
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceFromJSONCoord whereparseJSON=genericParseJSONcustomOptions
Instances
A type that can be converted to JSON.
Instances in general must specify toJSON and should (but don't need
to) specify toEncoding.
An example type and instance:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceToJSONCoord wheretoJSON(Coord x y) =object["x".=x, "y".=y]toEncoding(Coord x y) =pairs("x".=x<>"y".=y)
Instead of manually writing your ToJSON instance, there are two 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 it will probably be more efficient than the following option.
- The compiler can provide a default generic implementation for
toJSON.
To use the second, simply add a deriving clause to your
datatype and declare a GenericToJSON instance. If you require nothing other than
defaultOptions, it is sufficient to write (and this is the only
alternative where the default toJSON implementation is sufficient):
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord = Coord { x :: Double, y :: Double } deriving Generic
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
If on the other hand you wish to customize the generic decoding, you have to implement both methods:
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceToJSONCoord wheretoJSON=genericToJSONcustomOptionstoEncoding=genericToEncodingcustomOptions
Previous versions of this library only had the toJSON method. Adding
toEncoding had to reasons:
- toEncoding is more efficient for the common case that the output of
toJSONis directly serialized to aByteString. Further, expressing either method in terms of the other would be non-optimal. - The choice of defaults allows a smooth transition for existing users:
Existing instances that do not define
toEncodingstill compile and have the correct semantics. This is ensured by making the default implementation oftoEncodingusetoJSON. This produces correct results, but since it performs an intermediate conversion to aValue, it will be less efficient than directly emitting anEncoding. (this also means that specifying nothing more thaninstance ToJSON Coordwould be sufficient as a generically decoding instance, but there probably exists no good reason to not specifytoEncodingin new instances.)
Methods
Convert a Haskell value to a JSON-friendly intermediate type.
toEncoding :: a -> Encoding #
Encode a Haskell value as JSON.
The default implementation of this method creates an
intermediate Value using toJSON. This provides
source-level compatibility for people upgrading from older
versions of this library, but obviously offers no performance
advantage.
To benefit from direct encoding, you must provide an
implementation for this method. The easiest way to do so is by
having your types implement Generic using the DeriveGeneric
extension, and then have GHC generate a method body as follows.
instanceToJSONCoord wheretoEncoding=genericToEncodingdefaultOptions
toJSONList :: [a] -> Value #
toEncodingList :: [a] -> Encoding #
Instances
eitherDecode :: FromJSON a => ByteString -> Either String a #
Like decode but returns an error message when decoding fails.
decodeEither :: FromJSON a => LazyByteString -> Either String a Source #
eitherDecode' :: FromJSON a => ByteString -> Either String a #
Like decode' but returns an error message when decoding fails.
decodeEither' :: FromJSON a => LazyByteString -> Either String a Source #
decodeOrDie :: (FromJSON a, MonadIO m) => LazyByteString -> m a Source #
data DecodeError Source #
Instances
A JSON value represented as a Haskell value.
Instances
| Eq Value | |
| Data Value | |
| Read Value | |
| Show Value | |
| IsString Value | |
| Generic Value | |
| Lift Value | |
| FromString Encoding | |
| FromString Value | |
| Hashable Value | |
| ToJSON Value | |
| KeyValue Pair | |
| FromJSON Value | |
| NFData Value | |
| PGRep Value | |
| FromPairs Encoding Series | |
| GKeyValue Encoding Series | |
| PGParameter "json" Value | |
| PGParameter "jsonb" Value | |
| PGColumn "json" Value | |
| PGColumn "jsonb" Value | |
| GToJSON Encoding arity (U1 *) | |
| GToJSON Value arity (U1 *) | |
| ToJSON1 f => GToJSON Encoding One (Rec1 * f) | |
| ToJSON1 f => GToJSON Value One (Rec1 * f) | |
| ToJSON a => GToJSON Encoding arity (K1 * i a) | |
| (EncodeProduct arity a, EncodeProduct arity b) => GToJSON Encoding arity ((:*:) * a b) | |
| ToJSON a => GToJSON Value arity (K1 * i a) | |
| (WriteProduct arity a, WriteProduct arity b, ProductSize a, ProductSize b) => GToJSON Value arity ((:*:) * a b) | |
| (ToJSON1 f, GToJSON Encoding One g) => GToJSON Encoding One ((:.:) * * f g) | |
| (ToJSON1 f, GToJSON Value One g) => GToJSON Value One ((:.:) * * f g) | |
| FromPairs Value (DList Pair) | |
| ToJSON v => GKeyValue v (DList Pair) | |
| HasDefault ParamOtherSchema (Maybe Value) | |
| HasDefault Schema (Maybe Value) | |
| HasDefault Header (Maybe Value) | |
| HasParamSchema s (ParamSchema t) => HasEnum s (Maybe [Value]) | |
| HasExample Schema (Maybe Value) | |
| (GToJSON Encoding arity a, ConsToJSON Encoding arity a, Constructor Meta c) => SumToJSON' * TwoElemArray Encoding arity (C1 * c a) | |
| (GToJSON Value arity a, ConsToJSON Value arity a, Constructor Meta c) => SumToJSON' * TwoElemArray Value arity (C1 * c a) | |
| HasDefault (ParamSchema t) (Maybe Value) | |
| HasEnum (ParamSchema t) (Maybe [Value]) | |
| type Rep Value | |
| type PGRepType Value | |
encode :: ToJSON a => a -> ByteString #
Efficiently serialize a JSON value as a lazy ByteString.
This is implemented in terms of the ToJSON class's toEncoding method.
decode :: FromJSON a => ByteString -> Maybe a #
Efficiently deserialize a JSON value from a lazy ByteString.
If this fails due to incomplete or invalid input, Nothing is
returned.
The input must consist solely of a JSON document, with no trailing data except for whitespace.
This function parses immediately, but defers conversion. See
json for details.
module Data.Aeson.Types