| Copyright | (C) 2015 Oleg Grenrus |
|---|---|
| License | BSD3 |
| Maintainer | Oleg Grenrus <oleg.grenrus@iki.fi> |
| Safe Haskell | None |
| Language | Haskell2010 |
Data.Aeson.Compat
Contents
Description
Compatibility notices
decodeetc. work as inaeson >=0.9- but it is generalised to work in any
MonadThrow(that is extra) .:?works as inaeson ||=0.11.:!works as inaeson ||=0.11and as.:?did inaeson ==0.10.*- Orphan instances
FromJSONDayandFromJSONLocalTimeforaeson <0.10 Encodingrelated functionality is not added. It's present only withaeson >=0.10
- decode :: (FromJSON a, MonadThrow m) => ByteString -> m a
- decode' :: (FromJSON a, MonadThrow m) => ByteString -> m a
- newtype AesonException = AesonException String
- eitherDecode :: FromJSON a => ByteString -> Either String a
- eitherDecode' :: FromJSON a => ByteString -> Either String a
- encode :: ToJSON a => a -> ByteString
- decodeStrict :: (FromJSON a, MonadThrow m) => ByteString -> m a
- decodeStrict' :: (FromJSON a, MonadThrow m) => ByteString -> m a
- eitherDecodeStrict :: FromJSON a => ByteString -> Either String a
- eitherDecodeStrict' :: FromJSON a => ByteString -> Either String a
- data Value :: *
- type Encoding = Encoding' Value
- fromEncoding :: Encoding' tag -> Builder
- type Array = Vector Value
- type Object = HashMap Text Value
- newtype DotNetTime :: * = DotNetTime {}
- class FromJSON a where
- data Result a :: * -> *
- fromJSON :: FromJSON a => Value -> Result a
- class ToJSON a where
- class KeyValue kv where
- class GFromJSON arity f
- class GToJSON arity f
- class GToEncoding arity f
- genericToJSON :: (Generic a, GToJSON Zero (Rep a)) => Options -> a -> Value
- genericToEncoding :: (Generic a, GToEncoding Zero (Rep a)) => Options -> a -> Encoding
- genericParseJSON :: (Generic a, GFromJSON Zero (Rep a)) => Options -> Value -> Parser a
- defaultOptions :: Options
- withObject :: String -> (Object -> Parser a) -> Value -> Parser a
- withText :: String -> (Text -> Parser a) -> Value -> Parser a
- withArray :: String -> (Array -> Parser a) -> Value -> Parser a
- withNumber :: String -> (Number -> Parser a) -> Value -> Parser a
- withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser a
- withBool :: String -> (Bool -> Parser a) -> Value -> Parser a
- data Series :: *
- pairs :: Series -> Encoding
- foldable :: (Foldable t, ToJSON a) => t a -> Encoding
- (.:) :: FromJSON a => Object -> Text -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.:!) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- object :: [Pair] -> Value
- json :: Parser Value
- json' :: Parser Value
- value :: Parser Value
- value' :: Parser Value
- data Parser a :: * -> *
Encoding and decoding
Direct encoding
decode :: (FromJSON a, MonadThrow m) => ByteString -> m a #
Like original decode but in arbitrary MonadThrow.
Parse a top-level JSON value, i.e. also strings, numbers etc.
decode' :: (FromJSON a, MonadThrow m) => ByteString -> m a #
Like original decode' but in arbitrary MonadThrow.
newtype AesonException #
Exception thrown by decode - family of functions in this module.
Constructors
| AesonException String |
Instances
eitherDecode :: FromJSON a => ByteString -> Either String a #
Like decode but returns an error message when decoding fails.
eitherDecode' :: FromJSON a => ByteString -> Either String a #
Like decode' but returns an error message when decoding fails.
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.
Variants for strict bytestrings
decodeStrict :: (FromJSON a, MonadThrow m) => ByteString -> m a #
Like original decodeStrict but in arbitrary MonadThrow.
decodeStrict' :: (FromJSON a, MonadThrow m) => ByteString -> m a #
Like original decodeStrict' but in arbitrary MonadThrow.
eitherDecodeStrict :: FromJSON a => ByteString -> Either String a #
Like decodeStrict but returns an error message when decoding fails.
eitherDecodeStrict' :: FromJSON a => ByteString -> Either String a #
Like decodeStrict' but returns an error message when decoding fails.
Core JSON types
A JSON value represented as a Haskell value.
fromEncoding :: Encoding' tag -> Builder #
Acquire the underlying bytestring builder.
Convenience types
newtype DotNetTime :: * #
A newtype wrapper for UTCTime that uses the same non-standard
serialization format as Microsoft .NET, whose
System.DateTime
type is by default serialized to JSON as in the following example:
/Date(1302547608878)/
The number represents milliseconds since the Unix epoch.
Constructors
| DotNetTime | |
Fields
| |
Type conversion
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 uninformativefailyields a custom error messagetypeMismatchproduces an informative message for cases when the value encountered is not of the expected type
An example type and instance:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instance FromJSON Coord where parseJSON (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. parseJSON invalid =typeMismatch"Coord" invalid
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 will probably be more efficient than the following two options:
- 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
If DefaultSignatures doesn't give exactly the results you want,
you can customize the generic decoding with only a tiny amount of
effort, using genericParseJSON with your preferred Options:
instance FromJSON Coord where
parseJSON = genericParseJSON defaultOptions
Instances
The result of running a Parser.
fromJSON :: FromJSON a => Value -> Result a #
Convert a value from JSON, failing if the types do not match.
A type that can be converted to JSON.
An example type and instance:
-- Allow ourselves to writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instance ToJSON Coord where toJSON (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 will probably be more efficient than the following two options:
- 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 for your datatype without giving
definitions for toJSON or toEncoding.
For example, the previous example can be simplified to a more minimal instance:
{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics
data Coord = Coord { x :: Double, y :: Double } deriving Generic
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
Why do we provide an implementation for toEncoding here? The
toEncoding function is a relatively new addition to this class.
To allow users of older versions of this library to upgrade without
having to edit all of their instances or encounter surprising
incompatibilities, the default implementation of toEncoding uses
toJSON. This produces correct results, but since it performs an
intermediate conversion to a Value, it will be less efficient
than directly emitting an Encoding. Our one-liner definition of
toEncoding above bypasses the intermediate Value.
If DefaultSignatures doesn't give exactly the results you want,
you can customize the generic encoding with only a tiny amount of
effort, using genericToJSON and genericToEncoding with your
preferred Options:
instance ToJSON Coord where
toJSON = genericToJSON defaultOptions
toEncoding = genericToEncoding defaultOptions
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.
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
toJSONList :: [a] -> Value #
toEncodingList :: [a] -> Encoding #
Instances
A key-value pair for encoding a JSON object.
Minimal complete definition
Generic JSON classes and options
Class of generic representation types that can be converted from JSON.
Minimal complete definition
Instances
| GFromJSON arity U1 | |
| GFromJSON One Par1 | |
| FromJSON1 f => GFromJSON One (Rec1 f) | |
| ConsFromJSON arity a => GFromJSON arity (C1 c a) | |
| FromJSON a => GFromJSON arity (K1 i a) | |
| (AllNullary ((:+:) a b) allNullary, ParseSum * arity ((:+:) a b) allNullary) => GFromJSON arity ((:+:) a b) | |
| (FromProduct arity a, FromProduct arity b, ProductSize a, ProductSize b) => GFromJSON arity ((:*:) a b) | |
| (FromJSON1 f, GFromJSON One g) => GFromJSON One ((:.:) f g) | |
| GFromJSON arity a => GFromJSON arity (M1 i c a) | |
Class of generic representation types that can be converted to JSON.
Minimal complete definition
Instances
| GToJSON arity U1 | |
| GToJSON One Par1 | |
| ToJSON1 f => GToJSON One (Rec1 f) | |
| ConsToJSON arity a => GToJSON arity (C1 c a) | |
| ToJSON a => GToJSON arity (K1 i a) | |
| (AllNullary ((:+:) a b) allNullary, SumToJSON * arity ((:+:) a b) allNullary) => GToJSON arity ((:+:) a b) | |
| (WriteProduct arity a, WriteProduct arity b, ProductSize a, ProductSize b) => GToJSON arity ((:*:) a b) | |
| (ToJSON1 f, GToJSON One g) => GToJSON One ((:.:) f g) | |
| GToJSON arity a => GToJSON arity (M1 i c a) | |
class GToEncoding arity f #
Class of generic representation types that can be converted to
a JSON Encoding.
Minimal complete definition
Instances
| GToEncoding arity U1 | |
| GToEncoding One Par1 | |
| ToJSON1 f => GToEncoding One (Rec1 f) | |
| ConsToEncoding arity a => GToEncoding arity (C1 c a) | |
| ToJSON a => GToEncoding arity (K1 i a) | |
| (AllNullary ((:+:) a b) allNullary, SumToEncoding * arity ((:+:) a b) allNullary) => GToEncoding arity ((:+:) a b) | |
| (EncodeProduct arity a, EncodeProduct arity b) => GToEncoding arity ((:*:) a b) | |
| (ToJSON1 f, GToEncoding One g) => GToEncoding One ((:.:) f g) | |
| GToEncoding arity a => GToEncoding arity (M1 i c a) | |
genericToJSON :: (Generic a, GToJSON Zero (Rep a)) => Options -> a -> Value #
A configurable generic JSON creator. This function applied to
defaultOptions is used as the default for toJSON when the type
is an instance of Generic.
genericToEncoding :: (Generic a, GToEncoding Zero (Rep a)) => Options -> a -> Encoding #
A configurable generic JSON encoder. This function applied to
defaultOptions is used as the default for toEncoding when the type
is an instance of Generic.
genericParseJSON :: (Generic a, GFromJSON Zero (Rep a)) => Options -> Value -> Parser a #
A configurable generic JSON decoder. This function applied to
defaultOptions is used as the default for parseJSON when the
type is an instance of Generic.
Default encoding Options:
Options{fieldLabelModifier= id ,constructorTagModifier= id ,allNullaryToStringTag= True ,omitNothingFields= False ,sumEncoding=defaultTaggedObject,unwrapUnaryRecords= False }
Inspecting Values
ValueswithObject :: 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
withText :: String -> (Text -> Parser a) -> Value -> Parser a #
withText expected f value applies f to the Text when value is a String
and fails using typeMismatch expected
withArray :: String -> (Array -> Parser a) -> Value -> Parser a #
withArray expected f value applies f to the Array when value is an Array
and fails using typeMismatch expected
withNumber :: String -> (Number -> Parser a) -> Value -> Parser a #
withNumber expected f value applies f to the Number when value is a Number.
and fails using typeMismatch expected
withScientific :: String -> (Scientific -> Parser a) -> Value -> Parser a #
withScientific expected f value applies f to the Scientific number when value is a Number.
and fails using typeMismatch expected
withBool :: String -> (Bool -> Parser a) -> Value -> Parser a #
withBool expected f value applies f to the Bool when value is a Bool
and fails using typeMismatch expected
Constructors and accessors
A series of values that, when encoded, should be separated by
commas. Since 0.11.0.0, the .= operator is overloaded to create
either (Text, Value) or Series. You can use Series when
encoding directly to a bytestring builder as in the following
example:
toEncoding (Person name age) = pairs ("name" .= name <> "age" .= age)(.:) :: 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.
This operator is consistent in aeson >=0.7 && <0.11
(.!=) :: 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"
Parsing
Parse a top-level JSON value.
The conversion of a parsed value to a Haskell value is deferred until the Haskell value is needed. This may improve performance if only a subset of the results of conversions are needed, but at a cost in thunk allocation.
This function is an alias for value. In aeson 0.8 and earlier, it
parsed only object or array types, in conformance with the
now-obsolete RFC 4627.
Parse a top-level JSON value.
This is a strict version of json which avoids building up thunks
during parsing; it performs all conversions immediately. Prefer
this version if most of the JSON data needs to be accessed.
This function is an alias for value'. In aeson 0.8 and earlier, it
parsed only object or array types, in conformance with the
now-obsolete RFC 4627.
Parse any JSON value. You should usually json in preference to
this function, as this function relaxes the object-or-array
requirement of RFC 4627.
In particular, be careful in using this function if you think your
code might interoperate with Javascript. A naïve Javascript
library that parses JSON data using eval is vulnerable to attack
unless the encoded data represents an object or an array. JSON
implementations in other languages conform to that same restriction
to preserve interoperability and security.