Copyright | (C) 2015 Oleg Grenrus |
---|---|
License | BSD3 |
Maintainer | Oleg Grenrus <oleg.grenrus@iki.fi> |
Safe Haskell | None |
Language | Haskell2010 |
Compatibility notices
decode
etc. work as inaeson >=0.9
- but it is generalised to work in any
MonadThrow
(that is extra) .:?
works as inaeson <0.10
.:!
works as.:?
inaeson ==0.10
- Orphan instances
FromJSON
Day
andFromJSON
LocalTime
foraeson <0.10
Encoding
related 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
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- toJSONList :: [a] -> Value
- toEncodingList :: [a] -> Encoding
- class KeyValue kv where
- class GFromJSON arity f where
- gParseJSON :: Options -> FromArgs arity a -> Value -> Parser (f a)
- class GToJSON arity f where
- class GToEncoding arity f where
- gToEncoding :: Options -> ToArgs Encoding arity a -> f a -> Encoding
- 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 Source #
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 Source #
Like original decode'
but in arbitrary MonadThrow
.
newtype AesonException Source #
Exception thrown by decode
- family of functions in this module.
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 Source #
Like original decodeStrict
but in arbitrary MonadThrow
.
decodeStrict' :: (FromJSON a, MonadThrow m) => ByteString -> m a Source #
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.
DotNetTime | |
|
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:
empty
andmzero
work, but are terse and uninformativefail
yields a custom error messagetypeMismatch
produces an informative message for cases when the value encountered is not of the expected type
An example type and instance:
-- Allow ourselves to writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instance FromJSON Coord where parseJSON (Object
v) = Coord<$>
v.:
"x"<*>
v.:
"y" -- We do not expect a non-Object
value here. -- We could usemzero
to 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 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
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
Nothing
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 writeText
literals. {-# 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 Generic
ToJSON
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 } derivingGeneric
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
Nothing
Generic JSON classes and options
class GFromJSON arity f where #
Class of generic representation types that can be converted from JSON.
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.
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 where #
Class of generic representation types that can be converted to
a JSON Encoding
.
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 Value
s
Value
swithObject :: String -> (Object -> Parser a) -> Value -> Parser a #
withObject expected f value
applies f
to the Object
when value
is an Object
and fails using
otherwise.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
otherwise.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
otherwise.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
otherwise.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
otherwise.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
otherwise.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) Source #
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.