Copyright | (c) 2011, 2012 Bryan O'Sullivan (c) 2011 MailRank, Inc. |
---|---|
License | Apache |
Maintainer | Bryan O'Sullivan <bos@serpentine.com> |
Stability | experimental |
Portability | portable |
Safe Haskell | None |
Language | Haskell98 |
Types for working with JSON data.
- data Value
- type Array = Vector Value
- emptyArray :: Value
- type Pair = (Text, Value)
- type Object = HashMap Text Value
- emptyObject :: Value
- newtype DotNetTime = DotNetTime {}
- typeMismatch :: String -> Value -> Parser a
- data Parser a
- data Result a
- class FromJSON a where
- fromJSON :: FromJSON a => Value -> Result a
- parse :: (a -> Parser b) -> a -> Result b
- parseEither :: (a -> Parser b) -> a -> Either String b
- parseMaybe :: (a -> Parser b) -> a -> Maybe b
- class ToJSON a where
- modifyFailure :: (String -> String) -> Parser a -> Parser a
- class GFromJSON f where
- gParseJSON :: Options -> Value -> Parser (f a)
- class GToJSON f where
- genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> Value
- genericParseJSON :: (Generic a, GFromJSON (Rep a)) => Options -> Value -> Parser a
- 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
- (.=) :: ToJSON a => Text -> a -> Pair
- (.:) :: FromJSON a => Object -> Text -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- object :: [Pair] -> Value
- data Options = Options {}
- data SumEncoding
- camelTo :: Char -> String -> String
- defaultOptions :: Options
- defaultTaggedObject :: SumEncoding
Core JSON types
data Value
A JSON value represented as a Haskell value.
emptyArray :: Value
The empty array.
emptyObject :: Value
The empty object.
Convenience types and functions
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.
:: String | The name of the type you are trying to parse. |
-> Value | The actual value encountered. |
-> Parser a |
Fail parsing due to a type mismatch, with a descriptive message.
Type conversion
data Parser a
A continuation-based parser type.
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
fromJSON :: FromJSON a => Value -> Result a
Convert a value from JSON, failing if the types do not match.
parseMaybe :: (a -> Parser b) -> a -> Maybe b
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
modifyFailure :: (String -> String) -> Parser a -> Parser a
If the inner Parser
failed, modify the failure message using the
provided function. This allows you to create more descriptive error messages.
For example:
parseJSON (Object o) = modifyFailure ("Parsing of the Foo value failed: " ++) (Foo <$> o .: "someField")
Since 0.6.2.0
Generic JSON classes
class GFromJSON f where
Class of generic representation types (Rep
) that can be converted from JSON.
gParseJSON :: Options -> Value -> Parser (f a)
This method (applied to defaultOptions
) is used as the
default generic implementation of parseJSON
.
class GToJSON f where
Class of generic representation types (Rep
) that can be converted to JSON.
gToJSON :: Options -> f a -> Value
This method (applied to defaultOptions
) is used as the
default generic implementation of toJSON
.
genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> Value
A configurable generic JSON encoder. This function applied to
defaultOptions
is used as the default for toJSON
when the type
is an instance of Generic
.
genericParseJSON :: (Generic a, GFromJSON (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
.
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
Deprecated: Use withScientific instead
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
(.:) :: 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"
Generic and TH encoding configuration
data Options
Options that specify how to encode/decode your datatype to/from JSON.
Options | |
|
data SumEncoding
Specifies how to encode constructors of a sum datatype.
TaggedObject | A constructor will be encoded to an object with a field
|
ObjectWithSingleField | A constructor will be encoded to an object with a single
field named after the constructor tag (modified by the
|
TwoElemArray | A constructor will be encoded to a 2-element array where the
first element is the tag of the constructor (modified by the
|
camelTo :: Char -> String -> String
Converts from CamelCase to another lower case, interspersing
the character between all capital letters and their previous
entries, except those capital letters that appear together,
like API
.
For use by Aeson template haskell calls.
camelTo '_' 'CamelCaseAPI' == "camel_case_api"
Default encoding Options
:
Options
{fieldLabelModifier
= id ,constructorTagModifier
= id ,allNullaryToStringTag
= True ,omitNothingFields
= False ,sumEncoding
=defaultTaggedObject
}
defaultTaggedObject :: SumEncoding
Default TaggedObject
SumEncoding
options:
defaultTaggedObject =TaggedObject
{tagFieldName
= "tag" ,contentsFieldName
= "contents" }