Copyright | (c) 2011-2016 Bryan O'Sullivan (c) 2011 MailRank, Inc. |
---|---|
License | BSD3 |
Maintainer | Bryan O'Sullivan <bos@serpentine.com> |
Stability | experimental |
Portability | portable |
Safe Haskell | None |
Language | Haskell2010 |
Types for working with JSON data.
- data Value
- data Encoding
- unsafeToEncoding :: Builder -> Encoding
- fromEncoding :: Encoding -> Builder
- data Series
- 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
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- class KeyValue kv where
- modifyFailure :: (String -> String) -> Parser a -> Parser a
- class GFromJSON f where
- gParseJSON :: Options -> Value -> Parser (f a)
- class GToJSON f where
- class GToEncoding f where
- gToEncoding :: Options -> f a -> Encoding
- genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> Value
- genericToEncoding :: (Generic a, GToEncoding (Rep a)) => Options -> a -> Encoding
- 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
- 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
- data Options = Options {}
- data SumEncoding
- camelTo :: Char -> String -> String
- camelTo2 :: Char -> String -> String
- defaultOptions :: Options
- defaultTaggedObject :: SumEncoding
Core JSON types
A JSON value represented as a Haskell value.
An encoding of a JSON value.
unsafeToEncoding :: Builder -> Encoding Source
Make Encoding from Builder.
Use with care! You have to make sure that the passed Builder is a valid JSON Encoding!
fromEncoding :: Encoding -> Builder Source
Acquire the underlying bytestring builder.
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)
The empty array.
The empty object.
Convenience types and functions
newtype DotNetTime Source
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 | |
|
:: 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.
Example usage:
instance FromJSON Coord where parseJSON (Object
v) = {- type matches, life is good -} parseJSON wat =typeMismatch
"Coord" wat
Type conversion
A JSON parser.
The result of running a Parser
.
Monad Result Source | |
Functor Result Source | |
Applicative Result Source | |
Foldable Result Source | |
Traversable Result Source | |
Alternative Result Source | |
MonadPlus Result Source | |
MonadFail Result Source | |
Eq a => Eq (Result a) Source | |
Show a => Show (Result a) Source | |
Monoid (Result a) Source | |
NFData a => NFData (Result a) Source | |
Semigroup (Result a) Source |
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
fromJSON :: FromJSON a => Value -> Result a Source
Convert a value from JSON, failing if the types do not match.
parseEither :: (a -> Parser b) -> a -> Either String b Source
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
Convert a Haskell value to a JSON-friendly intermediate type.
toEncoding :: a -> Encoding Source
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
modifyFailure :: (String -> String) -> Parser a -> Parser a Source
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 Source
Class of generic representation types (Rep
) that can be converted from JSON.
gParseJSON :: Options -> Value -> Parser (f a) Source
This method (applied to defaultOptions
) is used as the
default generic implementation of parseJSON
.
Class of generic representation types (Rep
) that can be converted to
JSON.
class GToEncoding f where Source
gToEncoding :: Options -> f a -> Encoding Source
This method (applied to defaultOptions
) can be used as the
default generic implementation of toEncoding
.
genericToJSON :: (Generic a, GToJSON (Rep a)) => Options -> a -> Value Source
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 (Rep a)) => Options -> a -> Encoding Source
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 (Rep a)) => Options -> Value -> Parser a Source
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 Source
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 Source
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 Source
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 Source
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 Source
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 Source
withBool expected f value
applies f
to the Bool
when value
is a Bool
and fails using
otherwise.typeMismatch
expected
(.:) :: FromJSON a => Object -> Text -> Parser a Source
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.
(.!=) :: Parser (Maybe a) -> a -> Parser a Source
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
Options that specify how to encode/decode your datatype to/from JSON.
Options | |
|
data SumEncoding Source
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 Source
Deprecated: Use camelTo2 for better results
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"
camelTo2 :: Char -> String -> String Source
Better version of camelTo
. Example where it works better:
camelTo '_' 'CamelAPICase' == "camel_apicase" camelTo2 '_' 'CamelAPICase' == "camel_api_case"
defaultOptions :: Options Source
Default encoding Options
:
Options
{fieldLabelModifier
= id ,constructorTagModifier
= id ,allNullaryToStringTag
= True ,omitNothingFields
= False ,sumEncoding
=defaultTaggedObject
,unwrapUnaryRecords
= False }
defaultTaggedObject :: SumEncoding Source
Default TaggedObject
SumEncoding
options:
defaultTaggedObject =TaggedObject
{tagFieldName
= "tag" ,contentsFieldName
= "contents" }