Safe Haskell | None |
---|---|
Language | Haskell2010 |
Prolude.Aeson
Synopsis
- genericToJSON :: (Generic a, GToJSON' Value Zero (Rep a)) => Options -> a -> Value
- class ToJSON a where
- (.=) :: (KeyValue kv, ToJSON v) => Text -> v -> kv
- (.!=) :: Parser (Maybe a) -> a -> Parser a
- (.:?) :: FromJSON a => Object -> Text -> Parser (Maybe a)
- (.:) :: FromJSON a => Object -> Text -> Parser a
- fromJSON :: FromJSON a => Value -> Result a
- genericParseJSON :: (Generic a, GFromJSON Zero (Rep a)) => Options -> Value -> Parser a
- class FromJSON a where
- type JsonOptions = Options
- type JsonValue = Value
- pattern JsonArray :: Vector JsonValue -> JsonValue
- pattern JsonBoolean :: Bool -> JsonValue
- pattern JsonNull :: JsonValue
- pattern JsonNumber :: Scientific -> JsonValue
- pattern JsonObject :: Object -> JsonValue
- pattern JsonString :: Text -> JsonValue
Aeson re-exports
These are our re-exports from Aeson. They include: The FromJSON and ToJSON classes; Generic JSON parsing; (.!=), (.:), (.:?), (.=)
genericToJSON :: (Generic a, GToJSON' Value 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
.
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 writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceToJSON
Coord 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 Generic
ToJSON
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 } derivingGeneric
instanceToJSON
Coord wheretoEncoding
=genericToEncoding
defaultOptions
If on the other hand you wish to customize the generic decoding, you have to implement both methods:
customOptions =defaultOptions
{fieldLabelModifier
=map
toUpper
} instanceToJSON
Coord wheretoJSON
=genericToJSON
customOptionstoEncoding
=genericToEncoding
customOptions
Previous versions of this library only had the toJSON
method. Adding
toEncoding
had two reasons:
- toEncoding is more efficient for the common case that the output of
toJSON
is 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
toEncoding
still compile and have the correct semantics. This is ensured by making the default implementation oftoEncoding
usetoJSON
. 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 Coord
would be sufficient as a generically decoding instance, but there probably exists no good reason to not specifytoEncoding
in new instances.)
Minimal complete definition
Nothing
Instances
(.!=) :: 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"
(.:?) :: 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 if its value is Null
,
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.
(.:) :: 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 :: FromJSON a => Value -> Result a #
Convert a value from JSON, failing if the types do not match.
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
.
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:
fail
yields a custom error message: it is the recommended way of reporting a failure;empty
(ormzero
) is uninformative: use it when the error is meant to be caught by some(
;<|>
)typeMismatch
can be used to report a failure when the encountered value is not of the expected JSON type;unexpected
is an appropriate alternative when more than one type may be expected, or to keep the expected type implicit.
prependFailure
(or modifyFailure
) add more information to a parser's
error messages.
An example type and instance using typeMismatch
and prependFailure
:
-- Allow ourselves to writeText
literals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSON
Coord whereparseJSON
(Object
v) = Coord<$>
v.:
"x"<*>
v.:
"y" -- We do not expect a non-Object
value here. -- We could useempty
to fail, buttypeMismatch
-- gives a much more informative error message.parseJSON
invalid =prependFailure
"parsing Coord failed, " (typeMismatch
"Object" invalid)
For this common case of only being concerned with a single
type of JSON value, the functions withObject
, withScientific
, 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:
instanceFromJSON
Coord 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 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 } derivingGeneric
instanceFromJSON
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
=map
toUpper
} instanceFromJSON
Coord whereparseJSON
=genericParseJSON
customOptions
Minimal complete definition
Nothing
Instances
Alias types
We created type aliases for Aeson.Options and Aeson.Value.
type JsonOptions = Options Source #
Pattern synonyms
We created pattern synonyms for the Aeson.Value constructors.
pattern JsonBoolean :: Bool -> JsonValue Source #
pattern JsonNumber :: Scientific -> JsonValue Source #
pattern JsonObject :: Object -> JsonValue Source #
pattern JsonString :: Text -> JsonValue Source #