Copyright | Gautier DI FOLCO |
---|---|
License | BSD2 |
Maintainer | Gautier DI FOLCO <gautier.difolco@gmail.com> |
Stability | Unstable |
Portability | GHC |
Safe Haskell | None |
Language | Haskell2010 |
aeson instances for Sized
Synopsis
- class FromJSON a where
- class ToJSON a where
- toJSON :: a -> Value
- toEncoding :: a -> Encoding
- toJSONList :: [a] -> Value
- toEncodingList :: [a] -> Encoding
- class FromJSONKey a where
- class ToJSONKey a where
- toJSONKey :: ToJSONKeyFunction a
- toJSONKeyList :: ToJSONKeyFunction [a]
Documentation
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
or using the DerivingVia extension
deriving viaGenerically
Coord 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
Nothing
Instances
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
or more conveniently using the DerivingVia extension
deriving viaGenerically
Coord instanceToJSON
Coord
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 oftoJSON
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.)
Nothing
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.
instanceToJSON
Coord wheretoEncoding
=genericToEncoding
defaultOptions
toJSONList :: [a] -> Value #
toEncodingList :: [a] -> Encoding #
Instances
class FromJSONKey a where #
Read the docs for ToJSONKey
first. This class is a conversion
in the opposite direction. If you have a newtype wrapper around Text
,
the recommended way to define instances is with generalized newtype deriving:
newtype SomeId = SomeId { getSomeId :: Text } deriving (Eq,Ord,Hashable,FromJSONKey)
If you have a sum of nullary constructors, you may use the generic implementation:
data Color = Red | Green | Blue deriving Generic instanceFromJSONKey
Color wherefromJSONKey
=genericFromJSONKey
defaultJSONKeyOptions
Nothing
fromJSONKey :: FromJSONKeyFunction a #
Strategy for parsing the key of a map-like container.
fromJSONKeyList :: FromJSONKeyFunction [a] #
Instances
Typeclass for types that can be used as the key of a map-like container
(like Map
or HashMap
). For example, since Text
has a ToJSONKey
instance and Char
has a ToJSON
instance, we can encode a value of
type Map
Text
Char
:
>>>
LBC8.putStrLn $ encode $ Map.fromList [("foo" :: Text, 'a')]
{"foo":"a"}
Since Int
also has a ToJSONKey
instance, we can similarly write:
>>>
LBC8.putStrLn $ encode $ Map.fromList [(5 :: Int, 'a')]
{"5":"a"}
JSON documents only accept strings as object keys. For any type
from base
that has a natural textual representation, it can be
expected that its ToJSONKey
instance will choose that representation.
For data types that lack a natural textual representation, an alternative is provided. The map-like container is represented as a JSON array instead of a JSON object. Each value in the array is an array with exactly two values. The first is the key and the second is the value.
For example, values of type '[Text]' cannot be encoded to a
string, so a Map
with keys of type '[Text]' is encoded as follows:
>>>
LBC8.putStrLn $ encode $ Map.fromList [(["foo","bar","baz" :: Text], 'a')]
[[["foo","bar","baz"],"a"]]
The default implementation of ToJSONKey
chooses this method of
encoding a key, using the ToJSON
instance of the type.
To use your own data type as the key in a map, all that is needed
is to write a ToJSONKey
(and possibly a FromJSONKey
) instance
for it. If the type cannot be trivially converted to and from Text
,
it is recommended that ToJSONKeyValue
is used. Since the default
implementations of the typeclass methods can build this from a
ToJSON
instance, there is nothing that needs to be written:
data Foo = Foo { fooAge :: Int, fooName :: Text } deriving (Eq,Ord,Generic) instance ToJSON Foo instance ToJSONKey Foo
That's it. We can now write:
>>>
let m = Map.fromList [(Foo 4 "bar",'a'),(Foo 6 "arg",'b')]
>>>
LBC8.putStrLn $ encode m
[[{"fooName":"bar","fooAge":4},"a"],[{"fooName":"arg","fooAge":6},"b"]]
The next case to consider is if we have a type that is a
newtype wrapper around Text
. The recommended approach is to use
generalized newtype deriving:
newtype RecordId = RecordId { getRecordId :: Text } deriving (Eq,Ord,ToJSONKey)
Then we may write:
>>>
LBC8.putStrLn $ encode $ Map.fromList [(RecordId "abc",'a')]
{"abc":"a"}
Simple sum types are a final case worth considering. Suppose we have:
data Color = Red | Green | Blue deriving (Show,Read,Eq,Ord)
It is possible to get the ToJSONKey
instance for free as we did
with Foo
. However, in this case, we have a natural way to go to
and from Text
that does not require any escape sequences. So
ToJSONKeyText
can be used instead of ToJSONKeyValue
to encode maps
as objects instead of arrays of pairs. This instance may be
implemented using generics as follows:
instanceToJSONKey
Color wheretoJSONKey
=genericToJSONKey
defaultJSONKeyOptions
Low-level implementations
The Show
instance can be used to help write ToJSONKey
:
instance ToJSONKey Color where toJSONKey = ToJSONKeyText f g where f = Text.pack . show g = text . Text.pack . show -- text function is from Data.Aeson.Encoding
The situation of needing to turning function a -> Text
into
a ToJSONKeyFunction
is common enough that a special combinator
is provided for it. The above instance can be rewritten as:
instance ToJSONKey Color where toJSONKey = toJSONKeyText (Text.pack . show)
The performance of the above instance can be improved by
not using String
as an intermediate step when converting to
Text
. One option for improving performance would be to use
template haskell machinery from the text-show
package. However,
even with the approach, the Encoding
(a wrapper around a bytestring
builder) is generated by encoding the Text
to a ByteString
,
an intermediate step that could be avoided. The fastest possible
implementation would be:
-- Assuming that OverloadedStrings is enabled instance ToJSONKey Color where toJSONKey = ToJSONKeyText f g where f x = case x of {Red -> "Red";Green ->"Green";Blue -> "Blue"} g x = case x of {Red -> text "Red";Green -> text "Green";Blue -> text "Blue"} -- text function is from Data.Aeson.Encoding
This works because GHC can lift the encoded values out of the case statements, which means that they are only evaluated once. This approach should only be used when there is a serious need to maximize performance.
Nothing
toJSONKey :: ToJSONKeyFunction a #
Strategy for rendering the key for a map-like container.
toJSONKeyList :: ToJSONKeyFunction [a] #
Instances
Orphan instances
ToJSON a => ToJSON (Sized s a) Source # | |
ToJSONKey a => ToJSONKey (Sized s a) Source # | |
toJSONKey :: ToJSONKeyFunction (Sized s a) # toJSONKeyList :: ToJSONKeyFunction [Sized s a] # | |
(FromJSON a, Semigroup a, Size s, SizedSingleton a, SizedFromContainer a, FromJSON (SizedSingletonElement a)) => FromJSON (Sized s a) Source # | |
(FromJSON a, Size s, SizedSingleton a, SizedFromContainer a, FromJSON (SizedSingletonElement a), FromJSONKey (SizedSingletonElement a), FromJSONKey a, Semigroup a) => FromJSONKey (Sized s a) Source # | |
fromJSONKey :: FromJSONKeyFunction (Sized s a) # fromJSONKeyList :: FromJSONKeyFunction [Sized s a] # |