Safe Haskell | None |
---|---|
Language | Haskell2010 |
Deriving.Aeson.Stock
Contents
Synopsis
- type Prefixed str = CustomJSON '[FieldLabelModifier (StripPrefix str)]
- type PrefixedSnake str = CustomJSON '[FieldLabelModifier (StripPrefix str, CamelToSnake)]
- type Snake = CustomJSON '[FieldLabelModifier CamelToSnake]
- type Vanilla = CustomJSON '[]
- newtype CustomJSON t a = CustomJSON {
- unCustomJSON :: a
- class FromJSON a
- class ToJSON a
- class Generic a
Documentation
type Prefixed str = CustomJSON '[FieldLabelModifier (StripPrefix str)] Source #
Field names are prefixed by str
; strip them from JSON representation
type PrefixedSnake str = CustomJSON '[FieldLabelModifier (StripPrefix str, CamelToSnake)] Source #
Strip str
prefices and convert from CamelCase to snake_case
type Snake = CustomJSON '[FieldLabelModifier CamelToSnake] Source #
Convert from CamelCase to snake_case
type Vanilla = CustomJSON '[] Source #
No customisation
Reexports
newtype CustomJSON t a Source #
A newtype wrapper which gives FromJSON/ToJSON instances with modified options.
Constructors
CustomJSON | |
Fields
|
Instances
(AesonOptions t, Generic a, GToJSON Zero (Rep a), GToEncoding Zero (Rep a)) => ToJSON (CustomJSON t a) Source # | |
Defined in Deriving.Aeson Methods toJSON :: CustomJSON t a -> Value # toEncoding :: CustomJSON t a -> Encoding # toJSONList :: [CustomJSON t a] -> Value # toEncodingList :: [CustomJSON t a] -> Encoding # | |
(AesonOptions t, Generic a, GFromJSON Zero (Rep a)) => FromJSON (CustomJSON t a) Source # | |
Defined in Deriving.Aeson Methods parseJSON :: Value -> Parser (CustomJSON t a) # parseJSONList :: Value -> Parser [CustomJSON t a] # |
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
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
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.)
Instances
Representable types of kind *
.
This class is derivable in GHC with the DeriveGeneric
flag on.
A Generic
instance must satisfy the following laws:
from
.to
≡id
to
.from
≡id
Instances
Generic Bool | Since: base-4.6.0.0 |
Generic Ordering | Since: base-4.6.0.0 |
Generic () | Since: base-4.6.0.0 |
Generic Value | |
Generic ExitCode | |
Generic Fixity | Since: base-4.7.0.0 |
Generic Associativity | Since: base-4.7.0.0 |
Defined in GHC.Generics Associated Types type Rep Associativity :: Type -> Type # | |
Generic SourceUnpackedness | Since: base-4.9.0.0 |
Defined in GHC.Generics Associated Types type Rep SourceUnpackedness :: Type -> Type # Methods from :: SourceUnpackedness -> Rep SourceUnpackedness x # to :: Rep SourceUnpackedness x -> SourceUnpackedness # | |
Generic SourceStrictness | Since: base-4.9.0.0 |
Defined in GHC.Generics Associated Types type Rep SourceStrictness :: Type -> Type # Methods from :: SourceStrictness -> Rep SourceStrictness x # to :: Rep SourceStrictness x -> SourceStrictness # | |
Generic DecidedStrictness | Since: base-4.9.0.0 |
Defined in GHC.Generics Associated Types type Rep DecidedStrictness :: Type -> Type # Methods from :: DecidedStrictness -> Rep DecidedStrictness x # to :: Rep DecidedStrictness x -> DecidedStrictness # | |
Generic [a] | Since: base-4.6.0.0 |
Generic (Maybe a) | Since: base-4.6.0.0 |
Generic (Par1 p) | Since: base-4.7.0.0 |
Generic (Down a) | Since: base-4.12.0.0 |
Generic (NonEmpty a) | Since: base-4.6.0.0 |
Generic (Either a b) | Since: base-4.6.0.0 |
Generic (V1 p) | Since: base-4.9.0.0 |
Generic (U1 p) | Since: base-4.7.0.0 |
Generic (a, b) | Since: base-4.6.0.0 |
Generic (Proxy t) | Since: base-4.6.0.0 |
Generic (Rec1 f p) | Since: base-4.7.0.0 |
Generic (URec (Ptr ()) p) | Since: base-4.9.0.0 |
Generic (URec Char p) | Since: base-4.9.0.0 |
Generic (URec Double p) | Since: base-4.9.0.0 |
Generic (URec Float p) | |
Generic (URec Int p) | Since: base-4.9.0.0 |
Generic (URec Word p) | Since: base-4.9.0.0 |
Generic (a, b, c) | Since: base-4.6.0.0 |
Generic (K1 i c p) | Since: base-4.7.0.0 |
Generic ((f :+: g) p) | Since: base-4.7.0.0 |
Generic ((f :*: g) p) | Since: base-4.7.0.0 |
Generic (a, b, c, d) | Since: base-4.6.0.0 |
Generic (M1 i c f p) | Since: base-4.7.0.0 |
Generic ((f :.: g) p) | Since: base-4.7.0.0 |
Generic (a, b, c, d, e) | Since: base-4.6.0.0 |
Generic (a, b, c, d, e, f) | Since: base-4.6.0.0 |
Generic (a, b, c, d, e, f, g) | Since: base-4.6.0.0 |