| 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 ('[] :: [ Type ])
-
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 ('[] :: [ Type ]) 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 Source # toEncoding :: CustomJSON t a -> Encoding Source # toJSONList :: [ CustomJSON t a] -> Value Source # toEncodingList :: [ CustomJSON t a] -> Encoding Source # |
|
| ( AesonOptions t, Generic a, GFromJSON Zero ( Rep a)) => FromJSON ( CustomJSON t a) Source # | |
|
Defined in Deriving.Aeson Methods parseJSON :: Value -> Parser ( CustomJSON t a) Source # parseJSONList :: Value -> Parser [ CustomJSON t 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:
-
failyields 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(;<|>) -
typeMismatchcan be used to report a failure when the encountered value is not of the expected JSON type;unexpectedis 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 writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceFromJSONCoord whereparseJSON(Objectv) = Coord<$>v.:"x"<*>v.:"y" -- We do not expect a non-Objectvalue here. -- We could useemptyto fail, buttypeMismatch-- gives a much more informative error message.parseJSONinvalid =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:
instanceFromJSONCoord 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 } deriving Generic
instance FromJSON 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=maptoUpper} instanceFromJSONCoord whereparseJSON=genericParseJSONcustomOptions
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 writeTextliterals. {-# LANGUAGE OverloadedStrings #-} data Coord = Coord { x :: Double, y :: Double } instanceToJSONCoord 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 } deriving Generic
instance ToJSON Coord where
toEncoding = genericToEncoding defaultOptions
If on the other hand you wish to customize the generic decoding, you have to implement both methods:
customOptions =defaultOptions{fieldLabelModifier=maptoUpper} instanceToJSONCoord wheretoJSON=genericToJSONcustomOptionstoEncoding=genericToEncodingcustomOptions
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
toJSONis 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
toEncodingstill compile and have the correct semantics. This is ensured by making the default implementation oftoEncodingusetoJSON. 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 Coordwould be sufficient as a generically decoding instance, but there probably exists no good reason to not specifytoEncodingin 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≡idto.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 Methods from :: Associativity -> Rep Associativity x Source # to :: Rep Associativity x -> Associativity Source # |
|
| Generic SourceUnpackedness |
Since: base-4.9.0.0 |
|
Defined in GHC.Generics Methods from :: SourceUnpackedness -> Rep SourceUnpackedness x Source # to :: Rep SourceUnpackedness x -> SourceUnpackedness Source # |
|
| Generic SourceStrictness |
Since: base-4.9.0.0 |
|
Defined in GHC.Generics Methods from :: SourceStrictness -> Rep SourceStrictness x Source # to :: Rep SourceStrictness x -> SourceStrictness Source # |
|
| Generic DecidedStrictness |
Since: base-4.9.0.0 |
|
Defined in GHC.Generics Methods from :: DecidedStrictness -> Rep DecidedStrictness x Source # to :: Rep DecidedStrictness x -> DecidedStrictness Source # |
|
| 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 |