| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
ByOtherNames.Aeson
Description
A Rubric for JSON serialization using Aeson, along with some helper
newtypes and re-exports.
Required extensions:
- DataKinds
- DeriveGeneric
- DerivingVia
- FlexibleInstances
- MultiParamTypeClasses
- OverloadedStrings
- TypeApplications
- ScopedTypeVariables
Example of use for a record type:
>>>:{data Foo = Foo {aa :: Int, bb :: Bool, cc :: Char} deriving (Read, Show, Eq, Generic) deriving (FromJSON, ToJSON) via (JSONRecord "obj" Foo) instance Aliased JSON Foo where aliases = aliasListBegin $ alias @"aa" "aax" $ alias @"bb" "bbx" $ alias @"cc" "ccx" $ aliasListEnd :}
Example of use for a sum type:
>>>:{data Summy = Aa Int | Bb Bool | Cc deriving (Read, Show, Eq, Generic) deriving (FromJSON, ToJSON) via (JSONSum "sum" Summy) instance Aliased JSON Summy where aliases = aliasListBegin $ alias @"Aa" "Aax" $ alias @"Bb" "Bbx" $ alias @"Cc" "Ccx" $ aliasListEnd :}
Some limitations:
- Fields in branches of sum types can't have selectors. When there is more than one field in a branch, they are parsed as a JSON Array.
- For sum types, only the "object with a single key consisting in the branch tag" style of serialization is supported.
Synopsis
- data JSONRubric = JSON
- newtype JSONRecord objectName r = JSONRecord r
- newtype JSONSum objectName r = JSONSum r
- class (Rubric k, Generic r) => Aliased k r where
- aliasListBegin :: forall before a tree. AliasTree before tree '[] => AliasList before a -> Aliases tree a
- alias :: forall name a names. a -> AliasList names a -> AliasList (name ': names) a
- aliasListEnd :: AliasList '[] a
- class FromJSON a
- class ToJSON a
JSON helpers
data JSONRubric Source #
Aliases for JSON serialization fall under this Rubric.
The constructor JSON is used as a type, with DataKinds.
Constructors
| JSON |
newtype JSONRecord objectName r Source #
Helper newtype for deriving FromJSON and ToJSON for record types,
using DerivingVia.
The objectName type parameter of kind Symbol is used in parse error messages.
Constructors
| JSONRecord r |
Instances
| (Aliased 'JSON r, GRecord ToJSON (Rep r)) => ToJSON (JSONRecord objectName r) Source # | |
Defined in ByOtherNames.Aeson Methods toJSON :: JSONRecord objectName r -> Value # toEncoding :: JSONRecord objectName r -> Encoding # toJSONList :: [JSONRecord objectName r] -> Value # toEncodingList :: [JSONRecord objectName r] -> Encoding # | |
| (KnownSymbol objectName, Aliased 'JSON r, GRecord FromJSON (Rep r)) => FromJSON (JSONRecord objectName r) Source # | |
Defined in ByOtherNames.Aeson Methods parseJSON :: Value -> Parser (JSONRecord objectName r) # parseJSONList :: Value -> Parser [JSONRecord objectName r] # | |
newtype JSONSum objectName r Source #
Helper newtype for deriving FromJSON and ToJSON for sum types,
using DerivingVia.
The Symbol type parameter is used in parse error messages.
Constructors
| JSONSum r |
Re-exports from ByOtherNames
class (Rubric k, Generic r) => Aliased k r where Source #
Typeclass for datatypes r that have aliases for some Rubric k.
aliasListBegin :: forall before a tree. AliasTree before tree '[] => AliasList before a -> Aliases tree a Source #
Define the aliases for a type by listing them.
See also alias and aliasListEnd.
alias :: forall name a names. a -> AliasList names a -> AliasList (name ': names) a Source #
Add an alias to an AliasList.
TYPE APPLICATION REQUIRED! You must provide the field/branch name using a type application.
aliasListEnd :: AliasList '[] a Source #
The empty AliasList.
Re-exports from Data.Aeson
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 GenericFromJSON 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
or using the DerivingVia extension
deriving viaGenericallyCoord instanceFromJSONCoord
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 GenericToJSON 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
or more conveniently using the DerivingVia extension
deriving viaGenericallyCoord instanceToJSONCoord
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:
toEncodingis more efficient for the common case that the output oftoJSONis 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.)