| Safe Haskell | Safe-Inferred |
|---|---|
| Language | Haskell2010 |
ByOtherNamesH.Aeson
Description
A Rubric for JSON serialization using Aeson, along with some helper
newtypes and re-exports.
A more versatile version of the functionality provided by ByOtherNames.Aeson, in that it allows you to manually specify parsers/decoders for each field. But, because of that, it's also more verbose. And the error messages are worse.
If you plan to use both ByOtherNames.Aeson and ByOtherNamesH.Aeson, import this module qualified to avoid name collisions:
import qualified ByOthernamesH.Aeson as H
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, dd :: String, ee :: Int} deriving stock (Read, Show, Eq, Generic) deriving (FromJSON, ToJSON) via (JSONRecord "obj" Foo) instance Aliased JSON Foo where aliases = aliasListBegin . alias @"aa" "aax" (singleSlot fromToJSON) . alias @"bb" "bbx" (singleSlot fromToJSON) . alias @"cc" "ccx" (singleSlot fromToJSON) . alias @"dd" "ddx" (singleSlot fromToJSON) . alias @"ee" "eex" (singleSlot fromToJSON) $ aliasListEnd :}
Synopsis
- data JSONRubric = JSON
- newtype JSONRecord objectName r = JSONRecord r
- data FromToJSON v = FromToJSON {
- parseJSON' :: Value -> Parser v
- toJSON' :: v -> Value
- fromToJSON :: (ToJSON v, FromJSON v) => FromToJSON v
- newtype GeneralJSONRecord rubric objectName r = GeneralJSONRecord r
- class (Rubric k, Generic r) => Aliased k r where
- aliases :: Aliases (Rep r) (AliasType k) (WrapperType k)
- aliasListBegin :: forall names_slots a h rep. ToAliases names_slots rep '[] => AliasList names_slots a h -> Aliases rep a h
- alias :: forall name slots a h names_slots. a -> SlotList slots h -> AliasList names_slots a h -> AliasList ('(name, slots) ': names_slots) a h
- aliasListEnd :: AliasList '[] a h
- data SlotList :: [Type] -> (Type -> Type) -> Type
- singleSlot :: h v -> SlotList '[v] h
- slot :: h v -> SlotList rest h -> SlotList (v ': rest) h
- slotListEnd :: SlotList '[] h
- 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 |
Instances
| Rubric 'JSON Source # | The aliases will be of type Data.Aeson.Key. |
| type AliasType 'JSON Source # | |
Defined in ByOtherNamesH.Aeson | |
| type WrapperType 'JSON Source # | |
Defined in ByOtherNamesH.Aeson | |
newtype JSONRecord objectName r Source #
Constructors
| JSONRecord r |
Instances
| (KnownSymbol objectName, Aliased 'JSON r, GRecord (Rep r)) => FromJSON (JSONRecord objectName r) Source # | |
Defined in ByOtherNamesH.Aeson Methods parseJSON :: Value -> Parser (JSONRecord objectName r) # parseJSONList :: Value -> Parser [JSONRecord objectName r] # | |
| (Aliased 'JSON r, GRecord (Rep r)) => ToJSON (JSONRecord objectName r) Source # | |
Defined in ByOtherNamesH.Aeson Methods toJSON :: JSONRecord objectName r -> Value # toEncoding :: JSONRecord objectName r -> Encoding # toJSONList :: [JSONRecord objectName r] -> Value # toEncodingList :: [JSONRecord objectName r] -> Encoding # | |
data FromToJSON v Source #
Packs together a JSON parser and a encoder for some type.
Constructors
| FromToJSON | |
Fields
| |
fromToJSON :: (ToJSON v, FromJSON v) => FromToJSON v Source #
Advanced JSON helpers
newtype GeneralJSONRecord rubric objectName r Source #
A more flexible version of JSONRecord that lets you use any Rubric whose
AliasType is Key and its WrapperType is FromToJSON.
It allows deriving FromJSON and ToJSON for a newtype, using the generic
Rep and the aliases of the underlying type, but without defining
FromJSON and ToJSON instances for the underlying type.
>>>:{data Foo = Foo {aa :: Int, bb :: Bool, cc :: Char} deriving (Read, Show, Eq, Generic) data JSONLocal -- We define a local rubric type to avoid colliding "Aliased" instances over Foo. instance Rubric JSONLocal where type AliasType JSONLocal = Key type WrapperType JSONLocal = FromToJSON instance Aliased JSONLocal Foo where aliases = aliasListBegin $ alias @"aa" "aax" (singleSlot fromToJSON) $ alias @"bb" "bbx" (singleSlot fromToJSON) $ alias @"cc" "ccx" (singleSlot fromToJSON) $ aliasListEnd newtype FooN = FooN Foo deriving (FromJSON, ToJSON) via (GeneralJSONRecord JSONLocal "obj" Foo) :}
Constructors
| GeneralJSONRecord r |
Instances
| (KnownSymbol objectName, Rubric rubric2, Aliased rubric2 r, AliasType rubric2 ~ Key, WrapperType rubric2 ~ FromToJSON, GRecord (Rep r)) => FromJSON (GeneralJSONRecord rubric2 objectName r) Source # | |
Defined in ByOtherNamesH.Aeson Methods parseJSON :: Value -> Parser (GeneralJSONRecord rubric2 objectName r) # parseJSONList :: Value -> Parser [GeneralJSONRecord rubric2 objectName r] # | |
| (Rubric rubric2, Aliased rubric2 r, AliasType rubric2 ~ Key, WrapperType rubric2 ~ FromToJSON, GRecord (Rep r)) => ToJSON (GeneralJSONRecord rubric2 objectName r) Source # | |
Defined in ByOtherNamesH.Aeson Methods toJSON :: GeneralJSONRecord rubric2 objectName r -> Value # toEncoding :: GeneralJSONRecord rubric2 objectName r -> Encoding # toJSONList :: [GeneralJSONRecord rubric2 objectName r] -> Value # toEncodingList :: [GeneralJSONRecord rubric2 objectName r] -> Encoding # | |
Re-exports from ByOtherNames
aliasListEnd :: AliasList '[] a h Source #
The empty AliasList.
data SlotList :: [Type] -> (Type -> Type) -> Type Source #
A list of slots associated an alias. Indexed by the types of each slot and a type constructor that wraps each slot value.
For records, each field alias will have one and only one slot: the
corresponding record field. See singleSlot.
For sum types, each branch alias might have zero or more slots, depending on
the structure of the datatype. See slot and slotListEnd.
singleSlot :: h v -> SlotList '[v] h Source #
slotListEnd :: SlotList '[] h Source #
The empty SlotList.
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.)