| Copyright | © Frank Jung 2023 |
|---|---|
| License | GPL-3 |
| Safe Haskell | Safe-Inferred |
| Language | Haskell2010 |
MyJson
Description
Decode JSON which contains special characters like ° (ASCII decimal 176).
See also app/Json.hs for an example of how to parse JSON when the first key is variable. This is common in APIs such as the Alpha Vantage weekly stock prices. In that case, the first key is the date of that weeks summary data.
References
I found this a good guide when dealing with JSON and special characters: https://guide.aelve.com/haskell/aeson-cookbook-amra6lk6
Synopsis
- data MyJson = MyJson {}
- class FromJSON a
- class ToJSON a
- eitherDecodeSpecial :: FromJSON a => ByteString -> Either String a
- encodeSpecial :: ToJSON a => a -> ByteString
Documentation
Define a test data type.
Constructors
| MyJson | |
Instances
| FromJSON MyJson Source # | |
| ToJSON MyJson Source # | |
| Generic MyJson Source # | |
| Show MyJson Source # | |
| Eq MyJson Source # | |
| type Rep MyJson Source # | |
Defined in MyJson type Rep MyJson = D1 ('MetaData "MyJson" "MyJson" "Scrapbook-0.4.0-JoJPK52Rt3D24zdkn36vxS" 'False) (C1 ('MetaCons "MyJson" 'PrefixI 'True) ((S1 ('MetaSel ('Just "name") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 Text) :*: S1 ('MetaSel ('Just "identifier") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 Int)) :*: (S1 ('MetaSel ('Just "modifier") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 Float) :*: (S1 ('MetaSel ('Just "created") 'NoSourceUnpackedness 'SourceStrict 'DecidedStrict) (Rec0 UTCTime) :*: S1 ('MetaSel ('Just "series") 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 [Int]))))) | |
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.)
Instances
eitherDecodeSpecial :: FromJSON a => ByteString -> Either String a Source #
Decode Special Characters.
Where:
Data.Text.Lazy.Encoding.decodeLatin1 :: ByteString -> Text Data.Text.Lazy.Encoding.encodeUtf8 :: Text -> ByteString Data.Aeson.eitherDecode :: ByteString -> Either String a
This will successfully decode a ByteString containing special characters such
as ° (ASCII decimal 176).
encodeSpecial :: ToJSON a => a -> ByteString Source #
Encode Special Characters.
Where:
Data.Aeson.Text.encodeToLazyText :: ToJSON a => a -> Text Data.Text.Lazy.Encoding.encodeUtf8 :: Text -> ByteString