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deriving-aeson
====
[](https://hackage.haskell.org/package/deriving-aeson)
[](https://discord.gg/DG93Tgs)
This package provides a newtype wrapper where you can customise
[aeson](https://hackage.haskell.org/package/aeson)'s generic methods using a
type-level interface, which synergises well with DerivingVia.
```haskell
{-# LANGUAGE DerivingVia, DataKinds, DeriveGeneric #-}
import Data.Aeson
import Deriving.Aeson
import qualified Data.ByteString.Lazy.Char8 as BL
data User = User
{ userId :: Int
, userName :: String
, userAPIToken :: Maybe String
} deriving Generic
deriving (FromJSON, ToJSON)
via CustomJSON '[OmitNothingFields, FieldLabelModifier '[StripPrefix "user", CamelToSnake]] User
testData :: [User]
testData = [User 42 "Alice" Nothing, User 43 "Bob" (Just "xyz")]
main = BL.putStrLn $ encode testData
-- [{"name":"Alice","id":42},{"api_token":"xyz","name":"Bob","id":43}]
```
`Deriving.Aeson.Stock` contains some aliases for even less boilerplates.
* `Prefixed str` = `CustomJSON '[FieldLabelModifier (StripPrefix str)]`
* `PrefixedSnake str` = `CustomJSON '[FieldLabelModifier (StripPrefix str, CamelToSnake)]`
* `Suffixed str` = `CustomJSON '[FieldLabelModifier (StripSuffix str)]`
* `SuffixedSnake str` = `CustomJSON '[FieldLabelModifier (StripSuffix str, CamelToSnake)]`
* `Snake` = `CustomJSON '[FieldLabelModifier '[StripPrefix str, CamelToSnake]]`
* `Vanilla` = `CustomJSON '[]`
How it works
----
The wrapper type has a phantom type parameter `t`, a type-level builder of an [Option](http://hackage.haskell.org/package/aeson-1.4.6.0/docs/Data-Aeson.html#t:Options).
Type-level primitives are reduced to one `Option` by the `AesonOptions` class.
```haskell
newtype CustomJSON t a = CustomJSON { unCustomJSON :: a }
class AesonOptions xs where
aesonOptions :: Options
instance AesonOptions xs => AesonOptions (OmitNothingFields ': xs) where
aesonOptions = (aesonOptions @xs) { omitNothingFields = True }
...
```
You can use any (static) function for name modification by adding an instance of `StringModifier`.
```haskell
data ToLower
instance StringModifier ToLower where
getStringModifier "" = ""
getStringModifier (c : xs) = toLower c : xs
```
Previous studies
----
* [Type-driven safe derivation of ToJSON and FromJSON, using DerivingVia in GHC 8.6 and some type-level hacks](https://gist.github.com/konn/27c00f784dd883ec2b90eab8bc84a81d)
* [Strip prefices from JSON representation](https://gist.github.com/fumieval/5c89205d418d5f9cafac801afbe94969)
|
