aeson-possible

Three-valued possible types for use with aeson.

Useful for use in PATCH endpoints: use in records which have ToJSON and FromJSON instances.

Inspired by the possible package, but additionally provides To/FromJSON instances using aeson >= 2.2's omitField and omittedField machinery.

Usage

Use Possible a in your records

data MyRecord = MyRecord
    { myBool :: Possible Bool
    , myInt  :: Possible Int
    , myStr  :: Possible Text
    }
    deriving (Generic)

and then make sure to use omitNothingFields = True if you are generically deriving your To/FromJSON instances

instance FromJSON MyRecord where
    parseJSON = genericParseJSON $ defaultOptions{omitNothingFields = True}

instance ToJSON MyRecord where
    toJSON = genericToJSON $ defaultOptions{omitNothingFields = True}

If you are creating instances any other way, see aeson's documentation for how to make use of omitField and omittedField.

Caveat: if you are using Possible outside a record, even a Missing value will likely be encoded as null (e.g. if you have a list of Possible values).