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 the correct options if you are generically deriving your To/FromJSON instances:

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

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

Note that omitNothingFields affects ToJSON, and allowOmittedFields affects FromJSON. You can, of course, also set both to 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).