@@ -581,41 +581,53 @@ data Options = Options
581581 -- the constructor tag. If 'False' the encoding will always
582582 -- follow the `sumEncoding`.
583583 , omitNothingFields :: Bool
584− -- ^ If 'True' record fields with a 'Nothing' value will be
585− -- omitted from the resulting object. If 'False' the resulting
584+ -- ^ If 'True', record fields with a 'Nothing' value will be
585+ -- omitted from the resulting object. If 'False', the resulting
586586 -- object will include those fields mapping to @null@.
587587 --
588+ -- Note that this /does not/ affect parsing: 'Maybe' fields are
589+ -- optional regardless of the value of 'omitNothingFields', subject
590+ -- to the note below.
591+ --
588592 -- === Note
589593 --
590594 -- Setting 'omitNothingFields' to 'True' only affects fields which are of
591− -- type 'Maybe' /uniformly/ in the 'ToJSON' or 'FromJSON' instance. In
592− -- particular, if the type of a field is declared as a type variable, it
595+ -- type 'Maybe' /uniformly/ in the 'ToJSON' instance.
596+ -- In particular, if the type of a field is declared as a type variable, it
593597 -- will not be omitted from the JSON object, unless the field is
594598 -- specialized upfront in the instance.
595599 --
600+ -- The same holds for 'Maybe' fields being optional in the 'FromJSON' instance.
601+ --
596602 -- ==== __Example__
597603 --
598604 -- The generic instance for the following type @Fruit@ depends on whether
599605 -- the instance head is @Fruit a@ or @Fruit (Maybe a)@.
600606 --
601607 -- @
602− -- data Fruit a =
608+ -- data Fruit a = Fruit
603609 -- { apples :: a -- A field whose type is a type variable.
604610 -- , oranges :: 'Maybe' Int
605− -- }
606− --
607− -- options :: 'Options'
608− -- options = 'defaultOptions' { 'omitNothingFields' = 'True' }
611+ -- } deriving 'Generic'
609612 --
610613 -- -- apples required, oranges optional
611614 -- -- Even if 'Data.Aeson.fromJSON' is then specialized to (Fruit ('Maybe' a)).
612− -- instance 'Data.Aeson.FromJSON' a => 'Data.Aeson.FromJSON' (Fruit a) where
613− -- 'Data.Aeson.fromJSON' = 'Data.Aeson.genericFromJSON' options
615+ -- instance 'Data.Aeson.FromJSON' a => 'Data.Aeson.FromJSON' (Fruit a)
614616 --
615617 -- -- apples optional, oranges optional
616618 -- -- In this instance, the field apples is uniformly of type ('Maybe' a).
617− -- instance 'Data.Aeson.FromJSON' a => 'Data.Aeson.FromJSON' (Fruit ('Maybe' a)) where
618− -- 'Data.Aeson.fromJSON' = 'Data.Aeson.genericFromJSON' options
619+ -- instance 'Data.Aeson.FromJSON' a => 'Data.Aeson.FromJSON' (Fruit ('Maybe' a))
620+ --
621+ -- options :: 'Options'
622+ -- options = 'defaultOptions' { 'omitNothingFields' = 'True' }
623+ --
624+ -- -- apples always present in the output, oranges is omitted if 'Nothing'
625+ -- instance 'Data.Aeson.ToJSON' a => 'Data.Aeson.ToJSON' (Fruit a) where
626+ -- 'Data.Aeson.toJSON' = 'Data.Aeson.genericToJSON' options
627+ --
628+ -- -- both apples and oranges are omitted if 'Nothing'
629+ -- instance 'Data.Aeson.ToJSON' a => 'Data.Aeson.ToJSON' (Fruit ('Maybe' a)) where
630+ -- 'Data.Aeson.toJSON' = 'Data.Aeson.genericToJSON' options
619631 -- @
620632 , sumEncoding :: SumEncoding
621633 -- ^ Specifies how to encode constructors of a sum datatype.