A Maybe field will be invalid against a null

[parsonsmatt]

I'm getting validation errors against Maybe fields:

>>> :set -XDeriveGeneric
>>> data X = X { foo :: Maybe Int } deriving (Show, Generic)
>>> instance ToSchema X
>>> instance ToJSON X
>>> validateToJSON (X Nothing)
["expected JSON value of type SwaggerInteger"]
>>> encode (X Nothing)
"{\"foo\":null}"

Omitting "foo" from the required fields does not fix this issue.

What's the right way to get maybe fields to validate correctly?

[fizruk]

I personally use omitNothingFields for all my JSON instances, so I don't encounter this problem.

We could alter validateToJSON so that null is considered equivalent to a missing field.
Although I'm not 100% sure that's the right thing to do (I think it might break some code in theory).

A better solution would probably be to add some nullable support (e.g. see #128).

[parsonsmatt]

omitNothingFields is not helping this for me :(

>>> data X = X { foo :: Maybe Int } deriving (Show, Eq, Generic)
>>> instance ToJSON X where toJSON = genericToJSON defaultOptions { omitNothingFields = True }
>>> instance ToSchema X
>>> validateToJSON (X Nothing)
["property \"foo\" is required, but not found in \"{}\""]

[fizruk]

Ah, I see omitNothingFields actually has nothing to do with your problem.

Your problem is because of a weird behaviour feature (bug) of Generic-based ToSchema instance which only happens for single field data types with Maybe! It just so happens that I've never tried that case.

This is how everything works if you just add another field (even without omitNothingFields):

>>> data X = X { foo :: Maybe Int, bar :: Maybe String } deriving (Show, Eq, Generic)
>>> instance ToJSON X
>>> instance ToSchema X
>>> validateToJSON (X Nothing Nothing)
[]

What's causing this

The problem is specifically due to this line:

      recordSchema = gdeclareNamedSchema opts (Proxy :: Proxy (S1 s f)) s

Which calls something with S1 s f and then GHC just picks this instance:

instance OVERLAPPABLE_ (Selector s, GToSchema f) => GToSchema (S1 s f) where ...

over this one:

instance OVERLAPPING_ (Selector s, ToSchema c) => GToSchema (S1 s (K1 i (Maybe c))) where ...

even when there's a Maybe. That's because GHC has to pick an instance before it knows whether it can specialise to Maybe or not.

I think I'm going to send a PR fixing this right now.