[Feature proposal] Support for `nullable` in tapir `Schema`

[jnatten]

Hi!

I have a type which acts different when getting null instead of missing field so i would love a way to specify nullable: true in the openapi schema.

I have made a hacky workaround by modifying the schema after the fact in my application, but i would love if there was a way to do it a bit cleaner.

Is there a reason why there isn't a isNullable on the tapir Schema type that can trigger the nullable field in the same way isOptional + the global flag does?

For my use-case that would be perfect. Would a PR be accepted for that change?

[adamw]

Usually differentiating between value present / value null / value absent is quite fragile, hence it wasn't supported in Tapir. Also, I don't think there were requests for such a feature until now.

We could add support for this via an attribute on the schema, but there's still the question on how this would be represented in code - what would be the target deserialised type? It would need to have 3 states, I suppose?

[jnatten]

I saw this issue: #2953 which had some of the same wants.

I didn't really think the deserialized type would need to be a part of tapir itself, I just wanted the flexibility to say this type can be null as well (Mostly because i want to use openapi-ts to generate types for our frontend).

We have a custom type in our application that looks like this that deserializes differently for null and missing:
I guess something similar could be implemented here as well, but in my case I'm not really against just supporting customizing the schema.

sealed trait UpdateOrDelete[+T]
case object Missing                      extends UpdateOrDelete[Nothing]
case object Delete                       extends UpdateOrDelete[Nothing]
final case class UpdateWith[A](value: A) extends UpdateOrDelete[A]

[adamw]

Ah, so I guess an attribute (as e.g. Explode or EncodedDiscriminatorValue) would work? We could definitely add that, if you'd be open to creating a PR :)

[jnatten]

Submitted one now. Not very familiar with attributes or the code in general so let me know if there is a problem 😄