[Duplicate] Add null to Maybe a schemas

[rasheedja]

Duplicate of #91 <-- Use this PR, it's more thorough

Modifies instance ToSchema a => ToSchema (Maybe a) to not ignore the Maybe. Related to #71 .

• Generate a schema for a
• Create a schema with the null type
• Combine both schemas with a oneOf to emulate Maybe a (should we use anyOf instead?)
• Follows behaviour of other Maybe instances, e.g., ToJSON

I went with the oneOf approach rather than using nullable as this is compatible with OpenAPI 3.1.

Without these changes, something like Maybe (String, String) would generate a schema like this:

"application/json;charset=utf-8": {
    "schema": {
        "items": [
            {
                "type": "string"
            },
            {
                "type": "string"
            }
        ],
        "maxItems": 2,
        "minItems": 2,
        "type": "array"
    }
}

And with these changes:

"application/json;charset=utf-8": {
    "schema": {
        "oneOf": [
            {
                "items": [
                    {
                        "type": "string"
                    },
                    {
                        "type": "string"
                    }
                ],
                "maxItems": 2,
                "minItems": 2,
                "type": "array"
            },
            {
                "type": "null"
            }
        ]
    }
}

[stevladimir]

I think this duplicates #91

[rasheedja]

@stevladimir Yep, it does, thanks for pointing that out. I'll switch this one to use anyOf after seeing the issue with nested types.

Your branch is more thorough, though. I'll try out your branch and test that it generates the expected specs for me

EDIT: It does, I'm using your branch for my use case right now.