Support decoding of undefined record field to Nothing

[jvliwanag]

Is your change request related to a problem? Please describe.

Can we support optional fields to Maybe? In particular, treat undefined values as Nothing.

Both should be decode to { a: Nothing }

• {}
• {"a": null}

Describe the solution you'd like

I propose adding a new

class DecodeJsonField a where 
  decodeJsonField :: Maybe Json -> Maybe (Either JsonDecodeError a)

and using it within the decoding.

I am also of the opinion of encoding Nothing should encode to an undefined field - but that can be a separate matter.

[jvliwanag]

Note that this behavior aligns more with typical JSON based web api's, allowing us to use simple record types for such.