Add Support for Untagged Unwrapped Single Cases

[daniellittledev]

To be able to serialise into a typical (untagged) typescript union I propose adding support for a JsonUnionEncoding.Untagged ||| JsonUnionEncoding.UnwrapSingleFieldCases combination that support untagged and unwrapped cases.

Types such as the one below are quite common, and adding a tag is unnecessary and unwanted in this case.

type Value = number | number[]

The equivalent type in FSharp would be as follows.

type Value = Choice<decimal, decimal list> 

If we support primitive types with the constraint of one unique type per case we can also support deserializing without ambiguity.

Looking at the type type ChoiceOf5 = Choice<int, string, bool, int list, Object> we can deserialize the type knowing only the JsonTokenType. So the following type can be round trip-able without any tags.

Choice1Of5 1 = "1"
Choice2Of5 "F#" = "\"F#\""
Choice3Of5 false = "false"
Choice4Of5 [1,2] = "[1,2]"
Choice5Of5 { name = "Object" } = "{name:\"Object\"}"

Duplicate types could fail or attempt to pick the first matching type.

[ssiltanen]

Is there any reason this feature is not acknowledged or does it just need help to implement? I see there already is an old PR for this also that has been ignored. Is there some issue with the approach it is taking?

Although, since there is already a feature to unwrap untagged object, I think this feature should only target single non-object single values (Not sure how arrays behave with the other existing options).

E.g. a use case from the language server protocol definitions:

type A = Enabled of bool | Options of Object
type B = { a: A }
let b = { a: Enabled true }

b should be allowed to be serialized to

{ "a": true }