Add decoding from YamlNode

[dellisd]

This adds a method on YamlInput that allows for decoding a serializable value from a YamlNode.

The use case for this kind of functionality is the ability to manually interpret the structure of a YAML document and then delegate the deserialization of YamlNodes to other serializer implementations.

For example, in the test case I added we have this document:

keyA:
    host: A
keyB:
    host: B

which is a map of Database objects. Perhaps we want to decode this as a List<Database> instead of a Map<String, Database> (because reasons) and this enables us to write a custom serializer that looks at each YamlNode entry on the YamlMap and then delegates the deserialization of each Database object to the Database serializer rather than having to manually decode the rest of the Yaml structure.

[charleskorn]

Thanks for the contribution @dellisd.

I'm wondering if this would be better suited as a method on Yaml instead of on YamlInput, as the method doesn't strictly require a YamlInput instance. I'm imagining a method like decodeFromNode(node, serializer). What do you think?

Also, could you please add a test case for the scenario where the serializer is decoding something that isn't the root of the document? There might be an edge case there that I'd like to check.

[dellisd]

I agree adding a method to Yaml makes sense, but I think the YamlInput class will also need to be modified to expose the current Yaml instance so that custom settings are preserved and new Yaml instances don't need to be allocated during deserialization. This is similar to what is done with the JsonDecoder implementation.

I can make these changes, and I'll add that test case.

[charleskorn]

Overall looks good, just have a couple of questions...

[charleskorn]

Why is this yaml: Yaml property required? Is this so that custom deserializers can access it?

[dellisd]

Yes, that's correct. I explained above that the Json implementations in kotlinx.serialization expose the Json instances in the same way.

[charleskorn]

Thanks @dellisd!