add_multi_constructor equivalent / unknown tag handler

[akdor1154]

In PyYAML, I can use the add_multi_constructor feature to allow me to define a constructor for multiple tags. I am using it to handle any unknown tags. For example, I could use this to deserialize the document

thing1: !someTag asdf
thing2: !someOtherTag fdsa

into the JS structure

{
  thing1: UnknownTag { tag: 'someTag', value: 'asdf' },
  thing2: UnknownTag { tag: 'someOtherTag', value: 'fdsa' }
}

and be able to re-serialize that structure back into equivalent yaml.

My use case is to read and modify a tiny part of a big yaml file that has a bunch of domain-specific tags in it. I don't care how the tags are resolved as I'm not reading that part of the file, but currently with js-yaml I can't read it at all because to parse the file in the first place results in an unknown tag error. Even if I set it to skip unknown tags, I can't re-serialize the modified docjument.

This is fine with PyYAML as I can use add_multi_constructor as defined above to handle any unknown tags, but as far as I can tell this behvaiour is impossible to achieve with js-yaml.

[Thom1729]

This would be extremely useful to me. I'm processing AWS CloudFormation templates with js-yaml. CloudFormation uses a number of tags as macros, e.g. !Ref. These tags need to roundtrip when I load and dump the file.

Currently, in order to do that, I have to hard-code a list of tags used by AWS and define a type for each tag, for each kind of node (because some AWS tags are used for more than one kind of node). This is a pain, it will break if AWS starts supporting additional tags, and it makes otherwise-simple processing code harder to understand. This feature request would drastically simplify this task.

[tomodutch]

I'm interested in this feature as well. Currently working on deserializing various yaml scripts we get from customers. They're predictable but we could encounter an unknown tag

[rlidwka]

Added in 73ef02c (currently in dev branch).

See an example how to do it here:
https://github.com/nodeca/js-yaml/blob/a0d0caa5aa0f5354fefa9c637cfb7c4c17ef7d02/examples/handle_unknown_types.js