The go-yaml library in addition to custom unmarshalers, also allows partial unmarshaling into ast.Node struct members.
This can be useful when working with dynamic documents.

Unfortunately this is not enough when there is a need to have a custom type which needs to be constructed out of ast.Node but you also would like to avoid inner unmarshaling inside TextUnmarshaler.

In my case, this is also useful in order to provide a detailed information about position in returned syntax error.

Use-case demo

Let's assume you've an application which can accept some dynamic value which needs to be validated later.

Application accepts an input something like this:

tasks:
  build:
    - action: foo
      timeout: 10s
      with:
        # Here is a dynamic content
        key: value
        items:
          - 1
          - ${foo.bar.baz} # App supports might want to pre-process some fields
          - true

Most of the content is static except tasks.[*].with which can have different body depending on context.
Currently in order to process that, you would need to unmarshal whole document into a struct below and process ast.Node fields separately.

type Doc struct {
	Tasks map[string]struct{
		Action string
		With ast.Node
	}
}

This PR introduces NodeUnmarshaler and NodeUnmarshalerContext interfaces that allow moving AST traversal into an unmarshaler.

Testing

I've added tests for both new interfaces into yaml_test.go