Cannot unmarshal YAML that expands multiple aliases in a single map

[nmiyake]

yaml.v3 returns an error when trying to unmarshal valid YAML that expands multiple aliases in a single map:

letters: &letters
  a: "A"

numbers: &numbers
  one: "1"

combined:
  <<: *letters
  <<: *numbers

This results in the error line 10: mapping key "<<" already defined at line 9 -- it appears that yaml.v3 is interpreting "<<" as the name of the map key rather than as an expansion, and thus fails when multiple of them occur.

This is handled properly by yaml.v2.

Test that displays the behavior (and shows that it works in v2):

package example

import (
	"fmt"
	"testing"

	yamlv2 "gopkg.in/yaml.v2"
	"gopkg.in/yaml.v3"
)

func TestUnmarshal(t *testing.T) {
	in := `
letters: &letters
  a: "A"

numbers: &numbers
  one: "1"

combined:
  <<: *letters
  <<: *numbers
  key: value
`

	var obj interface{}
	err := yamlv2.UnmarshalStrict([]byte(in), &obj)
	if err != nil {
		panic(fmt.Sprintf("yaml.v2 unmarshal failed: %v", err))
	}
	fmt.Printf("yaml.v2 output: %#v\n", obj)

	obj = nil
	err = yaml.Unmarshal([]byte(in), &obj)
	if err != nil {
		panic(fmt.Sprintf("yaml.v3 unmarshal failed: %v", err))
	}
	fmt.Printf("yaml.v3 output: %#v\n", obj)
}

Output:

=== RUN   TestUnmarshal
yaml.v2 output: map[interface {}]interface {}{"combined":map[interface {}]interface {}{"a":"A", "key":"value", "one":"1"}, "letters":map[interface {}]interface {}{"a":"A"}, "numbers":map[interface {}]interface {}{"one":"1"}}
--- FAIL: TestUnmarshal (0.00s)
panic: yaml.v3 unmarshal failed: yaml: unmarshal errors:
  line 10: mapping key "<<" already defined at line 9 [recovered]
	panic: yaml.v3 unmarshal failed: yaml: unmarshal errors:
  line 10: mapping key "<<" already defined at line 9

[perlpunk]

The official way to do this is:

combined:
  <<: [ *letters, *numbers ]

instead of repeating the << merge key.

[6543]

my lib does support both: https://codeberg.org/6543/xyaml
and it's with in the yaml spec see answer

just in case anybody is interested ;)

[perlpunk]

@6543 I think this issue here has nothing to do with what you were mentioning in that yaml issue. This is simply about two ways of merging more than one mapping, not sequences.

[6543]

uh sorry missed that ... it's about v2 VS v3 ...

well I guess #624 (comment) should have closed that ?