Error when parsing YAML file on GOLANG when there's a dash before the square brackets list

When parsing a YAML file in GOLANG with gopkg.in/yaml.v3, if the YAML file has a dash before the brackets then it doesn't read it.

I have a YAML file with a syntax similar to this:

servers:

    - server_name: ratamahatta
      server_groups:
          - [r1, r2, r3]

    - server_name: kaiowas
      server_groups:
          - [c1, c2]

If I use the following struct in GO:

package main

import (
	"fmt"
	"io/ioutil"

	"gopkg.in/yaml.v3"
)

type S struct {
	Servers []struct {
		ServerName string   `yaml:"server_name"`
		ServerGroups []string `yaml:"server_groups"`
	} `yaml:"servers"`
}

func main() {
	file, err := ioutil.ReadFile("example2.yaml")

	if err != nil {
		panic(err)
	}

	var s S

	err = yaml.Unmarshal(file, &s)
	if err != nil {
		panic(err)
	}

	
	fmt.Printf("Value: %#v\n", s.Servers[0].ServerGroups[0])
}

I get:

  line 5: cannot unmarshal !!seq into string
  line 9: cannot unmarshal !!seq into string

But, if I remove the "-" before the brackets:

servers:

    - server_name: ratamahatta
      server_groups:
           [r1, r2, r3]

    - server_name: kaiowas
      server_groups:
           [c1, c2]

Then it works perfectly.

If I change this:

ServerGroups []string `yaml:"server_groups"`

To

ServerGroups struct {} `yaml:"server_groups"` 

Then I get

cannot unmarshal !!seq into struct {}

So basically I can't figure out how to read the YAML sequence in this case.

Thanks

The YAML you are processing - [r1, r2, r3] is a Sequence of Sequences meaning there could be additional entries e.g.

- server_name: ratamahatta
      server_groups:
          - [r1, r2, r3]
          - [r4, r5, r6]    

So to parse this you need an extra dimension i.e. [][]string. A working sample follows (playground):

const testYaml = `servers:
    - server_name: ratamahatta
      server_groups:
          - [r1, r2, r3]
          - [r4, r5, r6]
    - server_name: kaiowas
      server_groups:
          - [c1, c2]
`

type S struct {
	Servers []struct {
		ServerName   string     `yaml:"server_name"`
		ServerGroups [][]string `yaml:"server_groups"`
	} `yaml:"servers"`
}

func main() {
	var s S
	if err := yaml.Unmarshal([]byte(testYaml), &s); err != nil {
		panic(err)
	}
	fmt.Printf("Value: %#v\n", s.Servers[0].ServerGroups[0]) // []string{"r1", "r2", "r3"}
	fmt.Printf("Value: %#v\n", s.Servers[0].ServerGroups[1]) // []string{"r4", "r5", "r6"}
	fmt.Printf("Value: %#v\n", s.Servers[1].ServerGroups[0]) // []string{"c1", "c2"}
}