Parse YAML dual type field

go.mod

module lboulard.net/go/playfield/flexible-yaml

require gopkg.in/yaml.v2 v2.2.1

go.sum

gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
gopkg.in/yaml.v2 v2.2.1 h1:mUhvW9EsL+naU5Q3cakzfE91YhliOondGd6ZrsDBHQE=
gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

parse.go

package main

import (
	"fmt"
	"log"

	yaml "gopkg.in/yaml.v2"
)

const FILE = "t.yaml"

var test1 = `
- target: Single Target
- target:
  - Target 1
  - Target 2
- ignored: invalid key will appears empty (bug ?)
`

type TargetList []string

func (t *TargetList) UnmarshalYAML(unmarshall func(interface{}) error) error {
	var target interface{}
	if err := unmarshall(&target); err != nil {
		return err
	}
	switch v := target.(type) {
	case string: // value is a single string
		*t = append(*t, v)
	case []interface{}: // value is a list
		// We want a list of string. Nothing else.
		for _, w := range v {
			if s, ok := w.(string); ok {
				*t = append(*t, s)
			} else {
				return fmt.Errorf("cannot unmarshal %q to string", w)
			}
		}
	default:
		return fmt.Errorf("cannot unmarshal %q to string or list of strings", v)
	}
	return nil
}

type T []struct {
	Target TargetList
}

func main() {
	t := T{}
	err := yaml.Unmarshal([]byte(test1), &t)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("--- t:\n%+v\n", t)
}