Go YAML parsing: mandatory fields - go

Question

Summary:
I need to parse data in YAML format into golang struct. It there a way (library, attributes) to make some of the fields mandatory, i.e. to make Unmarshal function return the error in case if some field doesn't exist?
Example what is wanted:
Unmarshal function in this code should raise an error because input data doesn't contain 'b' field.
package main
import (
"fmt"
"gopkg.in/yaml.v2"
)
type TestStruct struct {
FieldA string `yaml:"a"`
FieldB string `yaml:"b"`
}
func main() {
input := []byte(`{a: 1}`)
var output TestStruct
_ = yaml.Unmarshal(input, &output)
}

Answer 1

You can use this library's HasZero method to check wether there are any missing values in a struct. This will return true or false depending wether the struct is completely filled or not. Please see the playground example to get an idea.
But if you specifically need to tell what field is missing, you need to check wether the value is nil like in the example below.
package main
import (
"fmt"
"errors"
"gopkg.in/yaml.v2"
)
type TestStruct struct {
FieldA string `yaml:"a"`
FieldB string `yaml:"b"`
}
func main() {
input := []byte(`{a: 1}`)
var output TestStruct
if err := output.ParseFromFile(input); err != nil {
fmt.Println(err)
}
fmt.Println(output)
}
func (output *TestStruct) ParseFromFile(data []byte) error {
if err := yaml.Unmarshal(data, output); err != nil {
return err
}
if output.FieldA == "" {
return errors.New("Blank Field A")
}
if output.FieldB == "" {
return errors.New("Blank Field B")
}
return nil
}
Playground example if you need to specifically return an error