How to handle missing fields? I don't want them to be populated with default values.

[omefire]

I have a set of yaml files I'm dealing with that follow a particular schema. However, sometimes, the fields are not present in the yaml file. But they do get populated with default values when unmarshalled. i.e: an int field gets populated with value 0.

Is there a way to deal with that? I want to be able to know whether a field was found or not in the yaml file.

[invidian]

@omefire for that, you should make your fields a pointers, so string becomes *string. This way you can do foo == nil instead of foo == "" (Golang zero value for string type). This is the Go way of marking struct fields as optional.

When you want to achieve the same when marshalling, use optional keyword.

[gurza]

@omefire try to use UnmarshalStrict() instead of Unmarshal(). This method is implemented in v2 https://pkg.go.dev/gopkg.in/yaml.v2#UnmarshalStrict. And add omitempty tag for optional fields.

For example

type Config struct {
    Port        int    `yaml:"port"`
    EmailSender string `yaml:"email_sender"`

    // AWS configuration
    AWS struct {
        Region          string  `yaml:"region"`
        AccessKeyID     *string `yaml:"access_key_id,omitempty"`
        SecretAccessKey *string `yaml:"secret_access_key,omitempty"`
    } `yaml:"aws"`
}

EDITED

Sorry, it will not work for you. UnmarshalStrict() check fields of config file, not fields of structure.

[AlexanderMakarov]

    cfg := &Config{}
    decoder := yaml.NewDecoder(strings.NewReader(string(buf)))
    decoder.KnownFields(true) // Disallow unknown fields
    if err = decoder.Decode(cfg); err != nil {
        if err.Error() == "EOF" {
            return nil, fmt.Errorf("can't decode YAML from configuration file '%s': %v", filename, err)
        }
        return nil, err
    }

Works for me in 2024/Go 1.22.2.