Add dynamic defaults and custom conversions

Use a Setter when one value depends on other loaded values. Use a Scanner when Fuda must convert text into your own Go type.

Compute a value with Setter

type Config struct {
    Host    string `yaml:"host" default:"localhost"`
    Port    int    `yaml:"port" default:"8080"`
    BaseURL string `yaml:"base_url"`
}

func (c *Config) SetDefaults() {
    if c.BaseURL == "" {
        c.BaseURL = fmt.Sprintf("http://%s:%d", c.Host, c.Port)
    }
}

Fuda calls SetDefaults() after it processes field tags. The method can use the final Host and Port values.

Convert text with Scanner

type LogLevel int

func (level *LogLevel) Scan(src any) error {
    value, ok := src.(string)
    if !ok {
        return fmt.Errorf("expected string, got %T", src)
    }

    switch value {
    case "debug":
        *level = 0
    case "info":
        *level = 1
    default:
        return fmt.Errorf("unknown log level: %s", value)
    }

    return nil
}

type Config struct {
    LogLevel LogLevel `default:"info"`
}

Fuda calls Scan(any) error when it converts a tag value for the custom field.

Know the order

Fuda processes environment, reference, and default tags before it calls a Setter. It validates the final struct after Setter methods run.

Next step

Review configuration precedence when you combine several features.