06-struct-tags

Struct Tags Example

This example demonstrates the RECOMMENDED way to handle defaults in confkit - using a DefaultConfig() function instead of struct tags.

Key Points

1. NO "default" tags in structs - Defaults belong in code, not in struct tags
2. Create a DefaultConfig() function - Returns a struct with sensible defaults
3. Use FromDefaults() helper - Converts the struct to a configuration layer
4. Defaults are queryable - Can be used for documentation generation

Benefits of This Approach

• Testable: Your defaults are in a function that can be unit tested
• Type-safe: Compiler ensures your defaults match the struct types
• Discoverable: Defaults are in one place, easy to find and review
• Flexible: Can compute defaults based on environment or other factors
• Documentation: Same defaults used at runtime and for generating docs

The Pattern

// 1. Define your config struct WITHOUT default tags
type Config struct {
    Server struct {
        Host string `mapstructure:"host" description:"Server bind address"`
        Port int    `mapstructure:"port" description:"Server listening port"`
    } `mapstructure:"server"`
}

// 2. Create a function that returns defaults
func DefaultConfig() *Config {
    return &Config{
        Server: struct{...}{
            Host: "localhost",
            Port: 8080,
        },
    }
}

// 3. Use it with confkit
cfg := confkit.NewConfig()
cfg.Add(confkit.FromDefaults("defaults", DefaultConfig()))

Why Not Struct Tags?

Struct tags for defaults are problematic because:

• They mix concerns (structure definition vs runtime behavior)
• They're strings, losing type safety
• They can't be computed dynamically
• They're harder to test
• They duplicate information if you need defaults elsewhere

This example shows how to do it right!