Enflag is a zero-dependency generics-based Golang package for configuring applications via environment variables and command-line flags.

go get -u github.com/atelpis/enflag

While many solutions are available for Go configuration, Enflag was created to address a specific gap. Most options fall into two extremes:

• Single-source tools limit flexibility by supporting only environment variables, flags, or a specific file type.
• All-in-one solutions rely on reflection and struct tags, which can lead to hidden runtime errors, require memorizing library-specific syntax, and make debugging more difficult.

Type-safe and balanced: Enflag uses Go's generics for compile-time type safety, avoiding runtime errors. By focusing on environment variables and command-line flags, it covers the majority of use cases with a clean, straightforward API, making configuration predictable and easy to manage.

• Type-safe – Generics-based design, no runtime reflection
• Container-optimized – Unified API for env vars and CLI flags
• Flexible – Handles primitives, slices, JSON, and binary formats
• Extensible – Custom parsers can be added easily
• Error handling – Built-in handlers with custom callback support
• Secure & lightweight – No external dependencies, suitable for security-sensitive applications

The Var function takes a pointer to a configuration variable and assigns its value according to the specified command-line flag or environment variable using the Bind method. Both sources are optional, but a flag has higher priority.

Additional methods like WithDefault, WithTimeLayout, etc. could be chained.

Behind the scenes, flags are handled by the standard library's flag.CommandLine flag set, meaning you get the same help-message output and error handling. Enflag uses generics to provide a cleaner and more convenient interface.

See the full runnable example

type MyServiceConf struct {
    BaseURL *url.URL
    DBHost  string
    Dates  []time.Time
}

func main() {
    var conf MyServiceConf

    // Basic usage
    enflag.Var(&conf.BaseURL).Bind("BASE_URL", "base-url")

    // Simple bindings can be defined using the less verbose BindVar shortcut
    enflag.BindVar(&conf.BaseURL, "BASE_URL", "base-url")

    // Add settings
    enflag.Var(&conf.DBHost).
        WithDefault("127.0.0.1").
        WithFlagUsage("db hostname").
        Bind("DB_HOST", "db-host")

    // Slice
    enflag.Var(&conf.Dates).
        WithSliceSeparator("|").       // Split the slice using a non-default separator
        WithTimeLayout(time.DateOnly). // Use a non-default time layout
        BindEnv("DATES")               // Bind only the env variable, ignore the flag

    enflag.Parse()
}

Enflag supports the most essential data types out of the box like binary, strings, numbers, time, URLs, IP and corresponding slices. You can also use VarFunc with a custom parser to work with other types:

See the full runnable example

func main() {
    var customVar int64

    parser := func(s string) (int64, error) {
        res, err := strconv.ParseInt(s, 10, 64)
        if err != nil {
            return 0, err
        }

        return res * 10, nil
    }
    enflag.VarFunc(&customVar, parser).Bind("CUSTOM", "custom")

    enflag.Parse()
}

While numerous packages handle complex configurations using YAML, TOML, JSON, etc., Enflag focuses strictly on supporting container-oriented applications with a reasonable configuration scope. It prioritizes simplicity and zero dependency, making it a solid choice for microservices and cloud-native deployments. For more complex configuration needs, consider using a dedicated configuration management tool.