A simple leveled logger

log.go

package foo

import "os"
import "log"

// Level indicates Logger verbosity
type Level int

// LogLevel specifies the minimum level logged
var LogLevel = Debug

// Logger is the common logger
var Logger = log.New(os.Stderr, "[FOO]", log.LstdFlags)

const (
	// Debug is used for diagnostic, developer information
	Debug Level = iota

	// Warn is used for automatic error fixing or potential problems
	Warn

	// Error is used for failures for the current operation, not to the system
	Error

	// Info is for domain-level notifications
	Info
)

// String returns a prefix
func (l Level) String() string {
	switch l {
	case Debug:
		return "[DEBUG]"
	case Warn:
		return "[WARN]"
	case Error:
		return "[ERROR]"
	case Info:
		return "[INFO]"
	default:
		panic("unknown level")
	}
}

// Println appends Level prefix and prints to Logger
func (l Level) Println(v ...interface{}) {
	if l >= LogLevel {
		Logger.Println(l.withPrefix(v)...)
	}
}

// Printf appends Level prefix and prints with format to Logger
func (l Level) Printf(format string, v ...interface{}) {
	if l >= LogLevel {
		Logger.Printf(format, l.withPrefix(v)...)
	}
}

func (l Level) withPrefix(v ...interface{}) []interface{} {
	return append([]interface{}{l.String()}, v...)
}