flags implementation and test

config.go

@@ -23,6 +23,7 @@ package config
 
 import (
 	"bufio"
+	"flag"
 	"fmt"
 	"os"
 	"reflect"
@@ -80,6 +81,23 @@ func (c *Builder) From(file string) *Builder {
 	return c
 }
 
+// FromFlags returns a new Builder, populated with the values from set flags.
+// Flags must be parsed before calling this function.
+func FromFlags() *Builder {
+	return newBuilder().FromFlags()
+}
+
+// FromFlags merges new values from set flag into the current config state, returning the Builder.
+// Flags must be parsed before calling this function.
+func (c *Builder) FromFlags() *Builder {
+	m := make(map[string]string)
+	flag.Visit(func(f *flag.Flag) {
+		m[strings.ToLower(f.Name)] = f.Value.String()
+	})
+	c.mergeConfig(m)
+	return c
+}
+
 // FromEnv returns a new Builder, populated with environment variables
 func FromEnv() *Builder {
 	return newBuilder().FromEnv()

example_test.go

@@ -1,6 +1,7 @@
 package config_test
 
 import (
+	"flag"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -66,3 +67,27 @@ func Example_fromFileWithOverride() {
 	// 5678
 	// true
 }
+
+func Example_flags() {
+	flag.String("databaseUrl", "", "")
+	flag.Int("port", 0, "")
+	// for slices, set the flag's type to the slices type
+	flag.String("subconfig__ipwhitelist", "", "")
+
+	flag.Set("databaseUrl", "db://")
+	flag.Set("port", "5678")
+	flag.Set("subconfig__ipwhitelist", "0.0.0.0 1.1.1.1 2.2.2.2")
+	flag.Parse()
+
+	var c MyConfig
+	config.FromFlags().To(&c)
+
+	fmt.Println(c.DatabaseURL)
+	fmt.Println(c.Port)
+	fmt.Println(c.SubConfig.IPWhitelist, len(c.SubConfig.IPWhitelist))
+
+	// Output:
+	// db://
+	// 5678
+	// [0.0.0.0 1.1.1.1 2.2.2.2] 3
+}