Add option for required fields (#49)

Author: vgough

README.md

@@ -25,6 +25,7 @@ import "github.com/itzg/go-flagsfiller"
 - Allows defaults to be given via struct tag `default`
 - Falls back to using instance field values as declared default
 - Declare flag usage via struct tag `usage`
+- Mark flags as required via struct tag `required` and validate with `Verify()` method (cannot be combined with `default` tag)
 - Can be combined with other modules, such as [google/subcommands](https://github.com/google/subcommands) for sub-command processing. Can also be integrated with [spf13/cobra](https://github.com/spf13/cobra) by using pflag's [AddGoFlagSet](https://godoc.org/github.com/spf13/pflag#FlagSet.AddGoFlagSet)
 - Beyond the standard types supported by flag.FlagSet also includes support for:
     - `[]string` where repetition of the argument appends to the slice and/or an argument value can contain a comma or newline-separated list of values. For example: `--arg one --arg two,three`
@@ -93,6 +94,36 @@ The following shows an example of the usage provided when passing `--help`:
     	How long to wait (default 5s)
 ```
 
+## Required flags
+
+Flags can be marked as required using the `required:"true"` struct tag. After parsing command-line arguments, call the `Verify()` method to ensure all required flags have been provided:
+
+```go
+type Config struct {
+    Host     string `required:"true" usage:"The remote host"`
+    Port     int    `default:"8080" usage:"The port"`
+    Username string `required:"true" usage:"Username for authentication"`
+}
+
+var config Config
+
+filler := flagsfiller.New()
+err := filler.Fill(flag.CommandLine, &config)
+if err != nil {
+    log.Fatal(err)
+}
+
+flag.Parse()
+
+// Verify all required fields are set
+err = filler.Verify()
+if err != nil {
+    log.Fatal(err) // Will fail if Host or Username not provided
+}
+```
+
+**Note:** A field cannot be both required and have a default value. Attempting to use both tags will result in an error during `Fill()`.
+
 ## Real world example
 
 [saml-auth-proxy](https://github.com/itzg/saml-auth-proxy) shows an end-to-end usage of flagsfiller where the main function fills the flags, maps those to environment variables with [envy](https://github.com/jamiealquiza/envy), and parses the command line:

example_test.go

@@ -3,17 +3,18 @@ package flagsfiller_test
 import (
 	"flag"
 	"fmt"
-	"github.com/itzg/go-flagsfiller"
 	"log"
 	"time"
+
+	"github.com/itzg/go-flagsfiller"
 )
 
 func Example() {
 	type Config struct {
 		Host      string        `default:"localhost" usage:"The remote host"`
 		Enabled   bool          `default:"true" usage:"Turn it on"`
 		Automatic bool          `default:"false" usage:"Make it automatic" aliases:"a"`
-		Retries   int          	`default:"1" usage:"Retry" aliases:"r,t"`
+		Retries   int           `default:"1" usage:"Retry" aliases:"r,t"`
 		Timeout   time.Duration `default:"5s" usage:"How long to wait"`
 	}
 
@@ -36,3 +37,38 @@ func Example() {
 	// Output:
 	// {Host:external.svc Enabled:true Automatic:true Retries:2 Timeout:10m0s}
 }
+
+func ExampleFlagSetFiller_Verify() {
+	type Config struct {
+		Host     string `required:"true" usage:"The remote host"`
+		Port     int    `default:"8080" usage:"The port"`
+		Username string `required:"true" usage:"Username for authentication"`
+	}
+
+	var config Config
+
+	flagset := flag.NewFlagSet("ExampleVerify", flag.ContinueOnError)
+
+	filler := flagsfiller.New()
+	err := filler.Fill(flagset, &config)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Parse with required fields provided
+	err = flagset.Parse([]string{"--host", "example.com", "--username", "admin"})
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Verify all required fields are set
+	err = filler.Verify()
+	if err != nil {
+		fmt.Printf("Error: %v\n", err)
+		return
+	}
+
+	fmt.Printf("Config validated: Host=%s, Port=%d, Username=%s\n", config.Host, config.Port, config.Username)
+	// Output:
+	// Config validated: Host=example.com, Port=8080, Username=admin
+}