flag: add examples

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5867049

src/pkg/flag/example_test.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// These examples demonstrate more intricate uses of the flag package.
+package flag_test
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// Example 1: A single string flag called "species" with default value "gopher".
+var species = flag.String("species", "gopher", "the species we are studying")
+
+// Example 2: Two flags sharing a variable, so we can have a shorthand.
+// The order of initialization is undefined, so make sure both use the
+// same default value. They must be set up with an init function.
+var gopherType string
+
+func init() {
+	const (
+		defaultGopher = "pocket"
+		usage         = "the variety of gopher"
+	)
+	flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
+	flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
+}
+
+// Example 3: A user-defined flag type, a slice of durations.
+type interval []time.Duration
+
+// String is the method to format the flag's value, part of the flag.Value interface.
+// The String method's output will be used in diagnostics.
+func (i *interval) String() string {
+	return fmt.Sprint(*i)
+}
+
+// Set is the method to set the flag value, part of the flag.Value interface.
+// Set's argument is a string to be parsed to set the flag.
+// It's a comma-separated list, so we split it.
+func (i *interval) Set(value string) error {
+	// If we wanted to allow the flag to be set multiple times,
+	// accumulating values, we would delete this if statement.
+	// That would permit usages such as
+	//	-deltaT 10s -deltaT 15s
+	// and other combinations.
+	if len(*i) > 0 {
+		return errors.New("interval flag already set")
+	}
+	for _, dt := range strings.Split(value, ",") {
+		duration, err := time.ParseDuration(dt)
+		if err != nil {
+			return err
+		}
+		*i = append(*i, duration)
+	}
+	return nil
+}
+
+// Define a flag to accumulate durations. Because it has a special type,
+// we need to use the Var function and therefore create the flag during
+// init.
+
+var intervalFlag interval
+
+func init() {
+	// Tie the command-line flag to the intervalFlag variable and
+	// set a usage message.
+	flag.Var(&intervalFlag, "deltaT", "comma-separated list of intervals to use between events")
+}
+
+func Example() {
+	// All the interesting pieces are with the variables declared above, but
+	// to enable the flag package to see the flags defined there, one must
+	// execute, typically at the start of main (not init!):
+	//	flag.Parse()
+	// We don't run it here because this is not a main function and
+	// the testing suite has already parsed the flags.
+}