flag: add TextVar function

The TextVar function makes it easier to integrate the flag package
with any Go type that implements encoding.Text{Marshaler,Unmarshaler}.

Fixes #45754

Change-Id: Id23c37d59cf8c9699a7943a22ce27a45eb685c0f
Reviewed-on: https://go-review.googlesource.com/c/go/+/313329
Trust: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 91 insertions, 0 deletions

diff --git a/src/flag/example_textvar_test.go b/src/flag/example_textvar_test.go
new file mode 100644
index 0000000000..8b8cbf6b6c
--- /dev/null
+++ b/src/flag/example_textvar_test.go
@@ -0,0 +1,35 @@
+// Copyright 2022 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.
+
+package flag_test
+
+import (
+	"flag"
+	"fmt"
+	"net"
+	"os"
+)
+
+func ExampleTextVar() {
+	fs := flag.NewFlagSet("ExampleTextVar", flag.ContinueOnError)
+	fs.SetOutput(os.Stdout)
+	var ip net.IP
+	fs.TextVar(&ip, "ip", net.IPv4(192, 168, 0, 100), "`IP address` to parse")
+	fs.Parse([]string{"-ip", "127.0.0.1"})
+	fmt.Printf("{ip: %v}\n\n", ip)
+
+	// 256 is not a valid IPv4 component
+	ip = nil
+	fs.Parse([]string{"-ip", "256.0.0.1"})
+	fmt.Printf("{ip: %v}\n\n", ip)
+
+	// Output:
+	// {ip: 127.0.0.1}
+	//
+	// invalid value "256.0.0.1" for flag -ip: invalid IP address: 256.0.0.1
+	// Usage of ExampleTextVar:
+	//   -ip IP address
+	//     	IP address to parse (default 192.168.0.100)
+	// {ip: <nil>}
+}
diff --git a/src/flag/flag.go b/src/flag/flag.go
index 4e2af450c5..c27a144434 100644
--- a/src/flag/flag.go
+++ b/src/flag/flag.go
@@ -68,6 +68,7 @@
 package flag
 
 import (
+	"encoding"
 	"errors"
 	"fmt"
 	"io"
@@ -278,6 +279,43 @@ func (d *durationValue) Get() any { return time.Duration(*d) }
 
 func (d *durationValue) String() string { return (*time.Duration)(d).String() }
 
+// -- encoding.TextUnmarshaler Value
+type textValue struct{ p encoding.TextUnmarshaler }
+
+func newTextValue(val encoding.TextMarshaler, p encoding.TextUnmarshaler) textValue {
+	ptrVal := reflect.ValueOf(p)
+	if ptrVal.Kind() != reflect.Ptr {
+		panic("variable value type must be a pointer")
+	}
+	defVal := reflect.ValueOf(val)
+	if defVal.Kind() == reflect.Ptr {
+		defVal = defVal.Elem()
+	}
+	if defVal.Type() != ptrVal.Type().Elem() {
+		panic(fmt.Sprintf("default type does not match variable type: %v != %v", defVal.Type(), ptrVal.Type().Elem()))
+	}
+	ptrVal.Elem().Set(defVal)
+	return textValue{p}
+}
+
+func (v textValue) Set(s string) error {
+	return v.p.UnmarshalText([]byte(s))
+}
+
+func (v textValue) Get() interface{} {
+	return v.p
+}
+
+func (v textValue) String() string {
+	if m, ok := v.p.(encoding.TextMarshaler); ok {
+		if b, err := m.MarshalText(); err == nil {
+			return string(b)
+		}
+	}
+	return ""
+}
+
+// -- func Value
 type funcValue func(string) error
 
 func (f funcValue) Set(s string) error { return f(s) }
@@ -838,6 +876,24 @@ func Duration(name string, value time.Duration, usage string) *time.Duration {
 	return CommandLine.Duration(name, value, usage)
 }
 
+// TextVar defines a flag with a specified name, default value, and usage string.
+// The argument p must be a pointer to a variable that will hold the value
+// of the flag, and p must implement encoding.TextUnmarshaler.
+// If the flag is used, the flag value will be passed to p's UnmarshalText method.
+// The type of the default value must be the same as the type of p.
+func (f *FlagSet) TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string) {
+	f.Var(newTextValue(value, p), name, usage)
+}
+
+// TextVar defines a flag with a specified name, default value, and usage string.
+// The argument p must be a pointer to a variable that will hold the value
+// of the flag, and p must implement encoding.TextUnmarshaler.
+// If the flag is used, the flag value will be passed to p's UnmarshalText method.
+// The type of the default value must be the same as the type of p.
+func TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string) {
+	CommandLine.Var(newTextValue(value, p), name, usage)
+}
+
 // Func defines a flag with the specified name and usage string.
 // Each time the flag is seen, fn is called with the value of the flag.
 // If fn returns a non-nil error, it will be treated as a flag value parsing error.