flag: add Func

Refs golang#39557

Author: earthboundkid

src/flag/example_value_test.go

@@ -42,3 +42,19 @@ func ExampleValue() {
 	// Output:
 	// {scheme: "https", host: "golang.org", path: "/pkg/flag/"}
 }
+
+func ExampleFunc() {
+	fs := flag.NewFlagSet("ExampleFunc", flag.ExitOnError)
+	var u *url.URL
+	fs.Func("url", "`URL` to parse", func(s string) error {
+		var err error
+		u, err = url.Parse(s)
+		return err
+	})
+
+	fs.Parse([]string{"-url", "https://golang.org/pkg/flag/"})
+	fmt.Printf(`{scheme: %q, host: %q, path: %q}`, u.Scheme, u.Host, u.Path)
+
+	// Output:
+	// {scheme: "https", host: "golang.org", path: "/pkg/flag/"}
+}

src/flag/flag.go

@@ -1043,3 +1043,21 @@ func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
 	f.name = name
 	f.errorHandling = errorHandling
 }
+
+// Func defines a flag with the specified name and usage string.
+// TODO: more here
+func (f *FlagSet) Func(name, usage string, cb func(string) error) {
+	f.Var(cbfunc(cb), name, usage)
+}
+
+// Func defines a flag with specified name, default value, and usage string.
+// TODO: more here
+func Func(name, usage string, f func(string) error) {
+	CommandLine.Func(name, usage, f)
+}
+
+type cbfunc func(string) error
+
+func (f cbfunc) Set(s string) error { return f(s) }
+
+func (f cbfunc) String() string { return "" }