add subcommands to usage string

3 files changed, 78 insertions, 25 deletions

diff --git a/example_test.go b/example_test.go
index 72807a7..eb8d315 100644
--- a/example_test.go
+++ b/example_test.go
@@ -135,3 +135,40 @@ func Example_usageString() {
 	//                          optimization level
 	//   --help, -h             display this help and exit
 }
+
+// This example shows the usage string generated by go-arg
+func Example_usageStringWithSubcommand() {
+	// These are the args you would pass in on the command line
+	os.Args = split("./example --help")
+
+	type getCmd struct {
+		Item string `arg:"positional" help:"item to fetch"`
+	}
+
+	type listCmd struct {
+		Format string `help:"output format"`
+		Limit  int
+	}
+
+	var args struct {
+		Verbose bool
+		Get     *getCmd  `arg:"subcommand" help:"fetch an item and print it"`
+		List    *listCmd `arg:"subcommand" help:"list available items"`
+	}
+
+	// This is only necessary when running inside golang's runnable example harness
+	osExit = func(int) {}
+
+	MustParse(&args)
+
+	// output:
+	// Usage: example [--verbose]
+	//
+	// Options:
+	//   --verbose
+	//   --help, -h             display this help and exit
+	//
+	// Commands:
+	//   get                    fetch an item and print it
+	//   list                   list available items
+}
diff --git a/parse.go b/parse.go
index d06b299..3a48880 100644
--- a/parse.go
+++ b/parse.go
@@ -59,6 +59,7 @@ type spec struct {
 // command represents a named subcommand, or the top-level command
 type command struct {
 	name        string
+	help        string
 	dest        path
 	specs       []*spec
 	subcommands []*command
@@ -296,6 +297,8 @@ func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
 						return false
 					}
 
+					subcmd.help = field.Tag.Get("help")
+
 					cmd.subcommands = append(cmd.subcommands, subcmd)
 					isSubcommand = true
 				default:
diff --git a/usage.go b/usage.go
index d73da71..42c564b 100644
--- a/usage.go
+++ b/usage.go
@@ -69,6 +69,23 @@ func (p *Parser) WriteUsage(w io.Writer) {
 	fmt.Fprint(w, "\n")
 }
 
+func printTwoCols(w io.Writer, left, help string, defaultVal *string) {
+	lhs := "  " + left
+	fmt.Fprint(w, lhs)
+	if help != "" {
+		if len(lhs)+2 < colWidth {
+			fmt.Fprint(w, strings.Repeat(" ", colWidth-len(lhs)))
+		} else {
+			fmt.Fprint(w, "\n"+strings.Repeat(" ", colWidth))
+		}
+		fmt.Fprint(w, help)
+	}
+	if defaultVal != nil {
+		fmt.Fprintf(w, " [default: %s]", *defaultVal)
+	}
+	fmt.Fprint(w, "\n")
+}
+
 // WriteHelp writes the usage string followed by the full help string for each option
 func (p *Parser) WriteHelp(w io.Writer) {
 	var positionals, options []*spec
@@ -89,17 +106,7 @@ func (p *Parser) WriteHelp(w io.Writer) {
 	if len(positionals) > 0 {
 		fmt.Fprint(w, "\nPositional arguments:\n")
 		for _, spec := range positionals {
-			left := "  " + strings.ToUpper(spec.long)
-			fmt.Fprint(w, left)
-			if spec.help != "" {
-				if len(left)+2 < colWidth {
-					fmt.Fprint(w, strings.Repeat(" ", colWidth-len(left)))
-				} else {
-					fmt.Fprint(w, "\n"+strings.Repeat(" ", colWidth))
-				}
-				fmt.Fprint(w, spec.help)
-			}
-			fmt.Fprint(w, "\n")
+			printTwoCols(w, strings.ToUpper(spec.long), spec.help, nil)
 		}
 	}
 
@@ -123,42 +130,44 @@ func (p *Parser) WriteHelp(w io.Writer) {
 			help:    "display version and exit",
 		})
 	}
+
+	// write the list of subcommands
+	if len(p.cmd.subcommands) > 0 {
+		fmt.Fprint(w, "\nCommands:\n")
+		for _, subcmd := range p.cmd.subcommands {
+			printTwoCols(w, subcmd.name, subcmd.help, nil)
+		}
+	}
 }
 
 func (p *Parser) printOption(w io.Writer, spec *spec) {
-	left := "  " + synopsis(spec, "--"+spec.long)
+	left := synopsis(spec, "--"+spec.long)
 	if spec.short != "" {
 		left += ", " + synopsis(spec, "-"+spec.short)
 	}
-	fmt.Fprint(w, left)
-	if spec.help != "" {
-		if len(left)+2 < colWidth {
-			fmt.Fprint(w, strings.Repeat(" ", colWidth-len(left)))
-		} else {
-			fmt.Fprint(w, "\n"+strings.Repeat(" ", colWidth))
-		}
-		fmt.Fprint(w, spec.help)
-	}
+
 	// If spec.dest is not the zero value then a default value has been added.
 	var v reflect.Value
 	if len(spec.dest.fields) > 0 {
 		v = p.readable(spec.dest)
 	}
+
+	var defaultVal *string
 	if v.IsValid() {
 		z := reflect.Zero(v.Type())
 		if (v.Type().Comparable() && z.Type().Comparable() && v.Interface() != z.Interface()) || v.Kind() == reflect.Slice && !v.IsNil() {
 			if scalar, ok := v.Interface().(encoding.TextMarshaler); ok {
 				if value, err := scalar.MarshalText(); err != nil {
-					fmt.Fprintf(w, " [default: error: %v]", err)
+					defaultVal = ptrTo(fmt.Sprintf("error: %v", err))
 				} else {
-					fmt.Fprintf(w, " [default: %v]", string(value))
+					defaultVal = ptrTo(fmt.Sprintf("%v", string(value)))
 				}
 			} else {
-				fmt.Fprintf(w, " [default: %v]", v)
+				defaultVal = ptrTo(fmt.Sprintf("%v", v))
 			}
 		}
 	}
-	fmt.Fprint(w, "\n")
+	printTwoCols(w, left, spec.help, defaultVal)
 }
 
 func synopsis(spec *spec, form string) string {
@@ -167,3 +176,7 @@ func synopsis(spec *spec, form string) string {
 	}
 	return form + " " + strings.ToUpper(spec.long)
 }
+
+func ptrTo(s string) *string {
+	return &s
+}