Merge pull request #82 from alexflint/subcommand-impl

Add support for subcommands

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 go:
   - "1.10"
-  - "1.11"
+  - "1.12"
   - tip
 env:
   - GO111MODULE=on  # will only be used in go 1.11

README.md

@@ -353,6 +353,59 @@ Options:
   --help, -h             display this help and exit
 ```
 
+### Subcommands
+
+*Introduced in `v1.1.0`*
+
+Subcommands are commonly used in tools that wish to group multiple functions into a single program. An example is the `git` tool:
+```shell
+$ git checkout [arguments specific to checking out code]
+$ git commit [arguments specific to committing]
+$ git push [arguments specific to pushing]
+```
+
+The strings "checkout", "commit", and "push" are different from simple positional arguments because the options available to the user change depending on which subcommand they choose.
+
+This can be implemented with `go-arg` as follows:
+
+```go
+type CheckoutCmd struct {
+	Branch string `arg:"positional"`
+	Track  bool   `arg:"-t"`
+}
+type CommitCmd struct {
+	All     bool   `arg:"-a"`
+	Message string `arg:"-m"`
+}
+type PushCmd struct {
+	Remote      string `arg:"positional"`
+	Branch      string `arg:"positional"`
+	SetUpstream bool   `arg:"-u"`
+}
+var args struct {
+	Checkout *CheckoutCmd `arg:"subcommand:checkout"`
+	Commit   *CommitCmd   `arg:"subcommand:commit"`
+	Push     *PushCmd     `arg:"subcommand:push"`
+	Quiet    bool         `arg:"-q"` // this flag is global to all subcommands
+}
+
+arg.MustParse(&args)
+
+switch {
+case args.Checkout != nil:
+	fmt.Printf("checkout requested for branch %s\n", args.Checkout.Branch)
+case args.Commit != nil:
+	fmt.Printf("commit requested with message \"%s\"\n", args.Commit.Message)
+case args.Push != nil:
+	fmt.Printf("push requested from %s to %s\n", args.Push.Branch, args.Push.Remote)
+}
+```
+
+Some additional rules apply when working with subcommands:
+* The `subcommand` tag can only be used with fields that are pointers to structs
+* Any struct that contains a subcommand must not contain any positionals
+
+
 ### API Documentation
 
 https://godoc.org/github.com/alexflint/go-arg

example_test.go

@@ -104,7 +104,7 @@ func Example_multipleMixed() {
 }
 
 // This example shows the usage string generated by go-arg
-func Example_usageString() {
+func Example_helpText() {
 	// These are the args you would pass in on the command line
 	os.Args = split("./example --help")
 
@@ -135,3 +135,167 @@ func Example_usageString() {
 	//                          optimization level
 	//   --help, -h             display this help and exit
 }
+
+// This example shows the usage string generated by go-arg when using subcommands
+func Example_helpTextWithSubcommand() {
+	// 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
+}
+
+// This example shows the usage string generated by go-arg when using subcommands
+func Example_helpTextForSubcommand() {
+	// These are the args you would pass in on the command line
+	os.Args = split("./example get --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 get ITEM
+	//
+	// Positional arguments:
+	//   ITEM                   item to fetch
+	//
+	// Options:
+	//   --help, -h             display this help and exit
+}
+
+// This example shows the error string generated by go-arg when an invalid option is provided
+func Example_errorText() {
+	// These are the args you would pass in on the command line
+	os.Args = split("./example --optimize INVALID")
+
+	var args struct {
+		Input    string   `arg:"positional"`
+		Output   []string `arg:"positional"`
+		Verbose  bool     `arg:"-v" help:"verbosity level"`
+		Dataset  string   `help:"dataset to use"`
+		Optimize int      `arg:"-O,help:optimization level"`
+	}
+
+	// This is only necessary when running inside golang's runnable example harness
+	osExit = func(int) {}
+	stderr = os.Stdout
+
+	MustParse(&args)
+
+	// output:
+	// Usage: example [--verbose] [--dataset DATASET] [--optimize OPTIMIZE] INPUT [OUTPUT [OUTPUT ...]]
+	// error: error processing --optimize: strconv.ParseInt: parsing "INVALID": invalid syntax
+}
+
+// This example shows the error string generated by go-arg when an invalid option is provided
+func Example_errorTextForSubcommand() {
+	// These are the args you would pass in on the command line
+	os.Args = split("./example get --count INVALID")
+
+	type getCmd struct {
+		Count int
+	}
+
+	var args struct {
+		Get *getCmd `arg:"subcommand"`
+	}
+
+	// This is only necessary when running inside golang's runnable example harness
+	osExit = func(int) {}
+	stderr = os.Stdout
+
+	MustParse(&args)
+
+	// output:
+	// Usage: example get [--count COUNT]
+	// error: error processing --count: strconv.ParseInt: parsing "INVALID": invalid syntax
+}
+
+// This example demonstrates use of subcommands
+func Example_subcommand() {
+	// These are the args you would pass in on the command line
+	os.Args = split("./example commit -a -m what-this-commit-is-about")
+
+	type CheckoutCmd struct {
+		Branch string `arg:"positional"`
+		Track  bool   `arg:"-t"`
+	}
+	type CommitCmd struct {
+		All     bool   `arg:"-a"`
+		Message string `arg:"-m"`
+	}
+	type PushCmd struct {
+		Remote      string `arg:"positional"`
+		Branch      string `arg:"positional"`
+		SetUpstream bool   `arg:"-u"`
+	}
+	var args struct {
+		Checkout *CheckoutCmd `arg:"subcommand:checkout"`
+		Commit   *CommitCmd   `arg:"subcommand:commit"`
+		Push     *PushCmd     `arg:"subcommand:push"`
+		Quiet    bool         `arg:"-q"` // this flag is global to all subcommands
+	}
+
+	// This is only necessary when running inside golang's runnable example harness
+	osExit = func(int) {}
+	stderr = os.Stdout
+
+	MustParse(&args)
+
+	switch {
+	case args.Checkout != nil:
+		fmt.Printf("checkout requested for branch %s\n", args.Checkout.Branch)
+	case args.Commit != nil:
+		fmt.Printf("commit requested with message \"%s\"\n", args.Commit.Message)
+	case args.Push != nil:
+		fmt.Printf("push requested from %s to %s\n", args.Push.Branch, args.Push.Remote)
+	}
+
+	// output:
+	// commit requested with message "what-this-commit-is-about"
+}

parse.go

@@ -1,7 +1,6 @@
 package arg
 
 import (
-	"encoding"
 	"encoding/csv"
 	"errors"
 	"fmt"
@@ -16,9 +15,36 @@ import (
 // to enable monkey-patching during tests
 var osExit = os.Exit
 
+// path represents a sequence of steps to find the output location for an
+// argument or subcommand in the final destination struct
+type path struct {
+	root   int      // index of the destination struct
+	fields []string // sequence of struct field names to traverse
+}
+
+// String gets a string representation of the given path
+func (p path) String() string {
+	if len(p.fields) == 0 {
+		return "args"
+	}
+	return "args." + strings.Join(p.fields, ".")
+}
+
+// Child gets a new path representing a child of this path.
+func (p path) Child(child string) path {
+	// copy the entire slice of fields to avoid possible slice overwrite
+	subfields := make([]string, len(p.fields)+1)
+	copy(subfields, append(p.fields, child))
+	return path{
+		root:   p.root,
+		fields: subfields,
+	}
+}
+
 // spec represents a command line option
 type spec struct {
-	dest       reflect.Value
+	dest       path
+	typ        reflect.Type
 	long       string
 	short      string
 	multiple   bool
@@ -30,6 +56,16 @@ type spec struct {
 	boolean    bool
 }
 
+// command represents a named subcommand, or the top-level command
+type command struct {
+	name        string
+	help        string
+	dest        path
+	specs       []*spec
+	subcommands []*command
+	parent      *command
+}
+
 // ErrHelp indicates that -h or --help were provided
 var ErrHelp = errors.New("help requested by user")
 
@@ -42,18 +78,19 @@ func MustParse(dest ...interface{}) *Parser {
 	if err != nil {
 		fmt.Println(err)
 		osExit(-1)
+		return nil // just in case osExit was monkey-patched
 	}
 
 	err = p.Parse(flags())
 	switch {
 	case err == ErrHelp:
-		p.WriteHelp(os.Stdout)
+		p.writeHelpForCommand(os.Stdout, p.lastCmd)
 		osExit(0)
 	case err == ErrVersion:
 		fmt.Println(p.version)
 		osExit(0)
 	case err != nil:
-		p.Fail(err.Error())
+		p.failWithCommand(err.Error(), p.lastCmd)
 	}
 
 	return p
@@ -83,10 +120,14 @@ type Config struct {
 
 // Parser represents a set of command line options with destination values
 type Parser struct {
-	specs       []*spec
+	cmd         *command
+	roots       []reflect.Value
 	config      Config
 	version     string
 	description string
+
+	// the following fields change curing processing of command line arguments
+	lastCmd *command
 }
 
 // Versioned is the interface that the destination struct should implement to
@@ -106,66 +147,180 @@ type Described interface {
 }
 
 // walkFields calls a function for each field of a struct, recursively expanding struct fields.
-func walkFields(v reflect.Value, visit func(field reflect.StructField, val reflect.Value, owner reflect.Type) bool) {
-	t := v.Type()
+func walkFields(t reflect.Type, visit func(field reflect.StructField, owner reflect.Type) bool) {
 	for i := 0; i < t.NumField(); i++ {
 		field := t.Field(i)
-		val := v.Field(i)
-		expand := visit(field, val, t)
+		expand := visit(field, t)
 		if expand && field.Type.Kind() == reflect.Struct {
-			walkFields(val, visit)
+			walkFields(field.Type, visit)
 		}
 	}
 }
 
 // NewParser constructs a parser from a list of destination structs
 func NewParser(config Config, dests ...interface{}) (*Parser, error) {
+	// first pick a name for the command for use in the usage text
+	var name string
+	switch {
+	case config.Program != "":
+		name = config.Program
+	case len(os.Args) > 0:
+		name = filepath.Base(os.Args[0])
+	default:
+		name = "program"
+	}
+
+	// construct a parser
 	p := Parser{
+		cmd:    &command{name: name},
 		config: config,
 	}
+
+	// make a list of roots
 	for _, dest := range dests {
+		p.roots = append(p.roots, reflect.ValueOf(dest))
+	}
+
+	// process each of the destination values
+	for i, dest := range dests {
+		t := reflect.TypeOf(dest)
+		if t.Kind() != reflect.Ptr {
+			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", t))
+		}
+
+		cmd, err := cmdFromStruct(name, path{root: i}, t)
+		if err != nil {
+			return nil, err
+		}
+		p.cmd.specs = append(p.cmd.specs, cmd.specs...)
+		p.cmd.subcommands = append(p.cmd.subcommands, cmd.subcommands...)
+
 		if dest, ok := dest.(Versioned); ok {
 			p.version = dest.Version()
 		}
 		if dest, ok := dest.(Described); ok {
 			p.description = dest.Description()
 		}
-		v := reflect.ValueOf(dest)
-		if v.Kind() != reflect.Ptr {
-			panic(fmt.Sprintf("%s is not a pointer (did you forget an ampersand?)", v.Type()))
-		}
-		v = v.Elem()
-		if v.Kind() != reflect.Struct {
-			panic(fmt.Sprintf("%T is not a struct pointer", dest))
+	}
+
+	return &p, nil
+}
+
+func cmdFromStruct(name string, dest path, t reflect.Type) (*command, error) {
+	// commands can only be created from pointers to structs
+	if t.Kind() != reflect.Ptr {
+		return nil, fmt.Errorf("subcommands must be pointers to structs but %s is a %s",
+			dest, t.Kind())
+	}
+
+	t = t.Elem()
+	if t.Kind() != reflect.Struct {
+		return nil, fmt.Errorf("subcommands must be pointers to structs but %s is a pointer to %s",
+			dest, t.Kind())
+	}
+
+	cmd := command{
+		name: name,
+		dest: dest,
+	}
+
+	var errs []string
+	walkFields(t, func(field reflect.StructField, t reflect.Type) bool {
+		// Check for the ignore switch in the tag
+		tag := field.Tag.Get("arg")
+		if tag == "-" {
+			return false
 		}
 
-		var errs []string
-		walkFields(v, func(field reflect.StructField, val reflect.Value, t reflect.Type) bool {
-			// Check for the ignore switch in the tag
-			tag := field.Tag.Get("arg")
-			if tag == "-" {
-				return false
-			}
+		// If this is an embedded struct then recurse into its fields
+		if field.Anonymous && field.Type.Kind() == reflect.Struct {
+			return true
+		}
 
-			// If this is an embedded struct then recurse into its fields
-			if field.Anonymous && field.Type.Kind() == reflect.Struct {
-				return true
-			}
+		// duplicate the entire path to avoid slice overwrites
+		subdest := dest.Child(field.Name)
+		spec := spec{
+			dest: subdest,
+			long: strings.ToLower(field.Name),
+			typ:  field.Type,
+		}
 
-			spec := spec{
-				long: strings.ToLower(field.Name),
-				dest: val,
-			}
+		help, exists := field.Tag.Lookup("help")
+		if exists {
+			spec.help = help
+		}
 
-			help, exists := field.Tag.Lookup("help")
-			if exists {
-				spec.help = help
-			}
+		// Look at the tag
+		var isSubcommand bool // tracks whether this field is a subcommand
+		if tag != "" {
+			for _, key := range strings.Split(tag, ",") {
+				key = strings.TrimLeft(key, " ")
+				var value string
+				if pos := strings.Index(key, ":"); pos != -1 {
+					value = key[pos+1:]
+					key = key[:pos]
+				}
+
+				switch {
+				case strings.HasPrefix(key, "---"):
+					errs = append(errs, fmt.Sprintf("%s.%s: too many hyphens", t.Name(), field.Name))
+				case strings.HasPrefix(key, "--"):
+					spec.long = key[2:]
+				case strings.HasPrefix(key, "-"):
+					if len(key) != 2 {
+						errs = append(errs, fmt.Sprintf("%s.%s: short arguments must be one character only",
+							t.Name(), field.Name))
+						return false
+					}
+					spec.short = key[1:]
+				case key == "required":
+					spec.required = true
+				case key == "positional":
+					spec.positional = true
+				case key == "separate":
+					spec.separate = true
+				case key == "help": // deprecated
+					spec.help = value
+				case key == "env":
+					// Use override name if provided
+					if value != "" {
+						spec.env = value
+					} else {
+						spec.env = strings.ToUpper(field.Name)
+					}
+				case key == "subcommand":
+					// decide on a name for the subcommand
+					cmdname := value
+					if cmdname == "" {
+						cmdname = strings.ToLower(field.Name)
+					}
+
+					// parse the subcommand recursively
+					subcmd, err := cmdFromStruct(cmdname, subdest, field.Type)
+					if err != nil {
+						errs = append(errs, err.Error())
+						return false
+					}
+
+					subcmd.parent = &cmd
+					subcmd.help = field.Tag.Get("help")
+
+					cmd.subcommands = append(cmd.subcommands, subcmd)
+					isSubcommand = true
+				default:
+					errs = append(errs, fmt.Sprintf("unrecognized tag '%s' on field %s", key, tag))
+					return false
+				}
+			}
+		}
+
+		// Check whether this field is supported. It's good to do this here rather than
+		// wait until ParseValue because it means that a program with invalid argument
+		// fields will always fail regardless of whether the arguments it received
+		// exercised those fields.
+		if !isSubcommand {
+			cmd.specs = append(cmd.specs, &spec)
 
-			// Check whether this field is supported. It's good to do this here rather than
-			// wait until ParseValue because it means that a program with invalid argument
-			// fields will always fail regardless of whether the arguments it received
-			// exercised those fields.
 			var parseable bool
 			parseable, spec.boolean, spec.multiple = canParse(field.Type)
 			if !parseable {
@@ -173,110 +328,50 @@ func NewParser(config Config, dests ...interface{}) (*Parser, error) {
 					t.Name(), field.Name, field.Type.String()))
 				return false
 			}
+		}
 
-			// Look at the tag
-			if tag != "" {
-				for _, key := range strings.Split(tag, ",") {
-					key = strings.TrimLeft(key, " ")
-					var value string
-					if pos := strings.Index(key, ":"); pos != -1 {
-						value = key[pos+1:]
-						key = key[:pos]
-					}
+		// if this was an embedded field then we already returned true up above
+		return false
+	})
 
-					switch {
-					case strings.HasPrefix(key, "---"):
-						errs = append(errs, fmt.Sprintf("%s.%s: too many hyphens", t.Name(), field.Name))
-					case strings.HasPrefix(key, "--"):
-						spec.long = key[2:]
-					case strings.HasPrefix(key, "-"):
-						if len(key) != 2 {
-							errs = append(errs, fmt.Sprintf("%s.%s: short arguments must be one character only",
-								t.Name(), field.Name))
-							return false
-						}
-						spec.short = key[1:]
-					case key == "required":
-						spec.required = true
-					case key == "positional":
-						spec.positional = true
-					case key == "separate":
-						spec.separate = true
-					case key == "help": // deprecated
-						spec.help = value
-					case key == "env":
-						// Use override name if provided
-						if value != "" {
-							spec.env = value
-						} else {
-							spec.env = strings.ToUpper(field.Name)
-						}
-					default:
-						errs = append(errs, fmt.Sprintf("unrecognized tag '%s' on field %s", key, tag))
-						return false
-					}
-				}
-			}
-			p.specs = append(p.specs, &spec)
+	if len(errs) > 0 {
+		return nil, errors.New(strings.Join(errs, "\n"))
+	}
 
-			// if this was an embedded field then we already returned true up above
-			return false
-		})
-
-		if len(errs) > 0 {
-			return nil, errors.New(strings.Join(errs, "\n"))
+	// check that we don't have both positionals and subcommands
+	var hasPositional bool
+	for _, spec := range cmd.specs {
+		if spec.positional {
+			hasPositional = true
 		}
 	}
-	if p.config.Program == "" {
-		p.config.Program = "program"
-		if len(os.Args) > 0 {
-			p.config.Program = filepath.Base(os.Args[0])
-		}
+	if hasPositional && len(cmd.subcommands) > 0 {
+		return nil, fmt.Errorf("%s cannot have both subcommands and positional arguments", dest)
 	}
-	return &p, nil
+
+	return &cmd, nil
 }
 
 // Parse processes the given command line option, storing the results in the field
 // of the structs from which NewParser was constructed
 func (p *Parser) Parse(args []string) error {
-	// If -h or --help were specified then print usage
-	for _, arg := range args {
-		if arg == "-h" || arg == "--help" {
-			return ErrHelp
-		}
-		if arg == "--version" {
-			return ErrVersion
-		}
-		if arg == "--" {
-			break
+	err := p.process(args)
+	if err != nil {
+		// If -h or --help were specified then make sure help text supercedes other errors
+		for _, arg := range args {
+			if arg == "-h" || arg == "--help" {
+				return ErrHelp
+			}
+			if arg == "--" {
+				break
+			}
 		}
 	}
-
-	// Process all command line arguments
-	return process(p.specs, args)
+	return err
 }
 
-// process goes through arguments one-by-one, parses them, and assigns the result to
-// the underlying struct field
-func process(specs []*spec, args []string) error {
-	// track the options we have seen
-	wasPresent := make(map[*spec]bool)
-
-	// construct a map from --option to spec
-	optionMap := make(map[string]*spec)
-	for _, spec := range specs {
-		if spec.positional {
-			continue
-		}
-		if spec.long != "" {
-			optionMap[spec.long] = spec
-		}
-		if spec.short != "" {
-			optionMap[spec.short] = spec
-		}
-	}
-
-	// deal with environment vars
+// process environment vars for the given arguments
+func (p *Parser) captureEnvVars(specs []*spec, wasPresent map[*spec]bool) error {
 	for _, spec := range specs {
 		if spec.env == "" {
 			continue
@@ -298,7 +393,7 @@ func process(specs []*spec, args []string) error {
 					err,
 				)
 			}
-			if err = setSlice(spec.dest, values, !spec.separate); err != nil {
+			if err = setSlice(p.val(spec.dest), values, !spec.separate); err != nil {
 				return fmt.Errorf(
 					"error processing environment variable %s with multiple values: %v",
 					spec.env,
@@ -306,13 +401,36 @@ func process(specs []*spec, args []string) error {
 				)
 			}
 		} else {
-			if err := scalar.ParseValue(spec.dest, value); err != nil {
+			if err := scalar.ParseValue(p.val(spec.dest), value); err != nil {
 				return fmt.Errorf("error processing environment variable %s: %v", spec.env, err)
 			}
 		}
 		wasPresent[spec] = true
 	}
 
+	return nil
+}
+
+// process goes through arguments one-by-one, parses them, and assigns the result to
+// the underlying struct field
+func (p *Parser) process(args []string) error {
+	// track the options we have seen
+	wasPresent := make(map[*spec]bool)
+
+	// union of specs for the chain of subcommands encountered so far
+	curCmd := p.cmd
+	p.lastCmd = curCmd
+
+	// make a copy of the specs because we will add to this list each time we expand a subcommand
+	specs := make([]*spec, len(curCmd.specs))
+	copy(specs, curCmd.specs)
+
+	// deal with environment vars
+	err := p.captureEnvVars(specs, wasPresent)
+	if err != nil {
+		return err
+	}
+
 	// process each string from the command line
 	var allpositional bool
 	var positionals []string
@@ -326,10 +444,44 @@ func process(specs []*spec, args []string) error {
 		}
 
 		if !isFlag(arg) || allpositional {
-			positionals = append(positionals, arg)
+			// each subcommand can have either subcommands or positionals, but not both
+			if len(curCmd.subcommands) == 0 {
+				positionals = append(positionals, arg)
+				continue
+			}
+
+			// if we have a subcommand then make sure it is valid for the current context
+			subcmd := findSubcommand(curCmd.subcommands, arg)
+			if subcmd == nil {
+				return fmt.Errorf("invalid subcommand: %s", arg)
+			}
+
+			// instantiate the field to point to a new struct
+			v := p.val(subcmd.dest)
+			v.Set(reflect.New(v.Type().Elem())) // we already checked that all subcommands are struct pointers
+
+			// add the new options to the set of allowed options
+			specs = append(specs, subcmd.specs...)
+
+			// capture environment vars for these new options
+			err := p.captureEnvVars(subcmd.specs, wasPresent)
+			if err != nil {
+				return err
+			}
+
+			curCmd = subcmd
+			p.lastCmd = curCmd
 			continue
 		}
 
+		// check for special --help and --version flags
+		switch arg {
+		case "-h", "--help":
+			return ErrHelp
+		case "--version":
+			return ErrVersion
+		}
+
 		// check for an equals sign, as in "--foo=bar"
 		var value string
 		opt := strings.TrimLeft(arg, "-")
@@ -338,9 +490,10 @@ func process(specs []*spec, args []string) error {
 			opt = opt[:pos]
 		}
 
-		// lookup the spec for this option
-		spec, ok := optionMap[opt]
-		if !ok {
+		// lookup the spec for this option (note that the "specs" slice changes as
+		// we expand subcommands so it is better not to use a map)
+		spec := findOption(specs, opt)
+		if spec == nil {
 			return fmt.Errorf("unknown argument %s", arg)
 		}
 		wasPresent[spec] = true
@@ -359,7 +512,7 @@ func process(specs []*spec, args []string) error {
 			} else {
 				values = append(values, value)
 			}
-			err := setSlice(spec.dest, values, !spec.separate)
+			err := setSlice(p.val(spec.dest), values, !spec.separate)
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", arg, err)
 			}
@@ -377,14 +530,14 @@ func process(specs []*spec, args []string) error {
 			if i+1 == len(args) {
 				return fmt.Errorf("missing value for %s", arg)
 			}
-			if !nextIsNumeric(spec.dest.Type(), args[i+1]) && isFlag(args[i+1]) {
+			if !nextIsNumeric(spec.typ, args[i+1]) && isFlag(args[i+1]) {
 				return fmt.Errorf("missing value for %s", arg)
 			}
 			value = args[i+1]
 			i++
 		}
 
-		err := scalar.ParseValue(spec.dest, value)
+		err := scalar.ParseValue(p.val(spec.dest), value)
 		if err != nil {
 			return fmt.Errorf("error processing %s: %v", arg, err)
 		}
@@ -400,13 +553,13 @@ func process(specs []*spec, args []string) error {
 		}
 		wasPresent[spec] = true
 		if spec.multiple {
-			err := setSlice(spec.dest, positionals, true)
+			err := setSlice(p.val(spec.dest), positionals, true)
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", spec.long, err)
 			}
 			positionals = nil
 		} else {
-			err := scalar.ParseValue(spec.dest, positionals[0])
+			err := scalar.ParseValue(p.val(spec.dest), positionals[0])
 			if err != nil {
 				return fmt.Errorf("error processing %s: %v", spec.long, err)
 			}
@@ -449,6 +602,30 @@ func isFlag(s string) bool {
 	return strings.HasPrefix(s, "-") && strings.TrimLeft(s, "-") != ""
 }
 
+// val returns a reflect.Value corresponding to the current value for the
+// given path
+func (p *Parser) val(dest path) reflect.Value {
+	v := p.roots[dest.root]
+	for _, field := range dest.fields {
+		if v.Kind() == reflect.Ptr {
+			if v.IsNil() {
+				return reflect.Value{}
+			}
+			v = v.Elem()
+		}
+
+		v = v.FieldByName(field)
+		if !v.IsValid() {
+			// it is appropriate to panic here because this can only happen due to
+			// an internal bug in this library (since we construct the path ourselves
+			// by reflecting on the same struct)
+			panic(fmt.Errorf("error resolving path %v: %v has no field named %v",
+				dest.fields, v.Type(), field))
+		}
+	}
+	return v
+}
+
 // parse a value as the appropriate type and store it in the struct
 func setSlice(dest reflect.Value, values []string, trunc bool) error {
 	if !dest.CanSet() {
@@ -480,56 +657,25 @@ func setSlice(dest reflect.Value, values []string, trunc bool) error {
 	return nil
 }
 
-// canParse returns true if the type can be parsed from a string
-func canParse(t reflect.Type) (parseable, boolean, multiple bool) {
-	parseable = scalar.CanParse(t)
-	boolean = isBoolean(t)
-	if parseable {
-		return
+// findOption finds an option from its name, or returns null if no spec is found
+func findOption(specs []*spec, name string) *spec {
+	for _, spec := range specs {
+		if spec.positional {
+			continue
+		}
+		if spec.long == name || spec.short == name {
+			return spec
+		}
 	}
-
-	// Look inside pointer types
-	if t.Kind() == reflect.Ptr {
-		t = t.Elem()
-	}
-	// Look inside slice types
-	if t.Kind() == reflect.Slice {
-		multiple = true
-		t = t.Elem()
-	}
-
-	parseable = scalar.CanParse(t)
-	boolean = isBoolean(t)
-	if parseable {
-		return
-	}
-
-	// Look inside pointer types (again, in case of []*Type)
-	if t.Kind() == reflect.Ptr {
-		t = t.Elem()
-	}
-
-	parseable = scalar.CanParse(t)
-	boolean = isBoolean(t)
-	if parseable {
-		return
-	}
-
-	return false, false, false
+	return nil
 }
 
-var textUnmarshalerType = reflect.TypeOf([]encoding.TextUnmarshaler{}).Elem()
-
-// isBoolean returns true if the type can be parsed from a single string
-func isBoolean(t reflect.Type) bool {
-	switch {
-	case t.Implements(textUnmarshalerType):
-		return false
-	case t.Kind() == reflect.Bool:
-		return true
-	case t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Bool:
-		return true
-	default:
-		return false
+// findSubcommand finds a subcommand using its name, or returns null if no subcommand is found
+func findSubcommand(cmds []*command, name string) *command {
+	for _, cmd := range cmds {
+		if cmd.name == name {
+			return cmd
+		}
 	}
+	return nil
 }

parse_test.go

@@ -19,15 +19,20 @@ func setenv(t *testing.T, name, val string) {
 }
 
 func parse(cmdline string, dest interface{}) error {
+	_, err := pparse(cmdline, dest)
+	return err
+}
+
+func pparse(cmdline string, dest interface{}) (*Parser, error) {
 	p, err := NewParser(Config{}, dest)
 	if err != nil {
-		return err
+		return nil, err
 	}
 	var parts []string
 	if len(cmdline) > 0 {
 		parts = strings.Split(cmdline, " ")
 	}
-	return p.Parse(parts)
+	return p, p.Parse(parts)
 }
 
 func TestString(t *testing.T) {
@@ -371,7 +376,7 @@ func TestNonsenseKey(t *testing.T) {
 	assert.Error(t, err)
 }
 
-func TestMissingValue(t *testing.T) {
+func TestMissingValueAtEnd(t *testing.T) {
 	var args struct {
 		Foo string
 	}
@@ -379,6 +384,24 @@ func TestMissingValue(t *testing.T) {
 	assert.Error(t, err)
 }
 
+func TestMissingValueInMIddle(t *testing.T) {
+	var args struct {
+		Foo string
+		Bar string
+	}
+	err := parse("--foo --bar=abc", &args)
+	assert.Error(t, err)
+}
+
+func TestNegativeValue(t *testing.T) {
+	var args struct {
+		Foo int
+	}
+	err := parse("--foo -123", &args)
+	require.NoError(t, err)
+	assert.Equal(t, -123, args.Foo)
+}
+
 func TestInvalidInt(t *testing.T) {
 	var args struct {
 		Foo int
@@ -462,11 +485,10 @@ func TestPanicOnNonPointer(t *testing.T) {
 	})
 }
 
-func TestPanicOnNonStruct(t *testing.T) {
+func TestErrorOnNonStruct(t *testing.T) {
 	var args string
-	assert.Panics(t, func() {
-		_ = parse("", &args)
-	})
+	err := parse("", &args)
+	assert.Error(t, err)
 }
 
 func TestUnsupportedType(t *testing.T) {
@@ -540,6 +562,15 @@ func TestEnvironmentVariable(t *testing.T) {
 	assert.Equal(t, "bar", args.Foo)
 }
 
+func TestEnvironmentVariableNotPresent(t *testing.T) {
+	var args struct {
+		NotPresent string `arg:"env"`
+	}
+	os.Args = []string{"example"}
+	MustParse(&args)
+	assert.Equal(t, "", args.NotPresent)
+}
+
 func TestEnvironmentVariableOverrideName(t *testing.T) {
 	var args struct {
 		Foo string `arg:"env:BAZ"`
@@ -584,7 +615,7 @@ func TestEnvironmentVariableSliceArgumentString(t *testing.T) {
 	var args struct {
 		Foo []string `arg:"env"`
 	}
-	setenv(t, "FOO", "bar,\"baz, qux\"")
+	setenv(t, "FOO", `bar,"baz, qux"`)
 	MustParse(&args)
 	assert.Equal(t, []string{"bar", "baz, qux"}, args.Foo)
 }
@@ -846,6 +877,28 @@ func TestEmbedded(t *testing.T) {
 	assert.Equal(t, true, args.Z)
 }
 
+func TestEmbeddedPtr(t *testing.T) {
+	// embedded pointer fields are not supported so this should return an error
+	var args struct {
+		*A
+	}
+	err := parse("--x=hello", &args)
+	require.Error(t, err)
+}
+
+func TestEmbeddedPtrIgnored(t *testing.T) {
+	// embedded pointer fields are not normally supported but here
+	// we explicitly exclude it so the non-nil embedded structs
+	// should work as expected
+	var args struct {
+		*A `arg:"-"`
+		B
+	}
+	err := parse("--y=321", &args)
+	require.NoError(t, err)
+	assert.Equal(t, 321, args.Y)
+}
+
 func TestEmptyArgs(t *testing.T) {
 	origArgs := os.Args
 
@@ -985,3 +1038,10 @@ func TestReuseParser(t *testing.T) {
 	err = p.Parse([]string{})
 	assert.Error(t, err)
 }
+
+func TestVersion(t *testing.T) {
+	var args struct{}
+	err := parse("--version", &args)
+	assert.Equal(t, ErrVersion, err)
+
+}

reflect.go

@@ -0,0 +1,62 @@
+package arg
+
+import (
+	"encoding"
+	"reflect"
+
+	scalar "github.com/alexflint/go-scalar"
+)
+
+var textUnmarshalerType = reflect.TypeOf([]encoding.TextUnmarshaler{}).Elem()
+
+// canParse returns true if the type can be parsed from a string
+func canParse(t reflect.Type) (parseable, boolean, multiple bool) {
+	parseable = scalar.CanParse(t)
+	boolean = isBoolean(t)
+	if parseable {
+		return
+	}
+
+	// Look inside pointer types
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+	// Look inside slice types
+	if t.Kind() == reflect.Slice {
+		multiple = true
+		t = t.Elem()
+	}
+
+	parseable = scalar.CanParse(t)
+	boolean = isBoolean(t)
+	if parseable {
+		return
+	}
+
+	// Look inside pointer types (again, in case of []*Type)
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+
+	parseable = scalar.CanParse(t)
+	boolean = isBoolean(t)
+	if parseable {
+		return
+	}
+
+	return false, false, false
+}
+
+// isBoolean returns true if the type can be parsed from a single string
+func isBoolean(t reflect.Type) bool {
+	switch {
+	case t.Implements(textUnmarshalerType):
+		return false
+	case t.Kind() == reflect.Bool:
+		return true
+	case t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Bool:
+		return true
+	default:
+		return false
+	}
+}

reflect_test.go

@@ -0,0 +1,55 @@
+package arg
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func assertCanParse(t *testing.T, typ reflect.Type, parseable, boolean, multiple bool) {
+	p, b, m := canParse(typ)
+	assert.Equal(t, parseable, p, "expected %v to have parseable=%v but was %v", typ, parseable, p)
+	assert.Equal(t, boolean, b, "expected %v to have boolean=%v but was %v", typ, boolean, b)
+	assert.Equal(t, multiple, m, "expected %v to have multiple=%v but was %v", typ, multiple, m)
+}
+
+func TestCanParse(t *testing.T) {
+	var b bool
+	var i int
+	var s string
+	var f float64
+	var bs []bool
+	var is []int
+
+	assertCanParse(t, reflect.TypeOf(b), true, true, false)
+	assertCanParse(t, reflect.TypeOf(i), true, false, false)
+	assertCanParse(t, reflect.TypeOf(s), true, false, false)
+	assertCanParse(t, reflect.TypeOf(f), true, false, false)
+
+	assertCanParse(t, reflect.TypeOf(&b), true, true, false)
+	assertCanParse(t, reflect.TypeOf(&s), true, false, false)
+	assertCanParse(t, reflect.TypeOf(&i), true, false, false)
+	assertCanParse(t, reflect.TypeOf(&f), true, false, false)
+
+	assertCanParse(t, reflect.TypeOf(bs), true, true, true)
+	assertCanParse(t, reflect.TypeOf(&bs), true, true, true)
+
+	assertCanParse(t, reflect.TypeOf(is), true, false, true)
+	assertCanParse(t, reflect.TypeOf(&is), true, false, true)
+}
+
+type implementsTextUnmarshaler struct{}
+
+func (*implementsTextUnmarshaler) UnmarshalText(text []byte) error {
+	return nil
+}
+
+func TestCanParseTextUnmarshaler(t *testing.T) {
+	var u implementsTextUnmarshaler
+	var su []implementsTextUnmarshaler
+	assertCanParse(t, reflect.TypeOf(u), true, false, false)
+	assertCanParse(t, reflect.TypeOf(&u), true, false, false)
+	assertCanParse(t, reflect.TypeOf(su), true, false, true)
+	assertCanParse(t, reflect.TypeOf(&su), true, false, true)
+}

subcommand.go

@@ -0,0 +1,37 @@
+package arg
+
+// Subcommand returns the user struct for the subcommand selected by
+// the command line arguments most recently processed by the parser.
+// The return value is always a pointer to a struct. If no subcommand
+// was specified then it returns the top-level arguments struct. If
+// no command line arguments have been processed by this parser then it
+// returns nil.
+func (p *Parser) Subcommand() interface{} {
+	if p.lastCmd == nil || p.lastCmd.parent == nil {
+		return nil
+	}
+	return p.val(p.lastCmd.dest).Interface()
+}
+
+// SubcommandNames returns the sequence of subcommands specified by the
+// user. If no subcommands were given then it returns an empty slice.
+func (p *Parser) SubcommandNames() []string {
+	if p.lastCmd == nil {
+		return nil
+	}
+
+	// make a list of ancestor commands
+	var ancestors []string
+	cur := p.lastCmd
+	for cur.parent != nil { // we want to exclude the root
+		ancestors = append(ancestors, cur.name)
+		cur = cur.parent
+	}
+
+	// reverse the list
+	out := make([]string, len(ancestors))
+	for i := 0; i < len(ancestors); i++ {
+		out[i] = ancestors[len(ancestors)-i-1]
+	}
+	return out
+}

subcommand_test.go

@@ -0,0 +1,355 @@
+package arg
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// This file contains tests for parse.go but I decided to put them here
+// since that file is getting large
+
+func TestSubcommandNotAPointer(t *testing.T) {
+	var args struct {
+		A string `arg:"subcommand"`
+	}
+	_, err := NewParser(Config{}, &args)
+	assert.Error(t, err)
+}
+
+func TestSubcommandNotAPointerToStruct(t *testing.T) {
+	var args struct {
+		A struct{} `arg:"subcommand"`
+	}
+	_, err := NewParser(Config{}, &args)
+	assert.Error(t, err)
+}
+
+func TestPositionalAndSubcommandNotAllowed(t *testing.T) {
+	var args struct {
+		A string    `arg:"positional"`
+		B *struct{} `arg:"subcommand"`
+	}
+	_, err := NewParser(Config{}, &args)
+	assert.Error(t, err)
+}
+
+func TestMinimalSubcommand(t *testing.T) {
+	type listCmd struct {
+	}
+	var args struct {
+		List *listCmd `arg:"subcommand"`
+	}
+	p, err := pparse("list", &args)
+	require.NoError(t, err)
+	assert.NotNil(t, args.List)
+	assert.Equal(t, args.List, p.Subcommand())
+	assert.Equal(t, []string{"list"}, p.SubcommandNames())
+}
+
+func TestNoSuchSubcommand(t *testing.T) {
+	type listCmd struct {
+	}
+	var args struct {
+		List *listCmd `arg:"subcommand"`
+	}
+	_, err := pparse("invalid", &args)
+	assert.Error(t, err)
+}
+
+func TestNamedSubcommand(t *testing.T) {
+	type listCmd struct {
+	}
+	var args struct {
+		List *listCmd `arg:"subcommand:ls"`
+	}
+	p, err := pparse("ls", &args)
+	require.NoError(t, err)
+	assert.NotNil(t, args.List)
+	assert.Equal(t, args.List, p.Subcommand())
+	assert.Equal(t, []string{"ls"}, p.SubcommandNames())
+}
+
+func TestEmptySubcommand(t *testing.T) {
+	type listCmd struct {
+	}
+	var args struct {
+		List *listCmd `arg:"subcommand"`
+	}
+	p, err := pparse("", &args)
+	require.NoError(t, err)
+	assert.Nil(t, args.List)
+	assert.Nil(t, p.Subcommand())
+	assert.Empty(t, p.SubcommandNames())
+}
+
+func TestTwoSubcommands(t *testing.T) {
+	type getCmd struct {
+	}
+	type listCmd struct {
+	}
+	var args struct {
+		Get  *getCmd  `arg:"subcommand"`
+		List *listCmd `arg:"subcommand"`
+	}
+	p, err := pparse("list", &args)
+	require.NoError(t, err)
+	assert.Nil(t, args.Get)
+	assert.NotNil(t, args.List)
+	assert.Equal(t, args.List, p.Subcommand())
+	assert.Equal(t, []string{"list"}, p.SubcommandNames())
+}
+
+func TestSubcommandsWithOptions(t *testing.T) {
+	type getCmd struct {
+		Name string
+	}
+	type listCmd struct {
+		Limit int
+	}
+	type cmd struct {
+		Verbose bool
+		Get     *getCmd  `arg:"subcommand"`
+		List    *listCmd `arg:"subcommand"`
+	}
+
+	{
+		var args cmd
+		err := parse("list", &args)
+		require.NoError(t, err)
+		assert.Nil(t, args.Get)
+		assert.NotNil(t, args.List)
+	}
+
+	{
+		var args cmd
+		err := parse("list --limit 3", &args)
+		require.NoError(t, err)
+		assert.Nil(t, args.Get)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, args.List.Limit, 3)
+	}
+
+	{
+		var args cmd
+		err := parse("list --limit 3 --verbose", &args)
+		require.NoError(t, err)
+		assert.Nil(t, args.Get)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, args.List.Limit, 3)
+		assert.True(t, args.Verbose)
+	}
+
+	{
+		var args cmd
+		err := parse("list --verbose --limit 3", &args)
+		require.NoError(t, err)
+		assert.Nil(t, args.Get)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, args.List.Limit, 3)
+		assert.True(t, args.Verbose)
+	}
+
+	{
+		var args cmd
+		err := parse("--verbose list --limit 3", &args)
+		require.NoError(t, err)
+		assert.Nil(t, args.Get)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, args.List.Limit, 3)
+		assert.True(t, args.Verbose)
+	}
+
+	{
+		var args cmd
+		err := parse("get", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Nil(t, args.List)
+	}
+
+	{
+		var args cmd
+		err := parse("get --name test", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Nil(t, args.List)
+		assert.Equal(t, args.Get.Name, "test")
+	}
+}
+
+func TestNestedSubcommands(t *testing.T) {
+	type child struct{}
+	type parent struct {
+		Child *child `arg:"subcommand"`
+	}
+	type grandparent struct {
+		Parent *parent `arg:"subcommand"`
+	}
+	type root struct {
+		Grandparent *grandparent `arg:"subcommand"`
+	}
+
+	{
+		var args root
+		p, err := pparse("grandparent parent child", &args)
+		require.NoError(t, err)
+		require.NotNil(t, args.Grandparent)
+		require.NotNil(t, args.Grandparent.Parent)
+		require.NotNil(t, args.Grandparent.Parent.Child)
+		assert.Equal(t, args.Grandparent.Parent.Child, p.Subcommand())
+		assert.Equal(t, []string{"grandparent", "parent", "child"}, p.SubcommandNames())
+	}
+
+	{
+		var args root
+		p, err := pparse("grandparent parent", &args)
+		require.NoError(t, err)
+		require.NotNil(t, args.Grandparent)
+		require.NotNil(t, args.Grandparent.Parent)
+		require.Nil(t, args.Grandparent.Parent.Child)
+		assert.Equal(t, args.Grandparent.Parent, p.Subcommand())
+		assert.Equal(t, []string{"grandparent", "parent"}, p.SubcommandNames())
+	}
+
+	{
+		var args root
+		p, err := pparse("grandparent", &args)
+		require.NoError(t, err)
+		require.NotNil(t, args.Grandparent)
+		require.Nil(t, args.Grandparent.Parent)
+		assert.Equal(t, args.Grandparent, p.Subcommand())
+		assert.Equal(t, []string{"grandparent"}, p.SubcommandNames())
+	}
+
+	{
+		var args root
+		p, err := pparse("", &args)
+		require.NoError(t, err)
+		require.Nil(t, args.Grandparent)
+		assert.Nil(t, p.Subcommand())
+		assert.Empty(t, p.SubcommandNames())
+	}
+}
+
+func TestSubcommandsWithPositionals(t *testing.T) {
+	type listCmd struct {
+		Pattern string `arg:"positional"`
+	}
+	type cmd struct {
+		Format string
+		List   *listCmd `arg:"subcommand"`
+	}
+
+	{
+		var args cmd
+		err := parse("list", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, "", args.List.Pattern)
+	}
+
+	{
+		var args cmd
+		err := parse("list --format json", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, "", args.List.Pattern)
+		assert.Equal(t, "json", args.Format)
+	}
+
+	{
+		var args cmd
+		err := parse("list somepattern", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, "somepattern", args.List.Pattern)
+	}
+
+	{
+		var args cmd
+		err := parse("list somepattern --format json", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, "somepattern", args.List.Pattern)
+		assert.Equal(t, "json", args.Format)
+	}
+
+	{
+		var args cmd
+		err := parse("list --format json somepattern", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, "somepattern", args.List.Pattern)
+		assert.Equal(t, "json", args.Format)
+	}
+
+	{
+		var args cmd
+		err := parse("--format json list somepattern", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.List)
+		assert.Equal(t, "somepattern", args.List.Pattern)
+		assert.Equal(t, "json", args.Format)
+	}
+
+	{
+		var args cmd
+		err := parse("--format json", &args)
+		require.NoError(t, err)
+		assert.Nil(t, args.List)
+		assert.Equal(t, "json", args.Format)
+	}
+}
+func TestSubcommandsWithMultiplePositionals(t *testing.T) {
+	type getCmd struct {
+		Items []string `arg:"positional"`
+	}
+	type cmd struct {
+		Limit int
+		Get   *getCmd `arg:"subcommand"`
+	}
+
+	{
+		var args cmd
+		err := parse("get", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Empty(t, args.Get.Items)
+	}
+
+	{
+		var args cmd
+		err := parse("get --limit 5", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Empty(t, args.Get.Items)
+		assert.Equal(t, 5, args.Limit)
+	}
+
+	{
+		var args cmd
+		err := parse("get item1", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Equal(t, []string{"item1"}, args.Get.Items)
+	}
+
+	{
+		var args cmd
+		err := parse("get item1 item2 item3", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Equal(t, []string{"item1", "item2", "item3"}, args.Get.Items)
+	}
+
+	{
+		var args cmd
+		err := parse("get item1 --limit 5 item2", &args)
+		require.NoError(t, err)
+		assert.NotNil(t, args.Get)
+		assert.Equal(t, []string{"item1", "item2"}, args.Get.Items)
+		assert.Equal(t, 5, args.Limit)
+	}
+}

usage.go

@@ -12,17 +12,30 @@ import (
 // the width of the left column
 const colWidth = 25
 
+// to allow monkey patching in tests
+var stderr = os.Stderr
+
 // Fail prints usage information to stderr and exits with non-zero status
 func (p *Parser) Fail(msg string) {
-	p.WriteUsage(os.Stderr)
-	fmt.Fprintln(os.Stderr, "error:", msg)
-	os.Exit(-1)
+	p.failWithCommand(msg, p.cmd)
+}
+
+// failWithCommand prints usage information for the given subcommand to stderr and exits with non-zero status
+func (p *Parser) failWithCommand(msg string, cmd *command) {
+	p.writeUsageForCommand(stderr, cmd)
+	fmt.Fprintln(stderr, "error:", msg)
+	osExit(-1)
 }
 
 // WriteUsage writes usage information to the given writer
 func (p *Parser) WriteUsage(w io.Writer) {
+	p.writeUsageForCommand(w, p.cmd)
+}
+
+// writeUsageForCommand writes usage information for the given subcommand
+func (p *Parser) writeUsageForCommand(w io.Writer, cmd *command) {
 	var positionals, options []*spec
-	for _, spec := range p.specs {
+	for _, spec := range cmd.specs {
 		if spec.positional {
 			positionals = append(positionals, spec)
 		} else {
@@ -34,7 +47,19 @@ func (p *Parser) WriteUsage(w io.Writer) {
 		fmt.Fprintln(w, p.version)
 	}
 
-	fmt.Fprintf(w, "Usage: %s", p.config.Program)
+	// make a list of ancestor commands so that we print with full context
+	var ancestors []string
+	ancestor := cmd
+	for ancestor != nil {
+		ancestors = append(ancestors, ancestor.name)
+		ancestor = ancestor.parent
+	}
+
+	// print the beginning of the usage string
+	fmt.Fprint(w, "Usage:")
+	for i := len(ancestors) - 1; i >= 0; i-- {
+		fmt.Fprint(w, " "+ancestors[i])
+	}
 
 	// write the option component of the usage message
 	for _, spec := range options {
@@ -69,10 +94,32 @@ 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) {
+	p.writeHelpForCommand(w, p.cmd)
+}
+
+// writeHelp writes the usage string for the given subcommand
+func (p *Parser) writeHelpForCommand(w io.Writer, cmd *command) {
 	var positionals, options []*spec
-	for _, spec := range p.specs {
+	for _, spec := range cmd.specs {
 		if spec.positional {
 			positionals = append(positionals, spec)
 		} else {
@@ -83,70 +130,74 @@ func (p *Parser) WriteHelp(w io.Writer) {
 	if p.description != "" {
 		fmt.Fprintln(w, p.description)
 	}
-	p.WriteUsage(w)
+	p.writeUsageForCommand(w, cmd)
 
 	// write the list of positionals
 	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)
 		}
 	}
 
 	// write the list of options
 	fmt.Fprint(w, "\nOptions:\n")
 	for _, spec := range options {
-		printOption(w, spec)
+		p.printOption(w, spec)
 	}
 
 	// write the list of built in options
-	printOption(w, &spec{boolean: true, long: "help", short: "h", help: "display this help and exit"})
+	p.printOption(w, &spec{
+		boolean: true,
+		long:    "help",
+		short:   "h",
+		help:    "display this help and exit",
+	})
 	if p.version != "" {
-		printOption(w, &spec{boolean: true, long: "version", help: "display version and exit"})
+		p.printOption(w, &spec{
+			boolean: true,
+			long:    "version",
+			help:    "display version and exit",
+		})
+	}
+
+	// write the list of subcommands
+	if len(cmd.subcommands) > 0 {
+		fmt.Fprint(w, "\nCommands:\n")
+		for _, subcmd := range cmd.subcommands {
+			printTwoCols(w, subcmd.name, subcmd.help, nil)
+		}
 	}
 }
 
-func printOption(w io.Writer, spec *spec) {
-	left := "  " + synopsis(spec, "--"+spec.long)
+func (p *Parser) printOption(w io.Writer, spec *spec) {
+	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.
-	v := spec.dest
+	var v reflect.Value
+	if len(spec.dest.fields) > 0 {
+		v = p.val(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 {
@@ -155,3 +206,7 @@ func synopsis(spec *spec, form string) string {
 	}
 	return form + " " + strings.ToUpper(spec.long)
 }
+
+func ptrTo(s string) *string {
+	return &s
+}