Add path, existingfile and existingdir types.

- Document custom types.
- Add docker example.

README.md

@@ -1,7 +1,7 @@
 <!-- markdownlint-disable MD013 MD033 -->
 <p align="center"><img width="90%" src="kong.png" /></p>
 
-# Kong is a command-line parser for Go [![CircleCI](https://circleci.com/gh/alecthomas/kong.svg?style=svg&circle-token=477fecac758383bf281453187269b913130f17d2)](https://circleci.com/gh/alecthomas/kong)
+# Kong is a command-line parser for Go [![CircleCI](https://img.shields.io/circleci/project/github/alecthomas/kong.svg)](https://circleci.com/gh/alecthomas/kong)
 
 <!-- MarkdownTOC -->
 
@@ -13,13 +13,14 @@
 1. [Terminating positional arguments](#terminating-positional-arguments)
 1. [Slices](#slices)
 1. [Maps](#maps)
+1. [Custom named types](#custom-named-types)
 1. [Supported tags](#supported-tags)
 1. [Modifying Kong's behaviour](#modifying-kongs-behaviour)
     1. [`Name(help)` and `Description(help)` - set the application name description](#namehelp-and-descriptionhelp---set-the-application-name-description)
     1. [`Configuration(loader, paths...)` - load defaults from configuration files](#configurationloader-paths---load-defaults-from-configuration-files)
     1. [`Resolver(...)` - support for default values from external sources](#resolver---support-for-default-values-from-external-sources)
     1. [`*Mapper(...)` - customising how the command-line is mapped to Go values](#mapper---customising-how-the-command-line-is-mapped-to-go-values)
-    1. [`HelpOptions(...HelpOption)` and `Help(HelpFunc)` - customising help](#helpoptionshelpoption-and-helphelpfunc---customising-help)
+    1. [`HelpOptions(HelpPrinterOptions)` and `Help(HelpFunc)` - customising help](#helpoptionshelpprinteroptions-and-helphelpfunc---customising-help)
     1. [`Hook(&field, HookFunc)` - callback hooks to execute when the command-line is parsed](#hookfield-hookfunc---callback-hooks-to-execute-when-the-command-line-is-parsed)
     1. [Other options](#other-options)
 
@@ -176,7 +177,7 @@ You would use the following:
 ```go
 var CLI struct {
   Ls struct {
-    Files []string `arg`
+    Files []string `arg type:"existingfile"`
   } `cmd`
 }
 ```
@@ -195,12 +196,30 @@ You would use the following:
 var CLI struct {
   Config struct {
     Set struct {
-      Config map[string]float64 `arg`
+      Config map[string]float64 `arg type:"file:"`
     } `cmd`
   } `cmd`
 }
 ```
 
+## Custom named types
+
+Kong includes a number of builtin custom type mappers. These can be used by
+specifying the tag `type:"<type>"`. They are registered with the option
+function `NamedMapper(name, mapper)`.
+
+| Name              | Description                                       |
+|-------------------|---------------------------------------------------|
+| `file`            | A path. ~ expansion is applied.                   |
+| `existingfile`    | An existing path. ~ expansion is applied.         |
+| `existingdir`     | An existing directory. ~ expansion is applied.    |
+
+
+Slices and maps treat type tags specially. For slices, the `type:""` tag
+specifies the element type. For maps, the tag has the format
+`tag:"[<key>]:[<value>]"` where either may be omitted.
+
+
 ## Supported tags
 
 Tags can be in two forms:
@@ -217,7 +236,7 @@ Both can coexist with standard Tag parsing.
 | `env:"X"`              | Specify envar to use for default value.
 | `name:"X"`             | Long name, for overriding field name.       |
 | `help:"X"`             | Help text.                                  |
-| `type:"X"`             | Specify named Mapper to use.                |
+| `type:"X"`             | Specify [named types](#custom-named-types) to use.                |
 | `placeholder:"X"`      | Placeholder text.                           |
 | `default:"X"`          | Default value.                              |
 | `short:"X"`            | Short name, if flag.                        |
@@ -279,11 +298,11 @@ All builtin Go types (as well as a bunch of useful stdlib types like `time.Time`
 3. `TypeMapper(reflect.Type, Mapper)`.
 4. `ValueMapper(interface{}, Mapper)`, passing in a pointer to a field of the grammar.
 
-### `HelpOptions(...HelpOption)` and `Help(HelpFunc)` - customising help
+### `HelpOptions(HelpPrinterOptions)` and `Help(HelpFunc)` - customising help
 
-The default help output is usually sufficient, but if it's not, there are two solutions.
+The default help output is usually sufficient, but if not there are two solutions.
 
-1. Use `HelpOptions(options...HelpOption)` to configure the default help (see [HelpOption](https://godoc.org/github.com/alecthomas/kong#HelpOption) for details).
+1. Use `HelpOptions(HelpPrinterOptions)` to configure how help is formatted (see [HelpPrinterOptions](https://godoc.org/github.com/alecthomas/kong#HelpPrinterOptions) for details).
 2. Custom help can be wired into Kong via the `Help(HelpFunc)` option. The `HelpFunc` is passed a `Context`, which contains the parsed context for the current command-line. See the implementation of `PrintHelp` for an example.
 
 ### `Hook(&field, HookFunc)` - callback hooks to execute when the command-line is parsed

_examples/docker/main.go

@@ -0,0 +1,178 @@
+// nolint
+package main
+
+import (
+	"fmt"
+
+	"github.com/alecthomas/kong"
+)
+
+type AttachCmd struct {
+	DetachKeys string `help:"Override the key sequence for detaching a container"`
+	NoStdin    bool   `help:"Do not attach STDIN"`
+	SigProxy   bool   `help:"Proxy all received signals to the process" default:"true"`
+
+	Container string `arg required help:"Container ID to attach to."`
+}
+
+func (a *AttachCmd) Run() error {
+	fmt.Printf("Attaching to: %v\n", a.Container)
+	fmt.Printf("SigProxy: %v\n", a.SigProxy)
+	return nil
+}
+
+var cli struct {
+	Config       string   `help:"Location of client config files" default:"~/.docker" type:"path"`
+	Debug        bool     `short:"D" help:"Enable debug mode"`
+	Host         []string `short:"H" help:"Daemon socket(s) to connect to"`
+	LogLevel     string   `short:"l" help:"Set the logging level (debug|info|warn|error|fatal)" default:"info"`
+	TLS          bool     `help:"Use TLS; implied by --tlsverify"`
+	TLSCACert    string   `name:"tls-ca-cert" help:"Trust certs signed only by this CA" default:"~/.docker/ca.pem" type:"path"`
+	TLSCert      string   `name:"tls-cert" help:"Path to TLS certificate file" default:"~/.docker/cert.pem" type:"path"`
+	TLSKey       string   `help:"Path to TLS key file" default:"~/.docker/key.pem" type:"path"`
+	TLSVerify    bool     `help:"Use TLS and verify the remote"`
+	PrintVersion bool     `name:"version" help:"Print version information and quit"`
+
+	Attach AttachCmd `cmd help:"Attach local standard input, output, and error streams to a running container"`
+
+	Build struct {
+		Arg string `arg required`
+	} `cmd help:"Build an image from a Dockerfile"`
+	Commit struct {
+		Arg string `arg required`
+	} `cmd help:"Create a new image from a container's changes"`
+	Cp struct {
+		Arg string `arg required`
+	} `cmd help:"Copy files/folders between a container and the local filesystem"`
+	Create struct {
+		Arg string `arg required`
+	} `cmd help:"Create a new container"`
+	Deploy struct {
+		Arg string `arg required`
+	} `cmd help:"Deploy a new stack or update an existing stack"`
+	Diff struct {
+		Arg string `arg required`
+	} `cmd help:"Inspect changes to files or directories on a container's filesystem"`
+	Events struct {
+		Arg string `arg required`
+	} `cmd help:"Get real time events from the server"`
+	Exec struct {
+		Arg string `arg required`
+	} `cmd help:"Run a command in a running container"`
+	Export struct {
+		Arg string `arg required`
+	} `cmd help:"Export a container's filesystem as a tar archive"`
+	History struct {
+		Arg string `arg required`
+	} `cmd help:"Show the history of an image"`
+	Images struct {
+		Arg string `arg required`
+	} `cmd help:"List images"`
+	Import struct {
+		Arg string `arg required`
+	} `cmd help:"Import the contents from a tarball to create a filesystem image"`
+	Info struct {
+		Arg string `arg required`
+	} `cmd help:"Display system-wide information"`
+	Inspect struct {
+		Arg string `arg required`
+	} `cmd help:"Return low-level information on Docker objects"`
+	Kill struct {
+		Arg string `arg required`
+	} `cmd help:"Kill one or more running containers"`
+	Load struct {
+		Arg string `arg required`
+	} `cmd help:"Load an image from a tar archive or STDIN"`
+	Login struct {
+		Arg string `arg required`
+	} `cmd help:"Log in to a Docker registry"`
+	Logout struct {
+		Arg string `arg required`
+	} `cmd help:"Log out from a Docker registry"`
+	Logs struct {
+		Arg string `arg required`
+	} `cmd help:"Fetch the logs of a container"`
+	Pause struct {
+		Arg string `arg required`
+	} `cmd help:"Pause all processes within one or more containers"`
+	Port struct {
+		Arg string `arg required`
+	} `cmd help:"List port mappings or a specific mapping for the container"`
+	Ps struct {
+		Arg string `arg required`
+	} `cmd help:"List containers"`
+	Pull struct {
+		Arg string `arg required`
+	} `cmd help:"Pull an image or a repository from a registry"`
+	Push struct {
+		Arg string `arg required`
+	} `cmd help:"Push an image or a repository to a registry"`
+	Rename struct {
+		Arg string `arg required`
+	} `cmd help:"Rename a container"`
+	Restart struct {
+		Arg string `arg required`
+	} `cmd help:"Restart one or more containers"`
+	Rm struct {
+		Arg string `arg required`
+	} `cmd help:"Remove one or more containers"`
+	Rmi struct {
+		Arg string `arg required`
+	} `cmd help:"Remove one or more images"`
+	Run struct {
+		Arg string `arg required`
+	} `cmd help:"Run a command in a new container"`
+	Save struct {
+		Arg string `arg required`
+	} `cmd help:"Save one or more images to a tar archive (streamed to STDOUT by default)"`
+	Search struct {
+		Arg string `arg required`
+	} `cmd help:"Search the Docker Hub for images"`
+	Start struct {
+		Arg string `arg required`
+	} `cmd help:"Start one or more stopped containers"`
+	Stats struct {
+		Arg string `arg required`
+	} `cmd help:"Display a live stream of container(s) resource usage statistics"`
+	Stop struct {
+		Arg string `arg required`
+	} `cmd help:"Stop one or more running containers"`
+	Tag struct {
+		Arg string `arg required`
+	} `cmd help:"Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE"`
+	Top struct {
+		Arg string `arg required`
+	} `cmd help:"Display the running processes of a container"`
+	Unpause struct {
+		Arg string `arg required`
+	} `cmd help:"Unpause all processes within one or more containers"`
+	Update struct {
+		Arg string `arg required`
+	} `cmd help:"Update configuration of one or more containers"`
+	Version struct {
+		Arg string `arg required`
+	} `cmd help:"Show the Docker version information"`
+	Wait struct {
+		Arg string `arg required`
+	} `cmd help:"Block until one or more containers stop, then print their exit codes"`
+}
+
+func main() {
+	cmd := kong.Parse(&cli,
+		kong.Name("docker"),
+		kong.Description("A self-sufficient runtime for containers"),
+		kong.UsageOnError(),
+		kong.HelpOptions(kong.HelpPrinterOptions{
+			Compact: true,
+		}))
+	var err error
+	switch cmd {
+	case "attach <container>":
+		fmt.Println(cli.Config)
+		err = cli.Attach.Run()
+
+	default:
+		panic("unsupported command " + cmd)
+	}
+	kong.FatalIfErrorf(err)
+}

_examples/shell/main.go

@@ -24,9 +24,13 @@ var cli struct {
 
 func main() {
 	cmd := kong.Parse(&cli, kong.Description("A shell-like example app."),
-		kong.HelpOptions(kong.CompactHelp()))
+		kong.UsageOnError(),
+		kong.HelpOptions(kong.HelpPrinterOptions{
+			Compact: true,
+			Summary: true,
+		}))
 	switch cmd {
-	case "rm <paths>":
+	case "rm <path>":
 		fmt.Println(cli.Rm.Paths, cli.Rm.Force, cli.Rm.Recursive)
 
 	case "ls":

build.go

@@ -138,7 +138,7 @@ func buildChild(k *Kong, node *Node, typ NodeType, v reflect.Value, ft reflect.S
 }
 
 func buildField(k *Kong, node *Node, v reflect.Value, ft reflect.StructField, fv reflect.Value, tag *Tag, name string, seenFlags map[string]bool) {
-	mapper := k.registry.ForNamedType(tag.Type, fv)
+	mapper := k.registry.ForNamedValue(tag.Type, fv)
 	if mapper == nil {
 		fail("unsupported field type %s.%s (of type %s)", v.Type(), ft.Name, ft.Type)
 	}

context.go

@@ -97,6 +97,16 @@ func Trace(k *Kong, args []string) (*Context, error) {
 	return c, c.traceResolvers()
 }
 
+// Empty returns true if there were no arguments provided.
+func (c *Context) Empty() bool {
+	for _, path := range c.Path {
+		if !path.Resolved && path.App == nil {
+			return false
+		}
+	}
+	return true
+}
+
 // Validate the current context.
 func (c *Context) Validate() error {
 	for _, path := range c.Path {

help.go

@@ -13,46 +13,54 @@ const (
 	defaultColumnPadding = 4
 )
 
-// HelpOption configures the default help.
+// HelpPrinterOptions for HelpPrinters.
-type HelpOption func(options *helpWriterOptions)
+type HelpPrinterOptions struct {
+	// Write a one-line summary of the context.
+	Summary bool
 
-// CompactHelp writes help in a more compact form.
+	// Write help in a more compact form, but still fully-specified.
-func CompactHelp() HelpOption {
+	Compact bool
-	return func(options *helpWriterOptions) {
-		options.compact = true
-	}
 }
 
-// HelpPrinter returns a HelpFunction configured with the given HelpOptions.
+// HelpPrinter is used to print context-sensitive help.
-func HelpPrinter(options ...HelpOption) HelpFunction {
+type HelpPrinter func(options HelpPrinterOptions, ctx *Context) error
-	return func(ctx *Context) error {
+
-		w := newHelpWriter(guessWidth(ctx.App.Stdout))
+// DefaultHelpPrinter is the default HelpPrinter.
-		for _, option := range options {
+func DefaultHelpPrinter(options HelpPrinterOptions, ctx *Context) error {
-			option(&w.options)
+	if ctx.Empty() {
-		}
+		options.Summary = false
-		selected := ctx.Selected()
-		if selected == nil {
-			printApp(w, ctx.App.Model)
-		} else {
-			printCommand(w, ctx.App.Model, selected)
-		}
-		return w.Write(ctx.App.Stdout)
 	}
+	w := newHelpWriter(ctx, options)
+	selected := ctx.Selected()
+	if selected == nil {
+		printApp(w, ctx.App.Model)
+	} else {
+		printCommand(w, ctx.App.Model, selected)
+	}
+	return w.Write(ctx.App.Stdout)
 }
 
 func printApp(w *helpWriter, app *Application) {
-	w.Printf("Usage: %s", app.Summary())
+	w.Printf("Usage:  %s", app.Summary())
 	printNodeDetail(w, &app.Node)
 	cmds := app.Leaves()
 	if len(cmds) > 0 {
 		w.Print("")
-		w.Printf(`Run "%s <command> --help" for more information on a command.`, app.Name)
+		if w.Summary {
+			w.Printf(`Run "%s --help" for more information.`, app.Name)
+		} else {
+			w.Printf(`Run "%s <command> --help" for more information on a command.`, app.Name)
+		}
 	}
 }
 
 func printCommand(w *helpWriter, app *Application, cmd *Command) {
-	w.Printf("Usage: %s %s", app.Name, cmd.Summary())
+	w.Printf("Usage:  %s %s", app.Name, cmd.Summary())
 	printNodeDetail(w, cmd)
+	if w.Summary {
+		w.Print("")
+		w.Printf(`Run "%s %s --help" for more information.`, app.Name, cmd.Path())
+	}
 }
 
 func printNodeDetail(w *helpWriter, node *Node) {
@@ -60,6 +68,9 @@ func printNodeDetail(w *helpWriter, node *Node) {
 		w.Print("")
 		w.Wrap(node.Help)
 	}
+	if w.Summary {
+		return
+	}
 	if len(node.Positional) > 0 {
 		w.Print("")
 		w.Print("Arguments:")
@@ -75,7 +86,7 @@ func printNodeDetail(w *helpWriter, node *Node) {
 		w.Print("")
 		w.Print("Commands:")
 		iw := w.Indent()
-		if w.options.compact {
+		if w.Compact {
 			rows := [][2]string{}
 			for _, cmd := range cmds {
 				rows = append(rows, [2]string{cmd.Path(), cmd.Help})
@@ -100,23 +111,21 @@ func printCommandSummary(w *helpWriter, cmd *Command) {
 }
 
 type helpWriter struct {
-	indent  string
+	indent string
-	width   int
+	width  int
-	lines   *[]string
+	lines  *[]string
-	options helpWriterOptions
+	HelpPrinterOptions
 }
 
-type helpWriterOptions struct {
+func newHelpWriter(ctx *Context, options HelpPrinterOptions) *helpWriter {
-	compact bool
-}
-
-func newHelpWriter(width int) *helpWriter {
 	lines := []string{}
-	return &helpWriter{
+	w := &helpWriter{
-		indent: "",
+		indent:             "",
-		width:  width,
+		width:              guessWidth(ctx.App.Stdout),
-		lines:  &lines,
+		lines:              &lines,
+		HelpPrinterOptions: options,
 	}
+	return w
 }
 
 func (h *helpWriter) Printf(format string, args ...interface{}) {
@@ -128,7 +137,7 @@ func (h *helpWriter) Print(text string) {
 }
 
 func (h *helpWriter) Indent() *helpWriter {
-	return &helpWriter{indent: h.indent + "  ", lines: h.lines, width: h.width - 2, options: h.options}
+	return &helpWriter{indent: h.indent + "  ", lines: h.lines, width: h.width - 2, HelpPrinterOptions: h.HelpPrinterOptions}
 }
 
 func (h *helpWriter) String() string {

help_test.go

@@ -53,7 +53,7 @@ func TestHelp(t *testing.T) {
 		})
 		require.True(t, exited)
 		t.Log(w.String())
-		require.Equal(t, `Usage: test-app --required <command>
+		require.Equal(t, `Usage:  test-app --required <command>
 
 A test app.
 
@@ -89,7 +89,7 @@ Run "test-app <command> --help" for more information on a command.
 		})
 		require.True(t, exited)
 		t.Log(w.String())
-		require.Equal(t, `Usage: test-app two <three> --required --required-two --required-three
+		require.Equal(t, `Usage:  test-app two <three> --required --required-two --required-three
 
 Sub-sub-arg.
 

kong.go

@@ -38,13 +38,13 @@ type Kong struct {
 	Stdout io.Writer
 	Stderr io.Writer
 
-	before         map[reflect.Value]HookFunc
+	before        map[reflect.Value]HookFunc
-	resolvers      []ResolverFunc
+	resolvers     []ResolverFunc
-	registry       *Registry
+	registry      *Registry
-	noDefaultHelp  bool
+	noDefaultHelp bool
-	noUsageOnError bool
+	usageOnError  bool
-	help           func(*Context) error
+	help          HelpPrinter
-	helpOptions    []HelpOption
+	helpOptions   HelpPrinterOptions
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
@@ -70,7 +70,7 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 	}
 
 	if k.help == nil {
-		k.help = HelpPrinter(k.helpOptions...)
+		k.help = DefaultHelpPrinter
 	}
 
 	model, err := build(k, grammar)
@@ -108,7 +108,9 @@ func (k *Kong) extraFlags() []*Flag {
 	}
 	helpFlag.Flag = helpFlag
 	hook := Hook(&helpValue, func(ctx *Context, path *Path) error {
-		err := k.help(ctx)
+		options := k.helpOptions
+		options.Summary = false
+		err := k.help(options, ctx)
 		if err != nil {
 			return err
 		}
@@ -119,15 +121,22 @@ func (k *Kong) extraFlags() []*Flag {
 	return []*Flag{helpFlag}
 }
 
-// Help writes help for the given args to the stdout io.Writer associated with this Kong.
+// Help writes help for the given error to the stdout io.Writer associated with this Kong.
+//
+// "err" should be the error returned by Parse().
 //
 // See Help() and Writers() for overriding the help function and stdout, respectively.
-func (k *Kong) Help(args []string) error {
+func (k *Kong) Help(err error) error {
-	ctx, err := Trace(k, args)
+	var ctx *Context
-	if err != nil {
+	if perr, ok := err.(*ParseError); ok {
-		return err
+		ctx = perr.Context
+	} else {
+		ctx, err = Trace(k, nil)
+		if err != nil {
+			return err
+		}
 	}
-	return k.help(ctx)
+	return k.help(k.helpOptions, ctx)
 }
 
 // Parse arguments into target.
@@ -215,9 +224,11 @@ func (k *Kong) FatalIfErrorf(err error, args ...interface{}) {
 	}
 	k.Errorf("%s", msg)
 	// Maybe display usage information.
-	if err, ok := err.(*ParseError); ok && !k.noUsageOnError {
+	if err, ok := err.(*ParseError); ok && k.usageOnError {
 		fmt.Fprintln(k.Stdout)
-		_ = k.help(err.Context)
+		options := k.helpOptions
+		options.Summary = true
+		_ = k.help(options, err.Context)
 	}
 	k.Exit(1)
 }

mapper.go

@@ -3,6 +3,7 @@ package kong
 import (
 	"fmt"
 	"math/bits"
+	"os"
 	"reflect"
 	"strconv"
 	"strings"
@@ -20,9 +21,9 @@ type DecodeContext struct {
 }
 
 // WithScanner creates a clone of this context with a new Scanner.
-func (d *DecodeContext) WithScanner(scan *Scanner) *DecodeContext {
+func (r *DecodeContext) WithScanner(scan *Scanner) *DecodeContext {
 	return &DecodeContext{
-		Value: d.Value,
+		Value: r.Value,
 		Scan:  scan,
 	}
 }
@@ -44,8 +45,8 @@ type BoolMapper interface {
 // A MapperFunc is a single function that complies with the Mapper interface.
 type MapperFunc func(ctx *DecodeContext, target reflect.Value) error
 
-func (d MapperFunc) Decode(ctx *DecodeContext, target reflect.Value) error { //nolint: golint
+func (m MapperFunc) Decode(ctx *DecodeContext, target reflect.Value) error { //nolint: golint
-	return d(ctx, target)
+	return m(ctx, target)
 }
 
 // A Registry contains a set of mappers and supporting lookup methods.
@@ -66,42 +67,52 @@ func NewRegistry() *Registry {
 	}
 }
 
-// ForNamedType finds a mapper for a value with a user-specified type.
+// ForNamedValue finds a mapper for a value with a user-specified name.
 //
 // Will return nil if a mapper can not be determined.
-func (d *Registry) ForNamedType(name string, value reflect.Value) Mapper {
+func (r *Registry) ForNamedValue(name string, value reflect.Value) Mapper {
-	if mapper, ok := d.names[name]; ok {
+	if mapper, ok := r.names[name]; ok {
 		return mapper
 	}
-	return d.ForValue(value)
+	return r.ForValue(value)
+}
+
+// ForNamedType finds a mapper for a type with a user-specified name.
+//
+// Will return nil if a mapper can not be determined.
+func (r *Registry) ForNamedType(name string, typ reflect.Type) Mapper {
+	if mapper, ok := r.names[name]; ok {
+		return mapper
+	}
+	return r.ForType(typ)
 }
 
 // ForValue looks up the Mapper for a reflect.Value.
-func (d *Registry) ForValue(value reflect.Value) Mapper {
+func (r *Registry) ForValue(value reflect.Value) Mapper {
-	if mapper, ok := d.values[value]; ok {
+	if mapper, ok := r.values[value]; ok {
 		return mapper
 	}
-	return d.ForType(value.Type())
+	return r.ForType(value.Type())
 }
 
 // ForType finds a mapper from a type, by type, then kind.
 //
 // Will return nil if a mapper can not be determined.
-func (d *Registry) ForType(typ reflect.Type) Mapper {
+func (r *Registry) ForType(typ reflect.Type) Mapper {
 	var mapper Mapper
 	var ok bool
-	if mapper, ok = d.types[typ]; ok {
+	if mapper, ok = r.types[typ]; ok {
 		return mapper
-	} else if mapper, ok = d.kinds[typ.Kind()]; ok {
+	} else if mapper, ok = r.kinds[typ.Kind()]; ok {
 		return mapper
 	}
 	return nil
 }
 
 // RegisterKind registers a Mapper for a reflect.Kind.
-func (d *Registry) RegisterKind(kind reflect.Kind, mapper Mapper) *Registry {
+func (r *Registry) RegisterKind(kind reflect.Kind, mapper Mapper) *Registry {
-	d.kinds[kind] = mapper
+	r.kinds[kind] = mapper
-	return d
+	return r
 }
 
 // RegisterName registeres a mapper to be used if the value mapper has a "type" tag matching name.
@@ -110,31 +121,31 @@ func (d *Registry) RegisterKind(kind reflect.Kind, mapper Mapper) *Registry {
 //
 // 		Mapper string `kong:"type='colour'`
 //   	registry.RegisterName("colour", ...)
-func (d *Registry) RegisterName(name string, mapper Mapper) *Registry {
+func (r *Registry) RegisterName(name string, mapper Mapper) *Registry {
-	d.names[name] = mapper
+	r.names[name] = mapper
-	return d
+	return r
 }
 
 // RegisterType registers a Mapper for a reflect.Type.
-func (d *Registry) RegisterType(typ reflect.Type, mapper Mapper) *Registry {
+func (r *Registry) RegisterType(typ reflect.Type, mapper Mapper) *Registry {
-	d.types[typ] = mapper
+	r.types[typ] = mapper
-	return d
+	return r
 }
 
 // RegisterValue registers a Mapper by pointer to the field value.
-func (d *Registry) RegisterValue(ptr interface{}, mapper Mapper) *Registry {
+func (r *Registry) RegisterValue(ptr interface{}, mapper Mapper) *Registry {
 	key := reflect.ValueOf(ptr)
 	if key.Kind() != reflect.Ptr {
 		panic("expected a pointer")
 	}
 	key = key.Elem()
-	d.values[key] = mapper
+	r.values[key] = mapper
-	return d
+	return r
 }
 
 // RegisterDefaults registers Mappers for all builtin supported Go types and some common stdlib types.
-func (d *Registry) RegisterDefaults() *Registry {
+func (r *Registry) RegisterDefaults() *Registry {
-	return d.RegisterKind(reflect.Int, intDecoder(bits.UintSize)).
+	return r.RegisterKind(reflect.Int, intDecoder(bits.UintSize)).
 		RegisterKind(reflect.Int8, intDecoder(8)).
 		RegisterKind(reflect.Int16, intDecoder(16)).
 		RegisterKind(reflect.Int32, intDecoder(32)).
@@ -153,8 +164,11 @@ func (d *Registry) RegisterDefaults() *Registry {
 		RegisterKind(reflect.Bool, boolMapper{}).
 		RegisterType(reflect.TypeOf(time.Time{}), timeDecoder()).
 		RegisterType(reflect.TypeOf(time.Duration(0)), durationDecoder()).
-		RegisterKind(reflect.Slice, sliceDecoder(d)).
+		RegisterKind(reflect.Slice, sliceDecoder(r)).
-		RegisterKind(reflect.Map, mapDecoder(d))
+		RegisterKind(reflect.Map, mapDecoder(r)).
+		RegisterName("path", pathMapper(r)).
+		RegisterName("existingfile", existingFileMapper(r)).
+		RegisterName("existingdir", existingDirMapper(r))
 }
 
 type boolMapper struct{}
@@ -167,11 +181,11 @@ func (boolMapper) IsBool() bool { return true }
 
 func durationDecoder() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		d, err := time.ParseDuration(ctx.Scan.PopValue("duration"))
+		r, err := time.ParseDuration(ctx.Scan.PopValue("duration"))
 		if err != nil {
 			return err
 		}
-		target.Set(reflect.ValueOf(d))
+		target.Set(reflect.ValueOf(r))
 		return nil
 	}
 }
@@ -227,7 +241,7 @@ func floatDecoder(bits int) MapperFunc {
 	}
 }
 
-func mapDecoder(d *Registry) MapperFunc {
+func mapDecoder(r *Registry) MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
 		if target.IsNil() {
 			target.Set(reflect.MakeMap(target.Type()))
@@ -240,15 +254,24 @@ func mapDecoder(d *Registry) MapperFunc {
 		}
 		key, value := parts[0], parts[1]
 
+		keyTypeName, valueTypeName := "", ""
+		if typ := ctx.Value.Tag.Type; typ != "" {
+			parts := strings.Split(typ, ":")
+			if len(parts) != 2 {
+				return fmt.Errorf("type:\"\" on map field must be in the form \"[<keytype>]:[<valuetype>]\"")
+			}
+			keyTypeName, valueTypeName = parts[0], parts[1]
+		}
+
 		keyScanner := Scan(key)
-		keyDecoder := d.ForType(el.Key())
+		keyDecoder := r.ForNamedType(keyTypeName, el.Key())
 		keyValue := reflect.New(el.Key()).Elem()
 		if err := keyDecoder.Decode(ctx.WithScanner(keyScanner), keyValue); err != nil {
 			return fmt.Errorf("invalid map key %q", key)
 		}
 
 		valueScanner := Scan(value)
-		valueDecoder := d.ForType(el.Elem())
+		valueDecoder := r.ForNamedType(valueTypeName, el.Elem())
 		valueValue := reflect.New(el.Elem()).Elem()
 		if err := valueDecoder.Decode(ctx.WithScanner(valueScanner), valueValue); err != nil {
 			return fmt.Errorf("invalid map value %q", value)
@@ -259,7 +282,7 @@ func mapDecoder(d *Registry) MapperFunc {
 	}
 }
 
-func sliceDecoder(d *Registry) MapperFunc {
+func sliceDecoder(r *Registry) MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
 		el := target.Type().Elem()
 		sep := ctx.Value.Tag.Sep
@@ -271,7 +294,7 @@ func sliceDecoder(d *Registry) MapperFunc {
 			tokens := ctx.Scan.PopWhile(func(t Token) bool { return t.IsValue() })
 			childScanner = Scan(tokens...)
 		}
-		childDecoder := d.ForType(el)
+		childDecoder := r.ForNamedType(ctx.Value.Tag.Type, el)
 		if childDecoder == nil {
 			return fmt.Errorf("no mapper for element type of %s", target.Type())
 		}
@@ -287,6 +310,56 @@ func sliceDecoder(d *Registry) MapperFunc {
 	}
 }
 
+func pathMapper(r *Registry) MapperFunc {
+	return func(ctx *DecodeContext, target reflect.Value) error {
+		if target.Kind() == reflect.Slice {
+			return sliceDecoder(r)(ctx, target)
+		}
+		path := ctx.Scan.PopValue("file")
+		path = expandPath(path)
+		target.SetString(path)
+		return nil
+	}
+}
+
+func existingFileMapper(r *Registry) MapperFunc {
+	return func(ctx *DecodeContext, target reflect.Value) error {
+		if target.Kind() == reflect.Slice {
+			return sliceDecoder(r)(ctx, target)
+		}
+		path := ctx.Scan.PopValue("file")
+		path = expandPath(path)
+		stat, err := os.Stat(path)
+		if err != nil {
+			return err
+		}
+		if stat.IsDir() {
+			return fmt.Errorf("%q exists but is a directory", path)
+		}
+		target.SetString(path)
+		return nil
+	}
+}
+
+func existingDirMapper(r *Registry) MapperFunc {
+	return func(ctx *DecodeContext, target reflect.Value) error {
+		if target.Kind() == reflect.Slice {
+			return sliceDecoder(r)(ctx, target)
+		}
+		path := ctx.Scan.PopValue("file")
+		path = expandPath(path)
+		stat, err := os.Stat(path)
+		if err != nil {
+			return err
+		}
+		if !stat.IsDir() {
+			return fmt.Errorf("%q exists but is not a directory", path)
+		}
+		target.SetString(path)
+		return nil
+	}
+}
+
 // SplitEscaped splits a string on a separator.
 //
 // It differs from strings.Split() in that the separator can exist in a field by escaping it with a \. eg.

mapper_test.go

@@ -34,10 +34,16 @@ func TestNamedMapper(t *testing.T) {
 	require.Equal(t, "MOO", cli.Flag)
 }
 
-type testMooMapper struct{}
+type testMooMapper struct {
+	text string
+}
 
-func (testMooMapper) Decode(ctx *DecodeContext, target reflect.Value) error {
+func (t testMooMapper) Decode(ctx *DecodeContext, target reflect.Value) error {
-	target.SetString("MOO")
+	if t.text == "" {
+		target.SetString("MOO")
+	} else {
+		target.SetString(t.text)
+	}
 	return nil
 }
 func (testMooMapper) IsBool() bool { return true }
@@ -75,3 +81,17 @@ func TestJoinEscaped(t *testing.T) {
 	require.Equal(t, `a\,b,c`, JoinEscaped([]string{`a,b`, `c`}, ','))
 	require.Equal(t, JoinEscaped(SplitEscaped(`a\,b,c`, ','), ','), `a\,b,c`)
 }
+
+func TestMapWithNamedTypes(t *testing.T) {
+	var cli struct {
+		TypedValue map[string]string `type:":moo"`
+		TypedKey   map[string]string `type:"upper:"`
+	}
+	k := mustNew(t, &cli, NamedMapper("moo", testMooMapper{}), NamedMapper("upper", testUppercaseMapper{}))
+	_, err := k.Parse([]string{"--typed-value", "first=5s", "--typed-value", "second=10s"})
+	require.NoError(t, err)
+	require.Equal(t, map[string]string{"first": "MOO", "second": "MOO"}, cli.TypedValue)
+	_, err = k.Parse([]string{"--typed-key", "first=5s", "--typed-key", "second=10s"})
+	require.NoError(t, err)
+	require.Equal(t, map[string]string{"FIRST": "5s", "SECOND": "10s"}, cli.TypedKey)
+}

options.go

@@ -111,31 +111,26 @@ func Hook(ptr interface{}, hook HookFunc) Option {
 	}
 }
 
-// HelpFunction is the type of a function used to display help.
+// Help printer to use.
-type HelpFunction func(*Context) error
+func Help(help HelpPrinter) Option {
-
-// Help function to use.
-//
-// Defaults to PrintHelp.
-func Help(help HelpFunction) Option {
 	return func(k *Kong) error {
 		k.help = help
 		return nil
 	}
 }
 
-// HelpOptions specifies options for the default help printer, if used.
+// HelpOptions sets the HelpPrinterOptions to use for printing help.
-func HelpOptions(options ...HelpOption) Option {
+func HelpOptions(options HelpPrinterOptions) Option {
 	return func(k *Kong) error {
 		k.helpOptions = options
 		return nil
 	}
 }
 
-// NoUsageOnError configures Kong to NOT display context-sensitive usage if FatalIfErrorf is called with an error.
+// UsageOnError configures Kong to display context-sensitive usage if FatalIfErrorf is called with an error.
-func NoUsageOnError() Option {
+func UsageOnError() Option {
 	return func(k *Kong) error {
-		k.noUsageOnError = true
+		k.usageOnError = true
 		return nil
 	}
 }