Decoders are now field mappers.

Mappers are responsible for mapping from command-line input to Go. This
is typically just decoding, but also includes other information such as
if the field is a bool.

build.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 )
 
-func build(ast interface{}, extraFlags []*Flag) (app *Application, err error) {
+func build(k *Kong, ast interface{}) (app *Application, err error) {
 	defer catch(&err)
 	v := reflect.ValueOf(ast)
 	iv := reflect.Indirect(v)
@@ -15,11 +15,12 @@ func build(ast interface{}, extraFlags []*Flag) (app *Application, err error) {
 	}
 
 	app = &Application{}
+	extraFlags := k.extraFlags()
 	seenFlags := map[string]bool{}
 	for _, flag := range extraFlags {
 		seenFlags[flag.Name] = true
 	}
-	node := buildNode(iv, ApplicationNode, seenFlags)
+	node := buildNode(k, iv, ApplicationNode, seenFlags)
 	if len(node.Positional) > 0 && len(node.Children) > 0 {
 		return nil, fmt.Errorf("can't mix positional arguments and branching arguments on %T", ast)
 	}
@@ -32,7 +33,7 @@ func dashedString(s string) string {
 	return strings.Join(camelCase(s), "-")
 }
 
-func buildNode(v reflect.Value, typ NodeType, seenFlags map[string]bool) *Node {
+func buildNode(k *Kong, v reflect.Value, typ NodeType, seenFlags map[string]bool) *Node {
 	node := &Node{
 		Type:   typ,
 		Target: v,
@@ -57,9 +58,9 @@ func buildNode(v reflect.Value, typ NodeType, seenFlags map[string]bool) *Node {
 			if tag.Arg {
 				typ = ArgumentNode
 			}
-			buildChild(node, typ, v, ft, fv, tag, name, seenFlags)
+			buildChild(k, node, typ, v, ft, fv, tag, name, seenFlags)
 		} else {
-			buildField(node, v, ft, fv, tag, name, seenFlags)
+			buildField(k, node, v, ft, fv, tag, name, seenFlags)
 		}
 	}
 
@@ -82,8 +83,8 @@ func buildNode(v reflect.Value, typ NodeType, seenFlags map[string]bool) *Node {
 	return node
 }
 
-func buildChild(node *Node, typ NodeType, v reflect.Value, ft reflect.StructField, fv reflect.Value, tag *Tag, name string, seenFlags map[string]bool) {
-	child := buildNode(fv, typ, seenFlags)
+func buildChild(k *Kong, node *Node, typ NodeType, v reflect.Value, ft reflect.StructField, fv reflect.Value, tag *Tag, name string, seenFlags map[string]bool) {
+	child := buildNode(k, fv, typ, seenFlags)
 	child.Parent = node
 	child.Help = tag.Help
 
@@ -118,10 +119,10 @@ func buildChild(node *Node, typ NodeType, v reflect.Value, ft reflect.StructFiel
 	}
 }
 
-func buildField(node *Node, v reflect.Value, ft reflect.StructField, fv reflect.Value, tag *Tag, name string, seenFlags map[string]bool) {
-	decoder := DecoderForField(tag.Type, ft)
-	if decoder == nil {
-		fail("no decoder for %s.%s (of type %s)", v.Type(), ft.Name, ft.Type)
+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)
+	if mapper == nil {
+		fail("no mapper for %s.%s (of type %s)", v.Type(), ft.Name, ft.Type)
 	}
 
 	flag := !tag.Arg
@@ -131,7 +132,7 @@ func buildField(node *Node, v reflect.Value, ft reflect.StructField, fv reflect.
 		Flag:    flag,
 		Help:    tag.Help,
 		Default: tag.Default,
-		Decoder: decoder,
+		Mapper:  mapper,
 		Tag:     tag,
 		Value:   fv,
 

decoders.go

@@ -1,250 +0,0 @@
-package kong
-
-import (
-	"fmt"
-	"reflect"
-	"strconv"
-	"strings"
-	"time"
-)
-
-type DecoderContext struct {
-	// Value being decoded into.
-	Value *Value
-}
-
-// A Decoder knows how to decode text into a Go value.
-type Decoder interface {
-	// Decode scan into target.
-	//
-	// "ctx" contains context about the value being decoded that may be useful
-	// to some decoders.
-	Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error
-}
-
-type DecoderFunc func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error
-
-func (d DecoderFunc) Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-	return d(ctx, scan, target)
-}
-
-var _ Decoder = DecoderFunc(nil)
-
-type TypeDecoder interface {
-	Type() reflect.Type
-	Decoder
-}
-
-func NewTypeDecoder(typ reflect.Type, decoder DecoderFunc) TypeDecoder {
-	return &typeDecoder{typ, decoder}
-}
-
-type typeDecoder struct {
-	typ reflect.Type
-	DecoderFunc
-}
-
-func (t *typeDecoder) Type() reflect.Type { return t.typ }
-
-var _ TypeDecoder = &typeDecoder{}
-
-type KindDecoder interface {
-	Kind() reflect.Kind
-	Decoder
-}
-
-func NewKindDecoder(kind reflect.Kind, decoder DecoderFunc) KindDecoder {
-	return &kindDecoder{kind, decoder}
-}
-
-type kindDecoder struct {
-	kind reflect.Kind
-	DecoderFunc
-}
-
-func (k *kindDecoder) Kind() reflect.Kind { return k.kind }
-
-var _ KindDecoder = &kindDecoder{}
-
-// A NamedDecoder will be used if the value field has a "type" tag matching Name().
-//
-// eg.
-//
-// 		Field string `kong:"type='colour'`
-//   	kong.RegisterDecoder(kong.NewNamedDecoder("colour", ...))
-type NamedDecoder interface {
-	Name() string
-	Decoder
-}
-
-func NewNamedDecoder(name string, decoder DecoderFunc) NamedDecoder {
-	return &namedDecoder{name, decoder}
-}
-
-type namedDecoder struct {
-	name string
-	DecoderFunc
-}
-
-func (n *namedDecoder) Name() string { return n.name }
-
-var _ NamedDecoder = &namedDecoder{}
-
-var (
-	namedDecoders = map[string]NamedDecoder{}
-	typeDecoders  = map[reflect.Type]TypeDecoder{}
-	kindDecoders  = map[reflect.Kind]KindDecoder{}
-)
-
-// DecoderForField finds a decoder for a struct field.
-//
-// Will return nil if a decoder can not be determined.
-func DecoderForField(name string, field reflect.StructField) Decoder {
-	if decoder, ok := namedDecoders[name]; ok {
-		return decoder
-	}
-	return DecoderForType(field.Type)
-}
-
-// DecoderForType finds a decoder from a type or kind.
-//
-// Will return nil if a decoder can not be determined.
-func DecoderForType(typ reflect.Type) Decoder {
-	var decoder Decoder
-	var ok bool
-	if decoder, ok = typeDecoders[typ]; ok {
-		return decoder
-	} else if decoder, ok = kindDecoders[typ.Kind()]; ok {
-		return decoder
-	}
-	return nil
-}
-
-// RegisterDecoder registers decoders.
-//
-// Decoders must be one of TypeDecoder, KindDecoder or NamedDecoder.
-func RegisterDecoder(decoders ...Decoder) {
-	for _, decoder := range decoders {
-		switch decoder := decoder.(type) {
-		case TypeDecoder:
-			typeDecoders[decoder.Type()] = decoder
-		case KindDecoder:
-			kindDecoders[decoder.Kind()] = decoder
-		case NamedDecoder:
-			namedDecoders[decoder.Name()] = decoder
-		default:
-			fail("unsupported decoder type %T", decoder)
-		}
-	}
-}
-
-func init() {
-	RegisterDecoder(
-		NewKindDecoder(reflect.Int, intDecoder),
-		NewKindDecoder(reflect.Int8, intDecoder),
-		NewKindDecoder(reflect.Int16, intDecoder),
-		NewKindDecoder(reflect.Int32, intDecoder),
-		NewKindDecoder(reflect.Int64, intDecoder),
-		NewKindDecoder(reflect.Uint, uintDecoder),
-		NewKindDecoder(reflect.Uint8, uintDecoder),
-		NewKindDecoder(reflect.Uint16, uintDecoder),
-		NewKindDecoder(reflect.Uint32, uintDecoder),
-		NewKindDecoder(reflect.Uint64, uintDecoder),
-		NewKindDecoder(reflect.Float32, floatDecoder(32)),
-		NewKindDecoder(reflect.Float64, floatDecoder(64)),
-		NewKindDecoder(reflect.String, func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-			target.SetString(scan.PopValue("string"))
-			return nil
-		}),
-		NewKindDecoder(reflect.Bool, func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-			target.SetBool(true)
-			return nil
-		}),
-		NewKindDecoder(reflect.Slice, sliceDecoder),
-		NewTypeDecoder(reflect.TypeOf(time.Time{}), timeDecoder),
-		NewTypeDecoder(reflect.TypeOf(time.Duration(0)), durationDecoder),
-	)
-}
-
-func durationDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-	d, err := time.ParseDuration(scan.PopValue("duration"))
-	if err != nil {
-		return err
-	}
-	target.Set(reflect.ValueOf(d))
-	return nil
-}
-
-func timeDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-	fmt := time.RFC3339
-	if ctx.Value.Format != "" {
-		fmt = ctx.Value.Format
-	}
-	t, err := time.Parse(fmt, scan.PopValue("time"))
-	if err != nil {
-		return err
-	}
-	target.Set(reflect.ValueOf(t))
-	return nil
-}
-
-func intDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-	value := scan.PopValue("int")
-	n, err := strconv.ParseInt(value, 10, 64)
-	if err != nil {
-		return fmt.Errorf("invalid int %q", value)
-	}
-	target.SetInt(n)
-	return nil
-}
-
-func uintDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-	value := scan.PopValue("uint")
-	n, err := strconv.ParseUint(value, 10, 64)
-	if err != nil {
-		return fmt.Errorf("invalid uint %q", value)
-	}
-	target.SetUint(n)
-	return nil
-}
-
-func floatDecoder(bits int) DecoderFunc {
-	return func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-		value := scan.PopValue("float")
-		n, err := strconv.ParseFloat(value, bits)
-		if err != nil {
-			return fmt.Errorf("invalid float %q", value)
-		}
-		target.SetFloat(n)
-		return nil
-	}
-}
-
-func sliceDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
-	el := target.Type().Elem()
-	sep, ok := ctx.Value.Tag.Get("sep")
-	if !ok {
-		sep = ","
-	}
-	var childScanner *Scanner
-	if ctx.Value.Flag {
-		// If decoding a flag, we need an argument.
-		childScanner = Scan(strings.Split(scan.PopValue("list"), sep)...)
-	} else {
-		tokens := scan.PopUntil(func(t Token) bool { return !t.IsValue() })
-		childScanner = Scan(tokens...)
-	}
-	childDecoder := DecoderForType(el)
-	if childDecoder == nil {
-		return fmt.Errorf("no decoder for element type of %s", target.Type())
-	}
-	for childScanner.Peek().Type != EOLToken {
-		childValue := reflect.New(el).Elem()
-		err := childDecoder.Decode(ctx, childScanner, childValue)
-		if err != nil {
-			return err
-		}
-		target.Set(reflect.Append(target, childValue))
-	}
-	return nil
-}

decoders_test.go

@@ -1,6 +0,0 @@
-package kong
-
-import "testing"
-
-func TestDecoders(t *testing.T) {
-}

help.go

@@ -82,11 +82,11 @@ func init() {
 	})
 }
 
-// Help returns a Hook that will display help and exit.
+// Help returns a Before hook that will display help and exit.
 //
 // tmpl receives a context with several top-level values, in addition to those passed through tmplctx:
 // .Context which is of type *Context and .Path which is of type *Path.
-func Help(tmpl *raymond.Template, tmplctx map[string]interface{}) HookFunction {
+func Help(tmpl *raymond.Template, tmplctx map[string]interface{}) Before {
 	return func(ctx *Context, path *Path) error {
 		merged := map[string]interface{}{
 			"App":     ctx.App,

kong.go

@@ -8,7 +8,8 @@ import (
 	"reflect"
 )
 
-type HookFunction func(ctx *Context, path *Path) error
+// Before is a callback tied to a field of the grammar, called before a value is applied.
+type Before func(ctx *Context, path *Path) error
 
 // Error reported by Kong.
 type Error struct{ msg string }
@@ -38,7 +39,8 @@ type Kong struct {
 	Stdout io.Writer
 	Stderr io.Writer
 
-	hooks         map[reflect.Value]HookFunction
+	before        map[reflect.Value]Before
+	registry      *Registry
 	noDefaultHelp bool
 }
 
@@ -47,13 +49,18 @@ type Kong struct {
 // See the README (https://github.com/alecthomas/kong) for usage instructions.
 func New(grammar interface{}, options ...Option) (*Kong, error) {
 	k := &Kong{
-		Exit:   os.Exit,
-		Stdout: os.Stdout,
-		Stderr: os.Stderr,
-		hooks:  map[reflect.Value]HookFunction{},
+		Exit:     os.Exit,
+		Stdout:   os.Stdout,
+		Stderr:   os.Stderr,
+		before:   map[reflect.Value]Before{},
+		registry: NewRegistry().RegisterDefaults(),
 	}
 
-	model, err := build(grammar, k.extraFlags())
+	for _, option := range options {
+		option(k)
+	}
+
+	model, err := build(k, grammar)
 	if err != nil {
 		return k, err
 	}
@@ -72,13 +79,14 @@ func (k *Kong) extraFlags() []*Flag {
 		return nil
 	}
 	helpValue := false
+	value := reflect.ValueOf(&helpValue).Elem()
 	helpFlag := &Flag{
 		Value: Value{
-			Name:    "help",
-			Help:    "Show context-sensitive help.",
-			Flag:    true,
-			Value:   reflect.ValueOf(&helpValue).Elem(),
-			Decoder: kindDecoders[reflect.Bool],
+			Name:   "help",
+			Help:   "Show context-sensitive help.",
+			Flag:   true,
+			Value:  value,
+			Mapper: k.registry.ForValue(value),
 		},
 	}
 	hook := Hook(&helpValue, Help(defaultHelpTemplate, nil))
@@ -133,7 +141,7 @@ func (k *Kong) applyHooks(ctx *Context) error {
 		if key.IsValid() {
 			key = key.Addr()
 		}
-		if hook := k.hooks[key]; hook != nil {
+		if hook := k.before[key]; hook != nil {
 			if err := hook(ctx, trace); err != nil {
 				return err
 			}

mapper.go

@@ -0,0 +1,229 @@
+package kong
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+	"strings"
+	"time"
+)
+
+type DecoderContext struct {
+	// Value being decoded into.
+	Value *Value
+}
+
+// A Mapper represents how a field is mapped from command-line values to Go.
+//
+// Mappers can be associated with concrete fields via pointer, reflect.Type, reflect.Kind, or via a "type" tag.
+type Mapper interface {
+	Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error
+}
+
+// A BoolMapper is a Mapper to a value that is a boolean.
+type BoolMapper interface {
+	Mapper
+	IsBool() bool
+}
+
+type MapperFunc func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error
+
+func (d MapperFunc) Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	return d(ctx, scan, target)
+}
+
+// A Registry encapsulates a set of fields and lookups to resolve them.
+type Registry struct {
+	names  map[string]Mapper
+	types  map[reflect.Type]Mapper
+	kinds  map[reflect.Kind]Mapper
+	values map[reflect.Value]Mapper
+}
+
+func NewRegistry() *Registry {
+	return &Registry{
+		names:  map[string]Mapper{},
+		types:  map[reflect.Type]Mapper{},
+		kinds:  map[reflect.Kind]Mapper{},
+		values: map[reflect.Value]Mapper{},
+	}
+}
+
+// ForNamedType finds a mapper for a value with a user-specified type.
+//
+// Will return nil if a mapper can not be determined.
+func (d *Registry) ForNamedType(name string, value reflect.Value) Mapper {
+	if mapper, ok := d.names[name]; ok {
+		return mapper
+	}
+	return d.ForValue(value)
+}
+
+func (d *Registry) ForValue(value reflect.Value) Mapper {
+	if mapper, ok := d.values[value]; ok {
+		return mapper
+	}
+	return d.ForType(value.Type())
+}
+
+// DecoderForType finds a mapper from a type or kind.
+//
+// Will return nil if a mapper can not be determined.
+func (d *Registry) ForType(typ reflect.Type) Mapper {
+	var mapper Mapper
+	var ok bool
+	if mapper, ok = d.types[typ]; ok {
+		return mapper
+	} else if mapper, ok = d.kinds[typ.Kind()]; ok {
+		return mapper
+	}
+	return nil
+}
+
+func (d *Registry) RegisterKind(kind reflect.Kind, mapper Mapper) *Registry {
+	d.kinds[kind] = mapper
+	return d
+}
+
+// RegisterName registeres a mapper to be used if the value mapper has a "type" tag matching name.
+//
+// eg.
+//
+// 		Mapper string `kong:"type='colour'`
+//   	registry.RegisterName("colour", ...)
+func (d *Registry) RegisterName(name string, mapper Mapper) *Registry {
+	d.names[name] = mapper
+	return d
+}
+
+func (d *Registry) RegisterType(typ reflect.Type, mapper Mapper) *Registry {
+	d.types[typ] = mapper
+	return d
+}
+
+// RegisterValue registers a mapper by a pointer to the mapper value.
+func (d *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
+	return d
+}
+
+func (d *Registry) RegisterDefaults() *Registry {
+	return d.RegisterKind(reflect.Int, MapperFunc(intDecoder)).
+		RegisterKind(reflect.Int8, MapperFunc(intDecoder)).
+		RegisterKind(reflect.Int16, MapperFunc(intDecoder)).
+		RegisterKind(reflect.Int32, MapperFunc(intDecoder)).
+		RegisterKind(reflect.Int64, MapperFunc(intDecoder)).
+		RegisterKind(reflect.Uint, MapperFunc(uintDecoder)).
+		RegisterKind(reflect.Uint8, MapperFunc(uintDecoder)).
+		RegisterKind(reflect.Uint16, MapperFunc(uintDecoder)).
+		RegisterKind(reflect.Uint32, MapperFunc(uintDecoder)).
+		RegisterKind(reflect.Uint64, MapperFunc(uintDecoder)).
+		RegisterKind(reflect.Float32, floatDecoder(32)).
+		RegisterKind(reflect.Float64, floatDecoder(64)).
+		RegisterKind(reflect.String, MapperFunc(func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+			target.SetString(scan.PopValue("string"))
+			return nil
+		})).
+		RegisterKind(reflect.Bool, boolMapper{}).
+		RegisterType(reflect.TypeOf(time.Time{}), MapperFunc(timeDecoder)).
+		RegisterType(reflect.TypeOf(time.Duration(0)), MapperFunc(durationDecoder)).
+		RegisterKind(reflect.Slice, sliceDecoder(d))
+}
+
+type boolMapper struct{}
+
+func (boolMapper) Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	target.SetBool(true)
+	return nil
+}
+func (boolMapper) IsBool() bool { return true }
+
+func durationDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	d, err := time.ParseDuration(scan.PopValue("duration"))
+	if err != nil {
+		return err
+	}
+	target.Set(reflect.ValueOf(d))
+	return nil
+}
+
+func timeDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	fmt := time.RFC3339
+	if ctx.Value.Format != "" {
+		fmt = ctx.Value.Format
+	}
+	t, err := time.Parse(fmt, scan.PopValue("time"))
+	if err != nil {
+		return err
+	}
+	target.Set(reflect.ValueOf(t))
+	return nil
+}
+
+func intDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	value := scan.PopValue("int")
+	n, err := strconv.ParseInt(value, 10, 64)
+	if err != nil {
+		return fmt.Errorf("invalid int %q", value)
+	}
+	target.SetInt(n)
+	return nil
+}
+
+func uintDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	value := scan.PopValue("uint")
+	n, err := strconv.ParseUint(value, 10, 64)
+	if err != nil {
+		return fmt.Errorf("invalid uint %q", value)
+	}
+	target.SetUint(n)
+	return nil
+}
+
+func floatDecoder(bits int) MapperFunc {
+	return func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+		value := scan.PopValue("float")
+		n, err := strconv.ParseFloat(value, bits)
+		if err != nil {
+			return fmt.Errorf("invalid float %q", value)
+		}
+		target.SetFloat(n)
+		return nil
+	}
+}
+
+func sliceDecoder(d *Registry) MapperFunc {
+	return func(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+		el := target.Type().Elem()
+		sep, ok := ctx.Value.Tag.Get("sep")
+		if !ok {
+			sep = ","
+		}
+		var childScanner *Scanner
+		if ctx.Value.Flag {
+			// If decoding a flag, we need an argument.
+			childScanner = Scan(strings.Split(scan.PopValue("list"), sep)...)
+		} else {
+			tokens := scan.PopUntil(func(t Token) bool { return !t.IsValue() })
+			childScanner = Scan(tokens...)
+		}
+		childDecoder := d.ForType(el)
+		if childDecoder == nil {
+			return fmt.Errorf("no mapper for element type of %s", target.Type())
+		}
+		for childScanner.Peek().Type != EOLToken {
+			childValue := reflect.New(el).Elem()
+			err := childDecoder.Decode(ctx, childScanner, childValue)
+			if err != nil {
+				return err
+			}
+			target.Set(reflect.Append(target, childValue))
+		}
+		return nil
+	}
+}

mapper_test.go

@@ -0,0 +1,42 @@
+package kong
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestValueMapper(t *testing.T) {
+	var cli struct {
+		Flag string
+	}
+	k := mustNew(t, &cli, ValueMapper(&cli.Flag, testMooMapper{}))
+	_, err := k.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "", cli.Flag)
+	_, err = k.Parse([]string{"--flag"})
+	require.NoError(t, err)
+	require.Equal(t, "MOO", cli.Flag)
+}
+
+func TestNamedMapper(t *testing.T) {
+	var cli struct {
+		Flag string `type:"moo"`
+	}
+	k := mustNew(t, &cli, NamedMapper("moo", testMooMapper{}))
+	_, err := k.Parse(nil)
+	require.NoError(t, err)
+	require.Equal(t, "", cli.Flag)
+	_, err = k.Parse([]string{"--flag"})
+	require.NoError(t, err)
+	require.Equal(t, "MOO", cli.Flag)
+}
+
+type testMooMapper struct{}
+
+func (testMooMapper) Decode(ctx *DecoderContext, scan *Scanner, target reflect.Value) error {
+	target.SetString("MOO")
+	return nil
+}
+func (testMooMapper) IsBool() bool { return true }

model.go

@@ -130,7 +130,7 @@ type Value struct {
 	Name     string
 	Help     string
 	Default  string
-	Decoder  Decoder
+	Mapper   Mapper
 	Tag      *Tag
 	Value    reflect.Value
 	Required bool
@@ -144,13 +144,16 @@ func (v *Value) IsCumulative() bool {
 }
 
 func (v *Value) IsBool() bool {
+	if m, ok := v.Mapper.(BoolMapper); ok && m.IsBool() {
+		return true
+	}
 	return v.Value.Kind() == reflect.Bool
 }
 
 // Parse tokens into value, parse, and validate, but do not write to the field.
 func (v *Value) Parse(scan *Scanner) (reflect.Value, error) {
 	value := reflect.New(v.Value.Type()).Elem()
-	err := v.Decoder.Decode(&DecoderContext{Value: v}, scan, value)
+	err := v.Mapper.Decode(&DecoderContext{Value: v}, scan, value)
 	if err == nil {
 		v.Set = true
 	}

options.go

@@ -32,6 +32,26 @@ func Name(name string) Option {
 	}
 }
 
+// TypeMapper registers a mapper to a type.
+func TypeMapper(typ reflect.Type, mapper Mapper) Option {
+	return func(k *Kong) { k.registry.RegisterType(typ, mapper) }
+}
+
+// KindMapper registers a mapper to a kind.
+func KindMapper(kind reflect.Kind, mapper Mapper) Option {
+	return func(k *Kong) { k.registry.RegisterKind(kind, mapper) }
+}
+
+// ValueMapper registers a mapper to a field value.
+func ValueMapper(ptr interface{}, mapper Mapper) Option {
+	return func(k *Kong) { k.registry.RegisterValue(ptr, mapper) }
+}
+
+// NamedMapper registers a mapper to a name.
+func NamedMapper(name string, mapper Mapper) Option {
+	return func(k *Kong) { k.registry.RegisterName(name, mapper) }
+}
+
 // Description sets the application description.
 func Description(description string) Option {
 	return func(k *Kong) {
@@ -49,13 +69,15 @@ func Writers(stdout, stderr io.Writer) Option {
 	}
 }
 
-// Hook to execute when a command, flag or positional argument is encountered.
-func Hook(ptr interface{}, hook HookFunction) Option {
+// Hook to aply before a command, flag or positional argument is encountered.
+//
+// "ptr" is a pointer to a field of the grammar.
+func Hook(ptr interface{}, hook Before) Option {
 	key := reflect.ValueOf(ptr)
 	if key.Kind() != reflect.Ptr {
 		panic("expected a pointer")
 	}
 	return func(k *Kong) {
-		k.hooks[key] = hook
+		k.before[key] = hook
 	}
 }