Use interface{} instead of string in tokens.

This allows the scanner and resolvers to pass Go types around rather
than having to serialise/deserialise to/from strings.

build.go

@@ -6,8 +6,6 @@ import (
 	"strings"
 )
 
-var helpProviderType = reflect.TypeOf((*HelpProvider)(nil)).Elem()
-
 func build(k *Kong, ast interface{}) (app *Application, err error) {
 	defer catch(&err)
 	v := reflect.ValueOf(ast)
@@ -135,8 +133,8 @@ func buildChild(k *Kong, node *Node, typ NodeType, v reflect.Value, ft reflect.S
 	child.Hidden = tag.Hidden
 	child.Group = tag.Group
 
-	if fv.Type().Implements(helpProviderType) {
+	if provider, ok := fv.Addr().Interface().(HelpProvider); ok {
-		child.Detail = fv.Interface().(HelpProvider).Help()
+		child.Detail = provider.Help()
 	}
 
 	// A branching argument. This is a bit hairy, as we let buildNode() do the parsing, then check that

context.go

@@ -258,9 +258,12 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 		token := c.scan.Peek()
 		switch token.Type {
 		case UntypedToken:
+			switch v := token.Value.(type) {
+			case string:
+
 				switch {
 				// Indicates end of parsing. All remaining arguments are treated as positional arguments only.
-			case token.Value == "--":
+				case v == "--":
 					c.scan.Pop()
 					args := []string{}
 					for {
@@ -268,7 +271,7 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 						if token.Type == EOLToken {
 							break
 						}
-					args = append(args, token.Value)
+						args = append(args, token.String())
 					}
 					// Note: tokens must be pushed in reverse order.
 					for i := range args {
@@ -276,44 +279,48 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 					}
 
 				// Long flag.
-			case strings.HasPrefix(token.Value, "--"):
+				case strings.HasPrefix(v, "--"):
 					c.scan.Pop()
 					// Parse it and push the tokens.
-				parts := strings.SplitN(token.Value[2:], "=", 2)
+					parts := strings.SplitN(v[2:], "=", 2)
 					if len(parts) > 1 {
 						c.scan.PushTyped(parts[1], FlagValueToken)
 					}
 					c.scan.PushTyped(parts[0], FlagToken)
 
 				// Short flag.
-			case strings.HasPrefix(token.Value, "-"):
+				case strings.HasPrefix(v, "-"):
 					c.scan.Pop()
 					// Note: tokens must be pushed in reverse order.
-				if tail := token.Value[2:]; tail != "" {
+					if tail := v[2:]; tail != "" {
 						c.scan.PushTyped(tail, ShortFlagTailToken)
 					}
-				c.scan.PushTyped(token.Value[1:2], ShortFlagToken)
+					c.scan.PushTyped(v[1:2], ShortFlagToken)
 
 				default:
 					c.scan.Pop()
 					c.scan.PushTyped(token.Value, PositionalArgumentToken)
 				}
+			default:
+				c.scan.Pop()
+				c.scan.PushTyped(token.Value, PositionalArgumentToken)
+			}
 
 		case ShortFlagTailToken:
 			c.scan.Pop()
 			// Note: tokens must be pushed in reverse order.
-			if tail := token.Value[1:]; tail != "" {
+			if tail := token.String()[1:]; tail != "" {
 				c.scan.PushTyped(tail, ShortFlagTailToken)
 			}
-			c.scan.PushTyped(token.Value[0:1], ShortFlagToken)
+			c.scan.PushTyped(token.String()[0:1], ShortFlagToken)
 
 		case FlagToken:
-			if err := c.parseFlag(flags, "--"+token.Value); err != nil {
+			if err := c.parseFlag(flags, token.String()); err != nil {
 				return err
 			}
 
 		case ShortFlagToken:
-			if err := c.parseFlag(flags, "-"+token.Value); err != nil {
+			if err := c.parseFlag(flags, token.String()); err != nil {
 				return err
 			}
 
@@ -369,7 +376,7 @@ func (c *Context) trace(node *Node) (err error) { // nolint: gocyclo
 				}
 			}
 
-			return findPotentialCandidates(token.Value, candidates, "unexpected argument %s", token)
+			return findPotentialCandidates(token.String(), candidates, "unexpected argument %s", token)
 		default:
 			return fmt.Errorf("unexpected token %s", token)
 		}
@@ -396,7 +403,7 @@ func (c *Context) Resolve() error {
 				if err != nil {
 					return err
 				}
-				if s == "" {
+				if s == nil {
 					continue
 				}
 

go.mod

@@ -2,6 +2,8 @@ module github.com/alecthomas/kong
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/mitchellh/mapstructure v1.1.2
+	github.com/pkg/errors v0.8.1
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/stretchr/testify v1.2.2
 )

go.sum

@@ -1,5 +1,9 @@
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/mitchellh/mapstructure v1.1.2 h1:fmNYVwqnSfB9mZU6OS2O6GsXM+wcskZDuKQzvN1EDeE=
+github.com/mitchellh/mapstructure v1.1.2/go.mod h1:FVVH3fgwuzCH5S8UJGiWEs2h04kUh9fWfEaFds41c1Y=
+github.com/pkg/errors v0.8.1 h1:iURUrRGxPUNPdy5/HRSm+Yj6okJ6UtLINN0Q9M4+h3I=
+github.com/pkg/errors v0.8.1/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1w=

mapper.go

@@ -1,7 +1,7 @@
 package kong
 
 import (
-	"fmt"
+	"encoding/json"
 	"io/ioutil"
 	"math/bits"
 	"net/url"
@@ -10,6 +10,8 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"github.com/pkg/errors"
 )
 
 var (
@@ -194,9 +196,8 @@ func (r *Registry) RegisterDefaults() *Registry {
 		RegisterKind(reflect.Float32, floatDecoder(32)).
 		RegisterKind(reflect.Float64, floatDecoder(64)).
 		RegisterKind(reflect.String, MapperFunc(func(ctx *DecodeContext, target reflect.Value) error {
-			token := ctx.Scan.PopValue("string")
+			_, err := ctx.Scan.PopValueInto("string", target.Addr().Interface())
-			target.SetString(token)
+			return err
-			return nil
 		})).
 		RegisterKind(reflect.Bool, boolMapper{}).
 		RegisterKind(reflect.Slice, sliceDecoder(r)).
@@ -214,7 +215,16 @@ type boolMapper struct{}
 func (boolMapper) Decode(ctx *DecodeContext, target reflect.Value) error {
 	if ctx.Scan.Peek().Type == FlagValueToken {
 		token := ctx.Scan.Pop()
-		target.SetBool(token.Value == "true")
+		switch v := token.Value.(type) {
+		case string:
+			target.SetBool(strings.ToLower(v) == "true")
+
+		case bool:
+			target.SetBool(v)
+
+		default:
+			return errors.Errorf("expected bool but got %q (%T)", token.Value, token.Value)
+		}
 	} else {
 		target.SetBool(true)
 	}
@@ -224,10 +234,13 @@ func (boolMapper) IsBool() bool { return true }
 
 func durationDecoder() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		value := ctx.Scan.PopValue("duration")
+		var value string
+		if _, err := ctx.Scan.PopValueInto("duration", &value); err != nil {
+			return err
+		}
 		r, err := time.ParseDuration(value)
 		if err != nil {
-			return fmt.Errorf("expected duration but got %q: %s", value, err)
+			return errors.Errorf("expected duration but got %q: %s", value, err)
 		}
 		target.Set(reflect.ValueOf(r))
 		return nil
@@ -240,7 +253,10 @@ func timeDecoder() MapperFunc {
 		if ctx.Value.Format != "" {
 			format = ctx.Value.Format
 		}
-		value := ctx.Scan.PopValue("time")
+		var value string
+		if _, err := ctx.Scan.PopValueInto("time", &value); err != nil {
+			return err
+		}
 		t, err := time.Parse(format, value)
 		if err != nil {
 			return err
@@ -250,38 +266,86 @@ func timeDecoder() MapperFunc {
 	}
 }
 
-func intDecoder(bits int) MapperFunc {
+func intDecoder(bits int) MapperFunc { // nolint: dupl
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		value := ctx.Scan.PopValue("int")
+		t, err := ctx.Scan.PopValue("int")
-		n, err := strconv.ParseInt(value, 10, bits)
 		if err != nil {
-			return fmt.Errorf("expected int but got %q", value)
+			return err
+		}
+		switch v := t.Value.(type) {
+		case string:
+			n, err := strconv.ParseInt(v, 10, bits)
+			if err != nil {
+				return errors.Errorf("expected an int but got %q (%T)", t, t.Value)
 			}
 			target.SetInt(n)
+
+		case float64:
+			target.SetInt(int64(v))
+
+		case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
+			target.Set(reflect.ValueOf(v))
+
+		default:
+			return errors.Errorf("expected an int but got %q (%T)", t, t.Value)
+		}
 		return nil
 	}
 }
 
-func uintDecoder(bits int) MapperFunc {
+func uintDecoder(bits int) MapperFunc { // nolint: dupl
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		value := ctx.Scan.PopValue("uint")
+		t, err := ctx.Scan.PopValue("uint")
-		n, err := strconv.ParseUint(value, 10, bits)
 		if err != nil {
-			return fmt.Errorf("expected unsigned int but got %q", value)
+			return err
+		}
+		switch v := t.Value.(type) {
+		case string:
+			n, err := strconv.ParseUint(v, 10, bits)
+			if err != nil {
+				return errors.Errorf("expected a uint but got %q (%T)", t, t.Value)
 			}
 			target.SetUint(n)
+
+		case float64:
+			target.SetUint(uint64(v))
+
+		case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
+			target.Set(reflect.ValueOf(v))
+
+		default:
+			return errors.Errorf("expected an int but got %q (%T)", t, t.Value)
+		}
 		return nil
 	}
 }
 
 func floatDecoder(bits int) MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		value := ctx.Scan.PopValue("float")
+		t, err := ctx.Scan.PopValue("float")
-		n, err := strconv.ParseFloat(value, bits)
 		if err != nil {
-			return fmt.Errorf("expected float but got %q", value)
+			return err
+		}
+		switch v := t.Value.(type) {
+		case string:
+			n, err := strconv.ParseFloat(v, bits)
+			if err != nil {
+				return errors.Errorf("expected a float but got %q (%T)", t, t.Value)
 			}
 			target.SetFloat(n)
+
+		case float32:
+			target.SetFloat(float64(v))
+
+		case float64:
+			target.SetFloat(v)
+
+		case int, int8, int16, int32, int64, uint, uint8, uint16, uint32, uint64:
+			target.Set(reflect.ValueOf(v))
+
+		default:
+			return errors.Errorf("expected an int but got %q (%T)", t, t.Value)
+		}
 		return nil
 	}
 }
@@ -294,17 +358,43 @@ func mapDecoder(r *Registry) MapperFunc {
 		el := target.Type()
 		var childScanner *Scanner
 		if ctx.Value.Flag != nil {
+			t := ctx.Scan.Pop()
 			// If decoding a flag, we need an argument.
-			childScanner = Scan(SplitEscaped(ctx.Scan.PopValue("map"), ';')...)
+			if t.IsEOL() {
+				return errors.Errorf("unexpected EOL")
+			}
+			switch v := t.Value.(type) {
+			case string:
+				childScanner = Scan(SplitEscaped(v, ';')...)
+
+			case []map[string]interface{}:
+				for _, m := range v {
+					err := jsonTranscode(m, target.Addr().Interface())
+					if err != nil {
+						return errors.WithStack(err)
+					}
+				}
+				return nil
+
+			case map[string]interface{}:
+				return jsonTranscode(v, target.Addr().Interface())
+
+			default:
+				return errors.Errorf("invalid map value %q (of type %T)", t, t.Value)
+			}
 		} else {
 			tokens := ctx.Scan.PopWhile(func(t Token) bool { return t.IsValue() })
 			childScanner = ScanFromTokens(tokens...)
 		}
 		for !childScanner.Peek().IsEOL() {
-			token := childScanner.PopValue("map")
+			var token string
+			_, err := childScanner.PopValueInto("map", &token)
+			if err != nil {
+				return err
+			}
 			parts := strings.SplitN(token, "=", 2)
 			if len(parts) != 2 {
-				return fmt.Errorf("expected \"<key>=<value>\" but got %q", token)
+				return errors.Errorf("expected \"<key>=<value>\" but got %q", token)
 			}
 			key, value := parts[0], parts[1]
 
@@ -312,7 +402,7 @@ func mapDecoder(r *Registry) MapperFunc {
 			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>]\"")
+					return errors.Errorf("type:\"\" on map field must be in the form \"[<keytype>]:[<valuetype>]\"")
 				}
 				keyTypeName, valueTypeName = parts[0], parts[1]
 			}
@@ -321,14 +411,14 @@ func mapDecoder(r *Registry) MapperFunc {
 			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)
+				return errors.Errorf("invalid map key %q", key)
 			}
 
 			valueScanner := Scan(value)
 			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)
+				return errors.Errorf("invalid map value %q", value)
 			}
 
 			target.SetMapIndex(keyValue, valueValue)
@@ -343,21 +433,35 @@ func sliceDecoder(r *Registry) MapperFunc {
 		sep := ctx.Value.Tag.Sep
 		var childScanner *Scanner
 		if ctx.Value.Flag != nil {
+			t := ctx.Scan.Pop()
 			// If decoding a flag, we need an argument.
-			childScanner = Scan(SplitEscaped(ctx.Scan.PopValue("list"), sep)...)
+			if t.IsEOL() {
+				return errors.Errorf("unexpected EOL")
+			}
+			switch v := t.Value.(type) {
+			case string:
+				childScanner = Scan(SplitEscaped(v, sep)...)
+
+			case []interface{}:
+				return jsonTranscode(v, target.Addr().Interface())
+
+			default:
+				v = []interface{}{v}
+				return jsonTranscode(v, target.Addr().Interface())
+			}
 		} else {
 			tokens := ctx.Scan.PopWhile(func(t Token) bool { return t.IsValue() })
 			childScanner = ScanFromTokens(tokens...)
 		}
 		childDecoder := r.ForNamedType(ctx.Value.Tag.Type, el)
 		if childDecoder == nil {
-			return fmt.Errorf("no mapper for element type of %s", target.Type())
+			return errors.Errorf("no mapper for element type of %s", target.Type())
 		}
 		for !childScanner.Peek().IsEOL() {
 			childValue := reflect.New(el).Elem()
 			err := childDecoder.Decode(ctx.WithScanner(childScanner), childValue)
 			if err != nil {
-				return err
+				return errors.WithStack(err)
 			}
 			target.Set(reflect.Append(target, childValue))
 		}
@@ -371,9 +475,13 @@ func pathMapper(r *Registry) MapperFunc {
 			return sliceDecoder(r)(ctx, target)
 		}
 		if target.Kind() != reflect.String {
-			return fmt.Errorf("\"path\" type must be applied to a string not %s", target.Type())
+			return errors.Errorf("\"path\" type must be applied to a string not %s", target.Type())
+		}
+		var path string
+		_, err := ctx.Scan.PopValueInto("file", &path)
+		if err != nil {
+			return err
 		}
-		path := ctx.Scan.PopValue("file")
 		path = ExpandPath(path)
 		target.SetString(path)
 		return nil
@@ -386,16 +494,20 @@ func existingFileMapper(r *Registry) MapperFunc {
 			return sliceDecoder(r)(ctx, target)
 		}
 		if target.Kind() != reflect.String {
-			return fmt.Errorf("\"existingfile\" type must be applied to a string not %s", target.Type())
+			return errors.Errorf("\"existingfile\" type must be applied to a string not %s", target.Type())
+		}
+		var path string
+		_, err := ctx.Scan.PopValueInto("file", &path)
+		if err != nil {
+			return err
 		}
-		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)
+			return errors.Errorf("%q exists but is a directory", path)
 		}
 		target.SetString(path)
 		return nil
@@ -408,16 +520,20 @@ func existingDirMapper(r *Registry) MapperFunc {
 			return sliceDecoder(r)(ctx, target)
 		}
 		if target.Kind() != reflect.String {
-			return fmt.Errorf("\"existingdir\" must be applied to a string not %s", target.Type())
+			return errors.Errorf("\"existingdir\" must be applied to a string not %s", target.Type())
+		}
+		var path string
+		_, err := ctx.Scan.PopValueInto("file", &path)
+		if err != nil {
+			return err
 		}
-		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)
+			return errors.Errorf("%q exists but is not a directory", path)
 		}
 		target.SetString(path)
 		return nil
@@ -426,10 +542,15 @@ func existingDirMapper(r *Registry) MapperFunc {
 
 func urlMapper() MapperFunc {
 	return func(ctx *DecodeContext, target reflect.Value) error {
-		url, err := url.Parse(ctx.Scan.PopValue("url"))
+		var urlStr string
+		_, err := ctx.Scan.PopValueInto("url", &urlStr)
 		if err != nil {
 			return err
 		}
+		url, err := url.Parse(urlStr)
+		if err != nil {
+			return errors.WithStack(err)
+		}
 		target.Set(reflect.ValueOf(url))
 		return nil
 	}
@@ -479,11 +600,24 @@ func JoinEscaped(s []string, sep rune) string {
 type FileContentFlag []byte
 
 func (f *FileContentFlag) Decode(ctx *DecodeContext) error { // nolint: golint
-	filename := ExpandPath(ctx.Scan.PopValue("filename"))
+	var filename string
+	_, err := ctx.Scan.PopValueInto("filename", &filename)
+	if err != nil {
+		return err
+	}
+	filename = ExpandPath(filename)
 	data, err := ioutil.ReadFile(filename) // nolint: gosec
 	if err != nil {
-		return fmt.Errorf("failed to open %q: %s", filename, err)
+		return errors.Errorf("failed to open %q: %s", filename, err)
 	}
 	*f = data
 	return nil
 }
+
+func jsonTranscode(in, out interface{}) error {
+	data, err := json.Marshal(in)
+	if err != nil {
+		return errors.WithStack(err)
+	}
+	return errors.Wrapf(json.Unmarshal(data, out), "%#v -> %T", in, out)
+}

mapper_test.go

@@ -140,8 +140,8 @@ type mappedValue struct {
 }
 
 func (m *mappedValue) Decode(ctx *kong.DecodeContext) error {
-	m.decoded = ctx.Scan.PopValue("mapped")
+	_, err := ctx.Scan.PopValueInto("mapped", &m.decoded)
-	return nil
+	return err
 }
 
 func TestMapperValue(t *testing.T) {

resolver.go

@@ -2,7 +2,6 @@ package kong
 
 import (
 	"encoding/json"
-	"fmt"
 	"io"
 	"strings"
 )
@@ -15,15 +14,15 @@ type Resolver interface {
 	Validate(app *Application) error
 
 	// Resolve the value for a Flag.
-	Resolve(context *Context, parent *Path, flag *Flag) (string, error)
+	Resolve(context *Context, parent *Path, flag *Flag) (interface{}, error)
 }
 
 // ResolverFunc is a convenience type for non-validating Resolvers.
-type ResolverFunc func(context *Context, parent *Path, flag *Flag) (string, error)
+type ResolverFunc func(context *Context, parent *Path, flag *Flag) (interface{}, error)
 
 var _ Resolver = ResolverFunc(nil)
 
-func (r ResolverFunc) Resolve(context *Context, parent *Path, flag *Flag) (string, error) { // nolint: golint
+func (r ResolverFunc) Resolve(context *Context, parent *Path, flag *Flag) (interface{}, error) { // nolint: golint
 	return r(context, parent, flag)
 }
 func (r ResolverFunc) Validate(app *Application) error { return nil } //  nolint: golint
@@ -37,54 +36,14 @@ func JSON(r io.Reader) (Resolver, error) {
 	if err != nil {
 		return nil, err
 	}
-	var f ResolverFunc = func(context *Context, parent *Path, flag *Flag) (string, error) {
+	var f ResolverFunc = func(context *Context, parent *Path, flag *Flag) (interface{}, error) {
 		name := strings.Replace(flag.Name, "-", "_", -1)
 		raw, ok := values[name]
 		if !ok {
-			return "", nil
+			return nil, nil
 		}
-		sep := flag.Tag.Sep
+		return raw, nil
-		value, err := jsonDecodeValue(sep, raw)
-		if err != nil {
-			return "", err
-		}
-		return value, nil
 	}
 
 	return f, nil
 }
-
-func jsonDecodeValue(sep rune, value interface{}) (string, error) {
-	switch v := value.(type) {
-	case string:
-		return v, nil
-	case float64:
-		return fmt.Sprintf("%v", v), nil
-	case []interface{}:
-		out := []string{}
-		for _, el := range v {
-			sel, err := jsonDecodeValue(sep, el)
-			if err != nil {
-				return "", err
-			}
-			out = append(out, sel)
-		}
-		return JoinEscaped(out, sep), nil
-	case map[string]interface{}:
-		out := []string{}
-		for key, el := range v {
-			sel, err := jsonDecodeValue(sep, el)
-			if err != nil {
-				return "", err
-			}
-			out = append(out, fmt.Sprintf("%s=%s", key, sel))
-		}
-		return JoinEscaped(out, ';'), nil
-	case bool:
-		if v {
-			return "true", nil
-		}
-		return "false", nil
-	}
-	return "", fmt.Errorf("unsupported JSON value %v (of type %T)", value, value)
-}

resolver_test.go

@@ -122,7 +122,11 @@ func TestJSONBasic(t *testing.T) {
 type testUppercaseMapper struct{}
 
 func (testUppercaseMapper) Decode(ctx *kong.DecodeContext, target reflect.Value) error {
-	value := ctx.Scan.PopValue("lowercase")
+	var value string
+	_, err := ctx.Scan.PopValueInto("lowercase", &value)
+	if err != nil {
+		return err
+	}
 	target.SetString(strings.ToUpper(value))
 	return nil
 }
@@ -148,11 +152,11 @@ func TestResolverWithBool(t *testing.T) {
 		Bool bool
 	}
 
-	var resolver kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var resolver kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (interface{}, error) {
 		if flag.Name == "bool" {
-			return "true", nil
+			return true, nil
 		}
-		return "", nil
+		return nil, nil
 	}
 
 	p := mustNew(t, &cli, kong.Resolvers(resolver))
@@ -167,18 +171,18 @@ func TestLastResolverWins(t *testing.T) {
 		Int []int
 	}
 
-	var first kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var first kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (interface{}, error) {
 		if flag.Name == "int" {
-			return "1", nil
+			return 1, nil
 		}
-		return "", nil
+		return nil, nil
 	}
 
-	var second kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var second kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (interface{}, error) {
 		if flag.Name == "int" {
-			return "2", nil
+			return 2, nil
 		}
-		return "", nil
+		return nil, nil
 	}
 
 	p := mustNew(t, &cli, kong.Resolvers(first, second))
@@ -192,11 +196,11 @@ func TestResolverSatisfiesRequired(t *testing.T) {
 	var cli struct {
 		Int int `required`
 	}
-	var resolver kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var resolver kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (interface{}, error) {
 		if flag.Name == "int" {
-			return "1", nil
+			return 1, nil
 		}
-		return "", nil
+		return nil, nil
 	}
 	_, err := mustNew(t, &cli, kong.Resolvers(resolver)).Parse(nil)
 	require.NoError(t, err)
@@ -210,18 +214,18 @@ func TestResolverTriggersHooks(t *testing.T) {
 		Flag hookValue
 	}
 
-	var first kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+	var first kong.ResolverFunc = func(context *kong.Context, parent *kong.Path, flag *kong.Flag) (interface{}, error) {
 		if flag.Name == "flag" {
-			return "1", nil
+			return "one", nil
 		}
-		return "", nil
+		return nil, nil
 	}
 
 	_, err := mustNew(t, &cli, kong.Bind(ctx), kong.Resolvers(first)).Parse(nil)
 	require.NoError(t, err)
 
-	require.Equal(t, "1", string(cli.Flag))
+	require.Equal(t, "one", string(cli.Flag))
-	require.Equal(t, []string{"before:", "after:1"}, ctx.values)
+	require.Equal(t, []string{"before:", "after:one"}, ctx.values)
 }
 
 type validatingResolver struct {
@@ -229,8 +233,8 @@ type validatingResolver struct {
 }
 
 func (v *validatingResolver) Validate(app *kong.Application) error { return v.err }
-func (v *validatingResolver) Resolve(context *kong.Context, parent *kong.Path, flag *kong.Flag) (string, error) {
+func (v *validatingResolver) Resolve(context *kong.Context, parent *kong.Path, flag *kong.Flag) (interface{}, error) {
-	return "", nil
+	return nil, nil
 }
 
 func TestValidatingResolverErrors(t *testing.T) {

scanner.go

@@ -1,8 +1,10 @@
 package kong
 
 import (
-	"strconv"
+	"fmt"
 	"strings"
+
+	"github.com/pkg/errors"
 )
 
 // TokenType is the type of a token.
@@ -41,23 +43,23 @@ func (t TokenType) String() string {
 
 // Token created by Scanner.
 type Token struct {
-	Value string
+	Value interface{}
 	Type  TokenType
 }
 
 func (t Token) String() string {
 	switch t.Type {
 	case FlagToken:
-		return "--" + t.Value
+		return fmt.Sprintf("--%v", t.Value)
 
 	case ShortFlagToken:
-		return "-" + t.Value
+		return fmt.Sprintf("-%v", t.Value)
 
 	case EOLToken:
 		return "EOL"
 
 	default:
-		return strconv.Quote(t.Value)
+		return fmt.Sprintf("%v", t.Value)
 	}
 }
 
@@ -67,9 +69,9 @@ func (t Token) IsEOL() bool {
 }
 
 // IsAny returns true if the token's type is any of those provided.
-func (t Token) IsAny(types ...TokenType) bool {
+func (t TokenType) IsAny(types ...TokenType) bool {
 	for _, typ := range types {
-		if t.Type == typ {
+		if t == typ {
 			return true
 		}
 	}
@@ -79,12 +81,14 @@ func (t Token) IsAny(types ...TokenType) bool {
 // InferredType tries to infer the type of a token.
 func (t Token) InferredType() TokenType {
 	if t.Type == UntypedToken {
-		if strings.HasPrefix(t.Value, "--") {
+		if v, ok := t.Value.(string); ok {
+			if strings.HasPrefix(v, "--") {
 				return FlagToken
-		} else if strings.HasPrefix(t.Value, "-") {
+			} else if strings.HasPrefix(v, "-") {
 				return ShortFlagToken
 			}
 		}
+	}
 	return t.Type
 }
 
@@ -92,8 +96,9 @@ func (t Token) InferredType() TokenType {
 //
 // A parseable value is either a value typed token, or an untyped token NOT starting with a hyphen.
 func (t Token) IsValue() bool {
-	return t.IsAny(FlagValueToken, ShortFlagTailToken, PositionalArgumentToken) ||
+	tt := t.InferredType()
-		(t.Type == UntypedToken && !strings.HasPrefix(t.Value, "-"))
+	return tt.IsAny(FlagValueToken, ShortFlagTailToken, PositionalArgumentToken) ||
+		(tt == UntypedToken && !strings.HasPrefix(t.String(), "-"))
 }
 
 // Scanner is a stack-based scanner over command-line tokens.
@@ -137,15 +142,26 @@ func (s *Scanner) Pop() Token {
 	return arg
 }
 
-// PopValue token, or panic with Error.
+// PopValue pops a value token, or returns an error.
 //
 // "context" is used to assist the user if the value can not be popped, eg. "expected <context> value but got <type>"
-func (s *Scanner) PopValue(context string) string {
+func (s *Scanner) PopValue(context string) (Token, error) {
 	t := s.Pop()
 	if !t.IsValue() {
-		fail("expected %s value but got %s (%s)", context, t, t.InferredType())
+		return t, errors.Errorf("expected %s value but got %q (%s)", context, t, t.InferredType())
 	}
-	return t.Value
+	return t, nil
+}
+
+// PopValueInto pops a value token into target or returns an error.
+//
+// "context" is used to assist the user if the value can not be popped, eg. "expected <context> value but got <type>"
+func (s *Scanner) PopValueInto(context string, target interface{}) (Token, error) {
+	t := s.Pop()
+	if !t.IsValue() {
+		return t, errors.Errorf("expected %s value but got %q (%s)", context, t, t.InferredType())
+	}
+	return t, jsonTranscode(t.Value, target)
 }
 
 // PopWhile predicate returns true.
@@ -173,13 +189,13 @@ func (s *Scanner) Peek() Token {
 }
 
 // Push an untyped Token onto the front of the Scanner.
-func (s *Scanner) Push(arg string) *Scanner {
+func (s *Scanner) Push(arg interface{}) *Scanner {
 	s.PushToken(Token{Value: arg})
 	return s
 }
 
 // PushTyped pushes a typed token onto the front of the Scanner.
-func (s *Scanner) PushTyped(arg string, typ TokenType) *Scanner {
+func (s *Scanner) PushTyped(arg interface{}, typ TokenType) *Scanner {
 	s.PushToken(Token{Value: arg, Type: typ})
 	return s
 }