diff --git a/decoders.go b/decoders.go
index 4cbb1f1..a5428ec 100644
--- a/decoders.go
+++ b/decoders.go
@@ -13,7 +13,12 @@ type DecoderContext struct {
 	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
 }
 
@@ -61,6 +66,12 @@ 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 `type:"colour"`
+//   	kong.RegisterDecoder(kong.NewNamedDecoder("colour", ...))
 type NamedDecoder interface {
 	Name() string
 	Decoder
@@ -98,7 +109,7 @@ func DecoderForField(field reflect.StructField) Decoder {
 	return DecoderForType(field.Type)
 }
 
-// DecoderForType finds a decoder via a type or kind.
+// 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 {
@@ -125,7 +136,7 @@ func RegisterDecoder(decoders ...Decoder) {
 		case NamedDecoder:
 			namedDecoders[decoder.Name()] = decoder
 		default:
-			fail("unsupported decoder type " + reflect.TypeOf(decoder).String())
+			fail("unsupported decoder type %T", decoder)
 		}
 	}
 }
@@ -220,11 +231,10 @@ func sliceDecoder(ctx *DecoderContext, scan *Scanner, target reflect.Value) erro
 	}
 	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() || (t.Type == UntypedToken && strings.HasPrefix(t.Value, "-"))
-		})
+		tokens := scan.PopUntil(func(t Token) bool { return !t.IsValue() })
 		childScanner = Scan(tokens...)
 	}
 	childDecoder := DecoderForType(el)
diff --git a/kong.go b/kong.go
index c382b40..5c06d55 100644
--- a/kong.go
+++ b/kong.go
@@ -61,7 +61,7 @@ func (k *Kong) reset(node *Node) {
 	for _, flag := range node.Flags {
 		flag.Value.Value.Set(reflect.Zero(flag.Value.Value.Type()))
 		if flag.Default != "" {
-			flag.Decoder.Decode(&DecoderContext{Value: &flag.Value}, Scan(flag.Default), flag.Value.Value)
+			flag.Decode(Scan(flag.Default))
 		}
 	}
 	for _, pos := range node.Positional {
@@ -144,7 +144,7 @@ func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error
 			// Ensure we've consumed all positional arguments.
 			if positional < len(node.Positional) {
 				arg := node.Positional[positional]
-				err := arg.Decoder.Decode(&DecoderContext{Value: arg}, scan, arg.Value)
+				err := arg.Decode(scan)
 				if err != nil {
 					return nil, err
 				}
@@ -169,7 +169,7 @@ func (k *Kong) applyNode(scan *Scanner, node *Node) (command []string, err error
 
 				case branch.Argument != nil:
 					arg := branch.Argument.Argument
-					if err := arg.Decoder.Decode(&DecoderContext{Value: arg}, scan, arg.Value); err == nil {
+					if err := arg.Decode(scan); err == nil {
 						command = append(command, "<"+arg.Name+">")
 						cmd, err := k.applyNode(scan, &branch.Argument.Node)
 						if err != nil {
@@ -200,7 +200,7 @@ func matchFlags(flags []*Flag, token Token, scan *Scanner, matcher func(f *Flag)
 	for _, flag := range flags {
 		// Found a matching flag.
 		if flag.Name == token.Value {
-			err := flag.Decoder.Decode(&DecoderContext{Value: &flag.Value}, scan, flag.Value.Value)
+			err := flag.Decode(scan)
 			if err != nil {
 				return err
 			}
diff --git a/model.go b/model.go
index 2583c00..50be9c2 100644
--- a/model.go
+++ b/model.go
@@ -31,6 +31,10 @@ type Value struct {
 	Format   string // Formatting directive, if applicable.
 }
 
+func (v *Value) Decode(scan *Scanner) error {
+	return v.Decoder.Decode(&DecoderContext{Value: v}, scan, v.Value)
+}
+
 type Positional = Value
 
 type Argument struct {
diff --git a/scanner.go b/scanner.go
index 9c7d86d..e653988 100644
--- a/scanner.go
+++ b/scanner.go
@@ -2,6 +2,7 @@ package kong
 
 import (
 	"strconv"
+	"strings"
 )
 
 //go:generate stringer -type=TokenType
@@ -48,8 +49,12 @@ func (t Token) IsAny(types ...TokenType) bool {
 	return false
 }
 
+// IsValue returns true if token is usable as a parseable value.
+//
+// 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, UntypedToken)
+	return t.IsAny(FlagValueToken, ShortFlagTailToken, PositionalArgumentToken) ||
+		(t.Type == UntypedToken && !strings.HasPrefix(t.Value, "-"))
 }
 
 type Scanner struct {
@@ -77,7 +82,7 @@ func (s *Scanner) Pop() Token {
 	return arg
 }
 
-// PopValue or panic with Error.
+// PopValue token, or panic with Error.
 func (s *Scanner) PopValue(context string) string {
 	t := s.Pop()
 	if !t.IsValue() {