Clean up error handling a bit.

kong_test.go

@@ -688,3 +688,12 @@ func TestParentBindings(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, "foo", cli.Command.value)
 }
+
+func TestNumericParamErrors(t *testing.T) {
+	var cli struct {
+		Name string
+	}
+	parser := mustNew(t, &cli)
+	_, err := parser.Parse([]string{"--name", "-10"})
+	require.EqualError(t, err, `expected string value but got "-10" (short flag)`)
+}

model.go

@@ -271,7 +271,12 @@ func (v *Value) IsBool() bool {
 func (v *Value) Parse(scan *Scanner, target reflect.Value) error {
 	defer func() {
 		if err := recover(); err != nil {
-			panic(fmt.Sprintf("mapper %T failed to apply to %s: %s", v.Mapper, v.Summary(), err))
+			switch err := err.(type) {
+			case Error:
+				panic(err)
+			default:
+				panic(fmt.Sprintf("mapper %T failed to apply to %s: %s", v.Mapper, v.Summary(), err))
+			}
 		}
 	}()
 	err := v.Mapper.Decode(&DecodeContext{Value: v, Scan: scan}, target)

scanner.go

@@ -5,8 +5,6 @@ import (
 	"strings"
 )
 
-//go:generate stringer -type=TokenType
-
 // TokenType is the type of a token.
 type TokenType int
 
@@ -21,6 +19,26 @@ const (
 	PositionalArgumentToken // <arg>
 )
 
+func (t TokenType) String() string {
+	switch t {
+	case UntypedToken:
+		return "untyped"
+	case EOLToken:
+		return "<EOL>"
+	case FlagToken: // --<flag>
+		return "long flag"
+	case FlagValueToken: // =<value>
+		return "flag value"
+	case ShortFlagToken: // -<short>[<tail]
+		return "short flag"
+	case ShortFlagTailToken: // <tail>
+		return "short flag remainder"
+	case PositionalArgumentToken: // <arg>
+		return "positional argument"
+	}
+	panic("unsupported type")
+}
+
 // Token created by Scanner.
 type Token struct {
 	Value string
@@ -58,6 +76,18 @@ func (t Token) IsAny(types ...TokenType) bool {
 	return false
 }
 
+// InferredType tries to infer the type of a token.
+func (t Token) InferredType() TokenType {
+	if t.Type == UntypedToken {
+		if strings.HasPrefix(t.Value, "--") {
+			return FlagToken
+		} else if strings.HasPrefix(t.Value, "-") {
+			return ShortFlagToken
+		}
+	}
+	return t.Type
+}
+
 // 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.
@@ -113,7 +143,7 @@ func (s *Scanner) Pop() Token {
 func (s *Scanner) PopValue(context string) string {
 	t := s.Pop()
 	if !t.IsValue() {
-		fail("expected %s value but got %s", context, t)
+		fail("expected %s value but got %s (%s)", context, t, t.InferredType())
 	}
 	return t.Value
 }

tokentype_string.go

@@ -1,16 +0,0 @@
-// Code generated by "stringer -type=TokenType"; DO NOT EDIT.
-
-package kong
-
-import "strconv"
-
-const _TokenType_name = "UntypedTokenEOLTokenFlagTokenFlagValueTokenShortFlagTokenShortFlagTailTokenPositionalArgumentToken"
-
-var _TokenType_index = [...]uint8{0, 12, 20, 29, 43, 57, 75, 98}
-
-func (i TokenType) String() string {
-	if i < 0 || i >= TokenType(len(_TokenType_index)-1) {
-		return "TokenType(" + strconv.FormatInt(int64(i), 10) + ")"
-	}
-	return _TokenType_name[_TokenType_index[i]:_TokenType_index[i+1]]
-}