chore: optionally allow parsing of hyphen-prefixied flag parameters

This allows for eg. `foo --number -10`, `foo --flag -bar`.

Fixes #478, #315.

context.go

@@ -99,7 +99,7 @@ type Context struct {
 // This just constructs a new trace. To fully apply the trace you must call Reset(), Resolve(),
 // Validate() and Apply().
 func Trace(k *Kong, args []string) (*Context, error) {
-	s := Scan(args...)
+	s := Scan(args...).AllowHyphenPrefixedParameters(k.allowHyphenated)
 	c := &Context{
 		Kong: k,
 		Args: args,

kong.go

@@ -56,16 +56,17 @@ type Kong struct {
 	registry     *Registry
 	ignoreFields []*regexp.Regexp
 
-	noDefaultHelp bool
-	usageOnError  usageOnError
-	help          HelpPrinter
-	shortHelp     HelpPrinter
-	helpFormatter HelpValueFormatter
-	helpOptions   HelpOptions
-	helpFlag      *Flag
-	groups        []Group
-	vars          Vars
-	flagNamer     func(string) string
+	noDefaultHelp   bool
+	allowHyphenated bool
+	usageOnError    usageOnError
+	help            HelpPrinter
+	shortHelp       HelpPrinter
+	helpFormatter   HelpValueFormatter
+	helpOptions     HelpOptions
+	helpFlag        *Flag
+	groups          []Group
+	vars            Vars
+	flagNamer       func(string) string
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option

kong_test.go

@@ -1043,15 +1043,6 @@ func TestParentBindings(t *testing.T) {
 	assert.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"})
-	assert.EqualError(t, err, `--name: expected string value but got "-10" (short flag); perhaps try --name="-10"?`)
-}
-
 func TestDefaultValueIsHyphen(t *testing.T) {
 	var cli struct {
 		Flag string `default:"-"`
@@ -2677,3 +2668,39 @@ func TestProviderWithoutError(t *testing.T) {
 	err = kctx.Run()
 	assert.NoError(t, err)
 }
+
+func TestParseHyphenParameter(t *testing.T) {
+	type shortFlag struct {
+		Flag    string `short:"f"`
+		Other   string `short:"o"`
+		Numeric int    `short:"n"`
+	}
+
+	t.Run("ShortFlag", func(t *testing.T) {
+		actual := &shortFlag{}
+		_, err := mustNew(t, actual, kong.WithHyphenPrefixedParameters(true)).Parse([]string{"-f", "-foo"})
+		assert.NoError(t, err)
+		assert.Equal(t, &shortFlag{Flag: "-foo"}, actual)
+	})
+
+	t.Run("LongFlag", func(t *testing.T) {
+		actual := &shortFlag{}
+		_, err := mustNew(t, actual, kong.WithHyphenPrefixedParameters(true)).Parse([]string{"--flag", "-foo"})
+		assert.NoError(t, err)
+		assert.Equal(t, &shortFlag{Flag: "-foo"}, actual)
+	})
+
+	t.Run("ParamMatchesFlag", func(t *testing.T) {
+		actual := &shortFlag{}
+		_, err := mustNew(t, actual, kong.WithHyphenPrefixedParameters(true)).Parse([]string{"--flag", "-oo"})
+		assert.NoError(t, err)
+		assert.Equal(t, &shortFlag{Flag: "-oo"}, actual)
+	})
+
+	t.Run("NegativeNumber", func(t *testing.T) {
+		actual := &shortFlag{}
+		_, err := mustNew(t, actual, kong.WithHyphenPrefixedParameters(true)).Parse([]string{"--numeric", "-10"})
+		assert.NoError(t, err)
+		assert.Equal(t, &shortFlag{Numeric: -10}, actual)
+	})
+}

options.go

@@ -55,6 +55,16 @@ func Exit(exit func(int)) Option {
 	})
 }
 
+// WithHyphenPrefixedParameters enables or disables hyphen-prefixed parameters.
+//
+// These are disabled by default.
+func WithHyphenPrefixedParameters(enable bool) Option {
+	return OptionFunc(func(k *Kong) error {
+		k.allowHyphenated = enable
+		return nil
+	})
+}
+
 type embedded struct {
 	strct any
 	tags  []string

scanner.go

@@ -111,7 +111,8 @@ func (t Token) IsValue() bool {
 //
 //	[{FlagToken, "foo"}, {FlagValueToken, "bar"}]
 type Scanner struct {
-	args []Token
+	allowHyphenated bool
+	args            []Token
 }
 
 // ScanAsType creates a new Scanner from args with the given type.
@@ -133,6 +134,14 @@ func ScanFromTokens(tokens ...Token) *Scanner {
 	return &Scanner{args: tokens}
 }
 
+// AllowHyphenPrefixedParameters enables or disables hyphen-prefixed flag parameters on this Scanner.
+//
+// Disabled by default.
+func (s *Scanner) AllowHyphenPrefixedParameters(enable bool) *Scanner {
+	s.allowHyphenated = enable
+	return s
+}
+
 // Len returns the number of input arguments.
 func (s *Scanner) Len() int {
 	return len(s.args)
@@ -162,7 +171,7 @@ func (e *expectedError) Error() string {
 // "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) (Token, error) {
 	t := s.Pop()
-	if !t.IsValue() {
+	if !s.allowHyphenated && !t.IsValue() {
 		return t, &expectedError{context, t}
 	}
 	return t, nil