feat: add FlagNamer option

Fixes #347

build.go

@@ -133,7 +133,7 @@ MAIN:
 		tag := field.tag
 		name := tag.Name
 		if name == "" {
-			name = tag.Prefix + strings.ToLower(dashedString(ft.Name))
+			name = tag.Prefix + k.flagNamer(ft.Name)
 		} else {
 			name = tag.Prefix + name
 		}

kong.go

@@ -65,6 +65,7 @@ type Kong struct {
 	helpFlag      *Flag
 	groups        []Group
 	vars          Vars
+	flagNamer     func(string) string
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
@@ -85,6 +86,9 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 		bindings:      bindings{},
 		helpFormatter: DefaultHelpValueFormatter,
 		ignoreFields:  make([]*regexp.Regexp, 0),
+		flagNamer: func(s string) string {
+			return strings.ToLower(dashedString(s))
+		},
 	}
 
 	options = append(options, Bind(k))

kong_test.go

@@ -642,17 +642,17 @@ func TestRun(t *testing.T) {
 
 func TestInterpolationIntoModel(t *testing.T) {
 	var cli struct {
-		Flag    string `default:"${default}" help:"Help, I need ${somebody}" enum:"${enum}"`
+		Flag    string `default:"${default_value}" help:"Help, I need ${somebody}" enum:"${enum}"`
 		EnumRef string `enum:"a,b" required:"" help:"One of ${enum}"`
 		EnvRef  string `env:"${env}" help:"God ${env}"`
 	}
 	_, err := kong.New(&cli)
 	assert.Error(t, err)
 	p, err := kong.New(&cli, kong.Vars{
-		"default":  "Some default value.",
+		"default_value": "Some default value.",
-		"somebody": "chickens!",
+		"somebody":      "chickens!",
-		"enum":     "a,b,c,d",
+		"enum":          "a,b,c,d",
-		"env":      "SAVE_THE_QUEEN",
+		"env":           "SAVE_THE_QUEEN",
 	})
 	assert.NoError(t, err)
 	assert.Equal(t, 4, len(p.Model.Flags))

options.go

@@ -483,3 +483,11 @@ func DefaultEnvars(prefix string) Option {
 		return nil
 	})
 }
+
+// FlagNamer allows you to override the default kebab-case automated flag name generation.
+func FlagNamer(namer func(fieldName string) string) Option {
+	return OptionFunc(func(k *Kong) error {
+		k.flagNamer = namer
+		return nil
+	})
+}

options_test.go

@@ -2,6 +2,7 @@ package kong
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/alecthomas/assert/v2"
@@ -112,3 +113,12 @@ func TestBindToProvider(t *testing.T) {
 	assert.NoError(t, err)
 	assert.True(t, cli.Called)
 }
+
+func TestFlagNamer(t *testing.T) {
+	var cli struct {
+		SomeFlag string
+	}
+	app, err := New(&cli, FlagNamer(strings.ToUpper))
+	assert.NoError(t, err)
+	assert.Equal(t, "SOMEFLAG", app.Model.Flags[1].Name)
+}