Support listing enums in specified order (#274)

context.go

@@ -891,16 +891,15 @@ func checkEnum(value *Value, target reflect.Value) error {
 		return errors.Errorf("enum can only be applied to a slice or value")
 
 	default:
-		enumMap := value.EnumMap()
+		enumSlice := value.EnumSlice()
 		v := fmt.Sprintf("%v", target)
-		if enumMap[v] {
-			return nil
-		}
 		enums := []string{}
-		for enum := range enumMap {
+		for _, enum := range enumSlice {
+			if enum == v {
+				return nil
+			}
 			enums = append(enums, fmt.Sprintf("%q", enum))
 		}
-		sort.Strings(enums)
 		return fmt.Errorf("%s must be one of %s but got %q", value.ShortSummary(), strings.Join(enums, ","), target.Interface())
 	}
 }

kong_test.go

@@ -663,6 +663,7 @@ func TestInterpolationIntoModel(t *testing.T) {
 	require.Equal(t, "Some default value.", flag.Default)
 	require.Equal(t, "Help, I need chickens!", flag.Help)
 	require.Equal(t, map[string]bool{"a": true, "b": true, "c": true, "d": true}, flag.EnumMap())
+	require.Equal(t, []string{"a", "b", "c", "d"}, flag.EnumSlice())
 	require.Equal(t, "One of a,b", flag2.Help)
 	require.Equal(t, "SAVE_THE_QUEEN", flag3.Env)
 	require.Equal(t, "God SAVE_THE_QUEEN", flag3.Help)
@@ -787,6 +788,14 @@ func TestEnum(t *testing.T) {
 	require.EqualError(t, err, "--flag must be one of \"a\",\"b\",\"c\" but got \"d\"")
 }
 
+func TestEnumMeaningfulOrder(t *testing.T) {
+	var cli struct {
+		Flag string `enum:"first,second,third,fourth,fifth" required:""`
+	}
+	_, err := mustNew(t, &cli).Parse([]string{"--flag", "sixth"})
+	require.EqualError(t, err, "--flag must be one of \"first\",\"second\",\"third\",\"fourth\",\"fifth\" but got \"sixth\"")
+}
+
 type commandWithHook struct {
 	value string
 }

model.go

@@ -257,6 +257,16 @@ func (v *Value) EnumMap() map[string]bool {
 	return out
 }
 
+// EnumSlice returns a slice of the enums in this value.
+func (v *Value) EnumSlice() []string {
+	parts := strings.Split(v.Enum, ",")
+	out := make([]string, len(parts))
+	for i, part := range parts {
+		out[i] = strings.TrimSpace(part)
+	}
+	return out
+}
+
 // ShortSummary returns a human-readable summary of the value, not including any placeholders/defaults.
 func (v *Value) ShortSummary() string {
 	if v.Flag != nil {