feat: add EnumDefault constructor for enums with an initial default value

Author: mfridman

CHANGELOG.md

@@ -7,6 +7,10 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+
+- `flagtype.EnumDefault` constructor for enums with an initial default value
+
 ## [v0.6.0] - 2026-02-18
 
 ### Added

flagtype/doc.go

@@ -5,6 +5,7 @@
 // The following types are available:
 //   - [StringSlice] - repeatable flag that collects values into []string
 //   - [Enum] - restricts values to a predefined set, retrieved as string
+//   - [EnumDefault] - like [Enum] but with an initial default value
 //   - [StringMap] - repeatable flag that parses key=value pairs into map[string]string
 //   - [URL] - parses and validates a URL (must have scheme and host), retrieved as *url.URL
 //   - [Regexp] - compiles a regular expression, retrieved as *regexp.Regexp
@@ -14,6 +15,7 @@
 //	Flags: cli.FlagsFunc(func(f *flag.FlagSet) {
 //	    f.Var(flagtype.StringSlice(), "tag", "add a tag (repeatable)")
 //	    f.Var(flagtype.Enum("json", "yaml", "table"), "format", "output format")
+//	    f.Var(flagtype.EnumDefault("sql", []string{"sql", "go"}), "type", "migration type")
 //	    f.Var(flagtype.StringMap(), "label", "key=value pair (repeatable)")
 //	})
 //

flagtype/enum.go

@@ -20,6 +20,18 @@ func Enum(allowed ...string) flag.Value {
 	return &enumValue{allowed: allowed}
 }
 
+// EnumDefault is like [Enum] but sets an initial default value. The default must be one of the
+// allowed values, otherwise EnumDefault panics.
+//
+// Use [cli.GetFlag] with type string to retrieve the value.
+func EnumDefault(defaultVal string, allowed []string) flag.Value {
+	if !slices.Contains(allowed, defaultVal) {
+		panic(fmt.Sprintf("flagtype: default value %q is not in allowed values: %s",
+			defaultVal, strings.Join(allowed, ", ")))
+	}
+	return &enumValue{val: defaultVal, allowed: allowed}
+}
+
 func (v *enumValue) String() string {
 	return v.val
 }

flagtype/flagtype_test.go

@@ -77,6 +77,36 @@ func TestEnum(t *testing.T) {
 	})
 }
 
+func TestEnumDefault(t *testing.T) {
+	t.Parallel()
+
+	t.Run("uses default when flag not set", func(t *testing.T) {
+		t.Parallel()
+		fs := flag.NewFlagSet("test", flag.ContinueOnError)
+		fs.Var(EnumDefault("sql", []string{"sql", "go"}), "type", "")
+		err := fs.Parse([]string{})
+		require.NoError(t, err)
+		got := fs.Lookup("type").Value.(flag.Getter).Get().(string)
+		assert.Equal(t, "sql", got)
+	})
+	t.Run("override default", func(t *testing.T) {
+		t.Parallel()
+		fs := flag.NewFlagSet("test", flag.ContinueOnError)
+		fs.Var(EnumDefault("sql", []string{"sql", "go"}), "type", "")
+		err := fs.Parse([]string{"--type=go"})
+		require.NoError(t, err)
+		got := fs.Lookup("type").Value.(flag.Getter).Get().(string)
+		assert.Equal(t, "go", got)
+	})
+	t.Run("invalid default panics", func(t *testing.T) {
+		t.Parallel()
+		assert.PanicsWithValue(t,
+			`flagtype: default value "xml" is not in allowed values: sql, go`,
+			func() { EnumDefault("xml", []string{"sql", "go"}) },
+		)
+	})
+}
+
 func TestStringMap(t *testing.T) {
 	t.Parallel()