fs: create fs.Enum for easy creation of parameters from a list of choices

fs/config/configstruct/configstruct.go

@@ -24,8 +24,7 @@ func camelToSnake(in string) string {
 // StringToInterface turns in into an interface{} the same type as def
 func StringToInterface(def interface{}, in string) (newValue interface{}, err error) {
 	typ := reflect.TypeOf(def)
-	switch typ.Kind() {
+	if typ.Kind() == reflect.String && typ.Name() == "string" {
-	case reflect.String:
 		// Pass strings unmodified
 		return in, nil
 	}

fs/enum.go

@@ -0,0 +1,107 @@
+package fs
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// Enum is an option which can only be one of the Choices.
+//
+// Suggested implementation is something like this:
+//
+//	type choice = Enum[choices]
+//
+//	const (
+//		choiceA choice = iota
+//		choiceB
+//		choiceC
+//	)
+//
+//	type choices struct{}
+//
+//	func (choices) Choices() []string {
+//		return []string{
+//			choiceA: "A",
+//			choiceB: "B",
+//			choiceC: "C",
+//		}
+//	}
+type Enum[C Choices] byte
+
+// Choices returns the valid choices for this type.
+//
+// It must work on the zero value.
+//
+// Note that when using this in an Option the ExampleChoices will be
+// filled in automatically.
+type Choices interface {
+	// Choices returns the valid choices for this type
+	Choices() []string
+}
+
+// String renders the Enum as a string
+func (e Enum[C]) String() string {
+	choices := e.Choices()
+	if int(e) >= len(choices) {
+		return fmt.Sprintf("Unknown(%d)", e)
+	}
+	return choices[e]
+}
+
+// Choices returns the possible values of the Enum.
+func (e Enum[C]) Choices() []string {
+	var c C
+	return c.Choices()
+}
+
+// Help returns a comma separated list of all possible states.
+func (e Enum[C]) Help() string {
+	return strings.Join(e.Choices(), ", ")
+}
+
+// Set the Enum entries
+func (e *Enum[C]) Set(s string) error {
+	for i, choice := range e.Choices() {
+		if strings.EqualFold(s, choice) {
+			*e = Enum[C](i)
+			return nil
+		}
+	}
+	return fmt.Errorf("invalid choice %q from: %s", s, e.Help())
+}
+
+// Type of the value.
+//
+// If C has a Type() string method then it will be used instead.
+func (e Enum[C]) Type() string {
+	var c C
+	if do, ok := any(c).(typer); ok {
+		return do.Type()
+	}
+	return strings.Join(e.Choices(), "|")
+}
+
+// Scan implements the fmt.Scanner interface
+func (e *Enum[C]) Scan(s fmt.ScanState, ch rune) error {
+	token, err := s.Token(true, nil)
+	if err != nil {
+		return err
+	}
+	return e.Set(string(token))
+}
+
+// UnmarshalJSON parses it as a string
+func (e *Enum[C]) UnmarshalJSON(in []byte) error {
+	var choice string
+	err := json.Unmarshal(in, &choice)
+	if err != nil {
+		return err
+	}
+	return e.Set(choice)
+}
+
+// MarshalJSON encodes it as string
+func (e *Enum[C]) MarshalJSON() ([]byte, error) {
+	return json.Marshal(e.String())
+}

fs/enum_test.go

@@ -0,0 +1,139 @@
+package fs
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type choices struct{}
+
+func (choices) Choices() []string {
+	return []string{
+		choiceA: "A",
+		choiceB: "B",
+		choiceC: "C",
+	}
+}
+
+type choice = Enum[choices]
+
+const (
+	choiceA choice = iota
+	choiceB
+	choiceC
+)
+
+// Check it satisfies the interfaces
+var (
+	_ flagger   = (*choice)(nil)
+	_ flaggerNP = choice(0)
+)
+
+func TestEnumString(t *testing.T) {
+	for _, test := range []struct {
+		in   choice
+		want string
+	}{
+		{choiceA, "A"},
+		{choiceB, "B"},
+		{choiceC, "C"},
+		{choice(100), "Unknown(100)"},
+	} {
+		got := test.in.String()
+		assert.Equal(t, test.want, got)
+	}
+}
+
+func TestEnumType(t *testing.T) {
+	assert.Equal(t, "A|B|C", choiceA.Type())
+}
+
+// Enum with Type() on the choices
+type choicestype struct{}
+
+func (choicestype) Choices() []string {
+	return []string{}
+}
+
+func (choicestype) Type() string {
+	return "potato"
+}
+
+type choicetype = Enum[choicestype]
+
+func TestEnumTypeWithFunction(t *testing.T) {
+	assert.Equal(t, "potato", choicetype(0).Type())
+}
+
+func TestEnumHelp(t *testing.T) {
+	assert.Equal(t, "A, B, C", choice(0).Help())
+}
+
+func TestEnumSet(t *testing.T) {
+	for _, test := range []struct {
+		in   string
+		want choice
+		err  bool
+	}{
+		{"A", choiceA, false},
+		{"B", choiceB, false},
+		{"C", choiceC, false},
+		{"D", choice(100), true},
+	} {
+		var got choice
+		err := got.Set(test.in)
+		if test.err {
+			require.Error(t, err)
+		} else {
+			require.NoError(t, err)
+			assert.Equal(t, test.want, got)
+		}
+	}
+}
+
+func TestEnumScan(t *testing.T) {
+	var v choice
+	n, err := fmt.Sscan(" A ", &v)
+	require.NoError(t, err)
+	assert.Equal(t, 1, n)
+	assert.Equal(t, choiceA, v)
+}
+
+func TestEnumUnmarshalJSON(t *testing.T) {
+	for _, test := range []struct {
+		in   string
+		want choice
+		err  bool
+	}{
+		{`"A"`, choiceA, false},
+		{`"B"`, choiceB, false},
+		{`"D"`, choice(0), true},
+	} {
+		var got choice
+		err := json.Unmarshal([]byte(test.in), &got)
+		if test.err {
+			require.Error(t, err, test.in)
+		} else {
+			require.NoError(t, err, test.in)
+		}
+		assert.Equal(t, test.want, got, test.in)
+	}
+}
+
+func TestEnumMarshalJSON(t *testing.T) {
+	for _, test := range []struct {
+		in   choice
+		want string
+	}{
+		{choiceA, `"A"`},
+		{choiceB, `"B"`},
+	} {
+		got, err := json.Marshal(&test.in)
+		require.NoError(t, err)
+		assert.Equal(t, test.want, string(got), fmt.Sprintf("%#v", test.in))
+	}
+}

fs/registry.go

@@ -60,6 +60,15 @@ func (os Options) setValues() {
 		if o.Default == nil {
 			o.Default = ""
 		}
+		// Create options for Enums
+		if do, ok := o.Default.(Choices); ok && len(o.Examples) == 0 {
+			o.Exclusive = true
+			o.Required = true
+			o.Examples = make(OptionExamples, len(do.Choices()))
+			for i, choice := range do.Choices() {
+				o.Examples[i].Value = choice
+			}
+		}
 	}
 }