feat: support enum field validation

relates to #6

Author: hans-m-song

env.go

@@ -107,6 +107,10 @@ func setField(t reflect.StructField, v reflect.Value, value string) (err error)
 		return setSlice(t, v, value)
 	}
 
+	if err = checkEnum(t, value); err != nil {
+		return fmt.Errorf("error setting %q: %v", t.Name, err)
+	}
+
 	if err = setBuiltInField(v, value); err != nil {
 		return fmt.Errorf("error setting %q: %v", t.Name, err)
 	}

env_test.go

@@ -675,6 +675,42 @@ func TestEnvPrefixed(t *testing.T) {
 	Equals(t, "hello", config.Prop)
 }
 
+func TestEnumUnmatched(t *testing.T) {
+	os.Setenv("PROP", "foo")
+
+	config := struct {
+		Prop string `env:"PROP" enum:"1,2,3"`
+	}{}
+
+	err := Set(&config)
+	ErrorNotNil(t, err)
+	Assert(t, strings.HasPrefix(err.Error(), `error setting "Prop": "foo" is not a member of [1,2,3]`))
+}
+
+func TestEnumMatchedString(t *testing.T) {
+	os.Setenv("PROP", "foo")
+
+	config := struct {
+		Prop string `env:"PROP" enum:"foo,bar,baz"`
+	}{}
+
+	err := Set(&config)
+	ErrorNil(t, err)
+	Assert(t, config.Prop == "foo")
+}
+
+func TestEnumMatchedInteger(t *testing.T) {
+	os.Setenv("PROP", "1")
+
+	config := struct {
+		Prop int `env:"PROP" enum:"1,2,3"`
+	}{}
+
+	err := Set(&config)
+	ErrorNil(t, err)
+	Assert(t, config.Prop == 1)
+}
+
 type configDuration struct {
 	Duration time.Duration
 }

set.go

@@ -3,6 +3,7 @@ package env
 import (
 	"fmt"
 	"reflect"
+	"slices"
 	"strconv"
 	"strings"
 	"time"
@@ -179,3 +180,18 @@ func getDelimiter(t reflect.StructField) string {
 	}
 	return ","
 }
+
+func checkEnum(t reflect.StructField, value string) (err error) {
+	rawChoices, ok := t.Tag.Lookup("enum")
+	if !ok {
+		return
+	}
+
+	delimiter := getDelimiter(t)
+	choices := split(rawChoices, delimiter)
+	if !slices.Contains(choices, value) {
+		return fmt.Errorf(`"%s" is not a member of [%s]`, value, strings.Join(choices, ","))
+	}
+
+	return
+}