enhancement: Expand with default values

README.md

@@ -180,7 +180,34 @@ type config struct {
 }
 ```
 
-This also works with `envDefault`.
+This also works with `envDefault`:
+```go
+import (
+	"fmt"
+	"github.com/caarlos0/env/v9"
+)
+
+type config struct {
+	Host     string `env:"HOST" envDefault:"localhost"`
+	Port     int    `env:"PORT" envDefault:"3000"`
+	Address  string `env:"ADDRESS,expand" envDefault:"$HOST:${PORT}"`
+}
+
+func main() {
+	cfg := config{}
+	if err := env.Parse(&cfg); err != nil {
+		fmt.Printf("%+v\n", err)
+	}
+	fmt.Printf("%+v\n", cfg)
+}
+```
+
+results in this:
+
+```sh
+$ PORT=8080 go run main.go
+{Host:localhost Port:8080 Address:localhost:8080}
+```
 
 ## Not Empty fields
 

env.go

@@ -122,13 +122,25 @@ type Options struct {
 
 	// Custom parse functions for different types.
 	FuncMap map[reflect.Type]ParserFunc
+
+	// Used internally. maps the env variable key to its resolved string value. (for env var expansion)
+	rawEnvVars map[string]string
+}
+
+func (opts *Options) getRawEnv(s string) string {
+	val := opts.rawEnvVars[s]
+	if val == "" {
+		return opts.Environment[s]
+	}
+	return val
 }
 
 func defaultOptions() Options {
 	return Options{
 		TagName:     "env",
 		Environment: toMap(os.Environ()),
 		FuncMap:     defaultTypeParsers(),
+		rawEnvVars:  make(map[string]string),
 	}
 }
 
@@ -143,6 +155,9 @@ func customOptions(opt Options) Options {
 	if opt.FuncMap == nil {
 		opt.FuncMap = map[reflect.Type]ParserFunc{}
 	}
+	if opt.rawEnvVars == nil {
+		opt.rawEnvVars = defOpts.rawEnvVars
+	}
 	for k, v := range defOpts.FuncMap {
 		if _, exists := opt.FuncMap[k]; !exists {
 			opt.FuncMap[k] = v
@@ -160,6 +175,7 @@ func optionsWithEnvPrefix(field reflect.StructField, opts Options) Options {
 		Prefix:                opts.Prefix + field.Tag.Get("envPrefix"),
 		UseFieldNameByDefault: opts.UseFieldNameByDefault,
 		FuncMap:               opts.FuncMap,
+		rawEnvVars:            opts.rawEnvVars,
 	}
 }
 
@@ -350,9 +366,11 @@ func get(fieldParams FieldParams, opts Options) (val string, err error) {
 	val, exists, isDefault = getOr(fieldParams.Key, fieldParams.DefaultValue, fieldParams.HasDefaultValue, opts.Environment)
 
 	if fieldParams.Expand {
-		val = os.ExpandEnv(val)
+		val = os.Expand(val, opts.getRawEnv)
 	}
 
+	opts.rawEnvVars[fieldParams.OwnKey] = val
+
 	if fieldParams.Unset {
 		defer os.Unsetenv(fieldParams.Key)
 	}
@@ -392,7 +410,7 @@ func getFromFile(filename string) (value string, err error) {
 	return string(b), err
 }
 
-func getOr(key, defaultValue string, defExists bool, envs map[string]string) (string, bool, bool) {
+func getOr(key, defaultValue string, defExists bool, envs map[string]string) (val string, exists bool, isDefault bool) {
 	value, exists := envs[key]
 	switch {
 	case (!exists || key == "") && defExists:

env_test.go

@@ -965,6 +965,38 @@ func TestParseExpandOption(t *testing.T) {
 	isEqual(t, "def1", cfg.Default)
 }
 
+func TestParseExpandWithDefaultOption(t *testing.T) {
+	type config struct {
+		Host            string `env:"HOST" envDefault:"localhost"`
+		Port            int    `env:"PORT,expand" envDefault:"3000"`
+		OtherPort       int    `env:"OTHER_PORT" envDefault:"4000"`
+		CompoundDefault string `env:"HOST_PORT,expand" envDefault:"${HOST}:${PORT}"`
+		SimpleDefault   string `env:"DEFAULT,expand" envDefault:"def1"`
+		MixedDefault    string `env:"MIXED_DEFAULT,expand" envDefault:"$USER@${HOST}:${OTHER_PORT}"`
+		OverrideDefault string `env:"OVERRIDE_DEFAULT,expand" envDefault:"$THIS_SHOULD_NOT_BE_USED"`
+		NoDefault       string `env:"NO_DEFAULT,expand"`
+	}
+
+	t.Setenv("OTHER_PORT", "5000")
+	t.Setenv("USER", "jhon")
+	t.Setenv("THIS_IS_USED", "this is used instead")
+	t.Setenv("OVERRIDE_DEFAULT", "msg: ${THIS_IS_USED}")
+	t.Setenv("NO_DEFAULT", "$PORT:$OTHER_PORT")
+
+	cfg := config{}
+	err := Parse(&cfg)
+
+	isNoErr(t, err)
+	isEqual(t, "localhost", cfg.Host)
+	isEqual(t, 3000, cfg.Port)
+	isEqual(t, 5000, cfg.OtherPort)
+	isEqual(t, "localhost:3000", cfg.CompoundDefault)
+	isEqual(t, "def1", cfg.SimpleDefault)
+	isEqual(t, "jhon@localhost:5000", cfg.MixedDefault)
+	isEqual(t, "msg: this is used instead", cfg.OverrideDefault)
+	isEqual(t, "3000:5000", cfg.NoDefault)
+}
+
 func TestParseUnsetRequireOptions(t *testing.T) {
 	type config struct {
 		Password string `env:"PASSWORD,unset,required"`