feat: implement NameSep option

README.md

@@ -93,14 +93,14 @@ In this case, the environment variables declared by its fields are prefixed with
 This rule is applied recursively to all nested structs.
 
 ```go
-os.Setenv("DB_HOST", "localhost")
-os.Setenv("DB_PORT", "5432")
+os.Setenv("DBHOST", "localhost")
+os.Setenv("DBPORT", "5432")
 
 var cfg struct {
     DB struct {
         Host string `env:"HOST"`
         Port int    `env:"PORT"`
-    } `env:"DB_"`
+    } `env:"DB"`
 }
 if err := env.Load(&cfg, nil); err != nil {
     fmt.Println(err)
@@ -182,6 +182,29 @@ if err := env.Load(&cfg, &env.Options{SliceSep: ","}); err != nil {
 fmt.Println(cfg.Ports) // [8080 8081 8082]
 ```
 
+### Name separator
+
+By default, environment variable names are concatenated from nested struct tags as is.
+If `Options.NameSep` is not empty, it is used as the separator:
+
+```go
+os.Setenv("DB_HOST", "localhost")
+os.Setenv("DB_PORT", "5432")
+
+var cfg struct {
+	DB struct {
+		Host string `env:"HOST"`
+		Port int    `env:"PORT"`
+	} `env:"DB"`
+}
+if err := env.Load(&cfg, &env.Options{NameSep: "_"}); err != nil {
+	fmt.Println(err)
+}
+
+fmt.Println(cfg.DB.Host) // localhost
+fmt.Println(cfg.DB.Port) // 5432
+```
+
 ### Source
 
 By default, `Load` retrieves environment variables directly from OS.

env.go

@@ -12,6 +12,7 @@ import (
 type Options struct {
 	Source   Source // The source of environment variables. The default is [OS].
 	SliceSep string // The separator used to parse slice values. The default is space.
+	NameSep  string // The separator used to concatenate environment variable names from nested struct tags. The default is an empty string.
 }
 
 // NotSetError is returned when environment variables are marked as required but not set.
@@ -76,7 +77,7 @@ func Load(cfg any, opts *Options) error {
 	}
 
 	v := pv.Elem()
-	vars := parseVars(v)
+	vars := parseVars(v, opts)
 	cache[v.Type()] = vars
 
 	var notset []string
@@ -111,7 +112,7 @@ func Load(cfg any, opts *Options) error {
 	return nil
 }
 
-func parseVars(v reflect.Value) []Var {
+func parseVars(v reflect.Value, opts *Options) []Var {
 	var vars []Var
 
 	for i := 0; i < v.NumField(); i++ {
@@ -126,9 +127,9 @@ func parseVars(v reflect.Value) []Var {
 			sf := v.Type().Field(i)
 			value, ok := sf.Tag.Lookup("env")
 			if ok {
-				prefix = value
+				prefix = value + opts.NameSep
 			}
-			for _, v := range parseVars(field) {
+			for _, v := range parseVars(field, opts) {
 				v.Name = prefix + v.Name
 				vars = append(vars, v)
 			}

env_test.go

@@ -61,6 +61,23 @@ func TestLoad(t *testing.T) {
 		assert.Panics[E](t, load, "env: `required` and `default` can't be used simultaneously")
 	})
 
+	t.Run("nested struct w/ and w/o tag", func(t *testing.T) {
+		m := env.Map{"A_FOO": "1", "BAR": "2"}
+
+		var cfg struct {
+			A struct {
+				Foo int `env:"FOO"`
+			} `env:"A"`
+			B struct {
+				Bar int `env:"BAR"`
+			}
+		}
+		err := env.Load(&cfg, &env.Options{Source: m, NameSep: "_"})
+		assert.NoErr[F](t, err)
+		assert.Equal[E](t, cfg.A.Foo, 1)
+		assert.Equal[E](t, cfg.B.Bar, 2)
+	})
+
 	t.Run("unsupported type", func(t *testing.T) {
 		m := env.Map{"FOO": "1+2i"}
 

example_test.go

@@ -52,17 +52,38 @@ func ExampleLoad_nestedStruct() {
 	// Output: 8080
 }
 
-func ExampleLoad_nestedStructPrefixed() {
+func ExampleLoad_nestedStructWithPrefix() {
+	os.Setenv("DBHOST", "localhost")
+	os.Setenv("DBPORT", "5432")
+
+	var cfg struct {
+		DB struct {
+			Host string `env:"HOST"`
+			Port int    `env:"PORT"`
+		} `env:"DB"`
+	}
+	if err := env.Load(&cfg, nil); err != nil {
+		fmt.Println(err)
+	}
+
+	fmt.Println(cfg.DB.Host)
+	fmt.Println(cfg.DB.Port)
+	// Output:
+	// localhost
+	// 5432
+}
+
+func ExampleLoad_nestedStructWithPrefixAndSeparator() {
 	os.Setenv("DB_HOST", "localhost")
 	os.Setenv("DB_PORT", "5432")
 
 	var cfg struct {
 		DB struct {
 			Host string `env:"HOST"`
 			Port int    `env:"PORT"`
-		} `env:"DB_"`
+		} `env:"DB"`
 	}
-	if err := env.Load(&cfg, nil); err != nil {
+	if err := env.Load(&cfg, &env.Options{NameSep: "_"}); err != nil {
 		fmt.Println(err)
 	}
 

usage.go

@@ -45,7 +45,7 @@ func Usage(cfg any, w io.Writer, opts *Options) {
 	v := pv.Elem()
 	vars, ok := cache[v.Type()]
 	if !ok {
-		vars = parseVars(v)
+		vars = parseVars(v, opts)
 	}
 
 	if u, ok := cfg.(interface {