feat: Implement NameSep option

README.md

@@ -91,6 +91,7 @@ fmt.Println(cfg.HTTP.Port) // 8080
 A nested struct can have the optional `env:"PREFIX"` tag.
 In this case, the environment variables declared by its fields are prefixed with PREFIX.
 This rule is applied recursively to all nested structs.
+Nested field names are separated by a string defined in `NameSep` option, empty string by default.
 
 ```go
 os.Setenv("DB_HOST", "localhost")
@@ -100,14 +101,16 @@ 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)
 }
 
-fmt.Println(cfg.DB.Host) // localhost
-fmt.Println(cfg.DB.Port) // 5432
+fmt.Println(cfg.DB.Host)
+fmt.Println(cfg.DB.Port)
 ```
 
 ### Default values
@@ -182,6 +185,32 @@ if err := env.Load(&cfg, &env.Options{SliceSep: ","}); err != nil {
 fmt.Println(cfg.Ports) // [8080 8081 8082]
 ```
 
+### Names separator
+By defaul variable names of nested structs are built with just simple concatenation.
+
+So given config like the following:
+
+```go
+var cfg struct {
+    DB struct {
+        Host string `env:"HOST"`
+        Port int    `env:"PORT"`
+    } `env:"DB"`
+}
+```
+
+would parse variables `DBHOST` and `DBPORT`.
+
+This can be changed with `Options.NameSep`, so parsing it like
+
+```go
+env.Load(&cfg, &env.Options{NameSep: "_"})
+```
+
+Will parse `DB_HOST` and `DB_PORT` variable names.
+Values like `"-"`, or any other string that builds valid environment variables
+are also possible
+
 ### 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 join nested struct names. The default is 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++ {
@@ -128,7 +129,10 @@ func parseVars(v reflect.Value) []Var {
 			if ok {
 				prefix = value
 			}
-			for _, v := range parseVars(field) {
+			if opts != nil {
+				prefix += opts.NameSep
+			}
+			for _, v := range parseVars(field, opts) {
 				v.Name = prefix + v.Name
 				vars = append(vars, v)
 			}

example_test.go

@@ -53,16 +53,37 @@ func ExampleLoad_nestedStruct() {
 }
 
 func ExampleLoad_nestedStructPrefixed() {
+	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_nestedStructPrefixedWithSeparator() {
 	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 {