From 625998fc966e5be56283ca3b032457c4e41d9676 Mon Sep 17 00:00:00 2001
From: Taras Bunyk <tbunyk@gmail.com>
Date: Wed, 13 Mar 2024 14:56:51 +0100
Subject: [PATCH] feat: support prefixes for nested structs (#53)

---
 README.md       | 42 ++++++++++++++++++++++++++++++++++++++++++
 env.go          | 17 ++++++++++++++++-
 example_test.go | 21 +++++++++++++++++++++
 3 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index f3c7840..4fb0b75 100644
--- a/README.md
+++ b/README.md
@@ -68,6 +68,48 @@ fmt.Println(cfg.Port) // 8080
 See the `strconv.Parse*` functions for the parsing rules.
 User-defined types can be used by implementing the `encoding.TextUnmarshaler` interface.
 
+### Nested structs
+
+Nested struct of any depth level are supported,
+allowing grouping of related environment variables.
+
+```go
+os.Setenv("HTTP_PORT", "8080")
+
+var cfg struct {
+    HTTP struct {
+        Port int `env:"HTTP_PORT"`
+    }
+}
+if err := env.Load(&cfg, nil); err != nil {
+    fmt.Println(err)
+}
+
+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.
+
+```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, nil); err != nil {
+    fmt.Println(err)
+}
+
+fmt.Println(cfg.DB.Host) // localhost
+fmt.Println(cfg.DB.Port) // 5432
+```
+
 ### Default values
 
 Default values can be specified using the `default:"VALUE"` struct tag:
diff --git a/env.go b/env.go
index b539302..e08a741 100644
--- a/env.go
+++ b/env.go
@@ -48,6 +48,12 @@ func (e *NotSetError) Error() string {
 // See the [strconv].Parse* functions for the parsing rules.
 // User-defined types can be used by implementing the [encoding.TextUnmarshaler] interface.
 //
+// Nested struct of any depth level are supported,
+// allowing grouping of related environment variables.
+// 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.
+//
 // Default values can be specified using the `default:"VALUE"` struct tag.
 //
 // The name of an environment variable can be followed by comma-separated options:
@@ -116,7 +122,16 @@ func parseVars(v reflect.Value) []Var {
 
 		// special case: a nested struct, parse its fields recursively.
 		if kindOf(field, reflect.Struct) && !implements(field, unmarshalerIface) {
-			vars = append(vars, parseVars(field)...)
+			var prefix string
+			sf := v.Type().Field(i)
+			value, ok := sf.Tag.Lookup("env")
+			if ok {
+				prefix = value
+			}
+			for _, v := range parseVars(field) {
+				v.Name = prefix + v.Name
+				vars = append(vars, v)
+			}
 			continue
 		}
 
diff --git a/example_test.go b/example_test.go
index 218b7cc..442652a 100644
--- a/example_test.go
+++ b/example_test.go
@@ -52,6 +52,27 @@ func ExampleLoad_nestedStruct() {
 	// Output: 8080
 }
 
+func ExampleLoad_nestedStructPrefixed() {
+	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, nil); err != nil {
+		fmt.Println(err)
+	}
+
+	fmt.Println(cfg.DB.Host)
+	fmt.Println(cfg.DB.Port)
+	// Output:
+	// localhost
+	// 5432
+}
+
 func ExampleLoad_required() {
 	os.Unsetenv("PORT")