From 56beaf08c06f610087f119460d51ce26b721ff18 Mon Sep 17 00:00:00 2001
From: Andrei Avram <avramandrei@ymail.com>
Date: Mon, 6 Jan 2020 11:06:34 +0200
Subject: [PATCH] Add default value option.

---
 README.md        |  2 ++
 config.go        |  1 -
 config_test.go   |  7 +++++--
 examples_test.go |  3 +++
 parser.go        | 13 +++++++++++++
 5 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index 2fafac5..5dff1ea 100644
--- a/README.md
+++ b/README.md
@@ -22,6 +22,7 @@ import (
 
 type Config struct {
 	Username string `env:"USERNAME"`
+	Tag      string `env:"TAG" default:"none"`
 }
 
 func main() {
@@ -33,6 +34,7 @@ func main() {
 	}
 
 	fmt.Println(cfg.Username)
+	fmt.Println(cfg.Tag)
 }
 ```
 
diff --git a/config.go b/config.go
index 0a94db2..6e4cf64 100644
--- a/config.go
+++ b/config.go
@@ -17,7 +17,6 @@ import (
 	"os"
 )
 
-const tag = "env"
 const dotEnvFile = ".env"
 
 // Loader provides methods to load configuration values into a struct
diff --git a/config_test.go b/config_test.go
index 65948d7..7cf4faf 100644
--- a/config_test.go
+++ b/config_test.go
@@ -28,7 +28,7 @@ type Config struct {
 			Port int `env:"REDIS_PORT"`
 		}
 	}
-	String       string `env:"ABC"`
+	String       string `env:"ABC" default:"ignored"`
 	Struct       Struct
 	StructPtr    *Struct
 	D            int64
@@ -47,6 +47,7 @@ type Config struct {
 	UA           uint8
 	IsSet        bool
 	Interpolated string
+	Default      string `default:"default value"`
 }
 
 type Struct struct {
@@ -185,7 +186,8 @@ func TestJson(t *testing.T) {
 	   "Struct":{
 		  "Field":"Value"
 	   },
-	   "Interpolated":"$B env_1 $ $B \\3 6379 + $"
+	   "Interpolated":"$B env_1 $ $B \\3 6379 + $",
+	   "Default":"default value"
 	}`)
 
 	_, expected, err := testdata()
@@ -372,6 +374,7 @@ func testdata() ([]byte, Config, error) {
 			},
 		}},
 		Interpolated: "$B env_1 $ $B \\3 6379 + $",
+		Default:      "default value",
 	}
 
 	return input, expected, nil
diff --git a/examples_test.go b/examples_test.go
index 1a7b015..22f7dd4 100644
--- a/examples_test.go
+++ b/examples_test.go
@@ -10,6 +10,7 @@ import (
 
 type Config struct {
 	Username string `env:"USERNAME"`
+	Tag      string `env:"TAG" default:"none"`
 }
 
 func ExampleLoader_Env() {
@@ -23,9 +24,11 @@ func ExampleLoader_Env() {
 	}
 
 	fmt.Println(cfg.Username)
+	fmt.Println(cfg.Tag)
 
 	// Output:
 	// msd
+	// none
 }
 
 func ExampleLoader_Bytes() {
diff --git a/parser.go b/parser.go
index b0454c6..c7a3a91 100644
--- a/parser.go
+++ b/parser.go
@@ -8,6 +8,11 @@ import (
 	"strings"
 )
 
+const (
+	tag             = "env"
+	defaultValueTag = "default"
+)
+
 type getValue func(string) string
 
 func parseIntoStruct(i interface{}, data getValue) error {
@@ -51,6 +56,10 @@ func parse(typ reflect.Type, val reflect.Value, getValue getValue, path string)
 
 		value := value(&field, getValue, path)
 
+		if value == "" {
+			value = defaultValue(&field)
+		}
+
 		if value == "" {
 			continue
 		}
@@ -81,6 +90,10 @@ func key(field *reflect.StructField, path string) string {
 	return key
 }
 
+func defaultValue(field *reflect.StructField) string {
+	return field.Tag.Get(defaultValueTag)
+}
+
 func setFieldValue(field *reflect.StructField, fieldValue reflect.Value, value string) error {
 	switch field.Type.Kind() {
 	case reflect.String: