Add variable key to error message and allow for custom error

README.md

@@ -37,6 +37,7 @@ type Environment struct {
 	Duration      time.Duration `env:"TYPE_DURATION"`
 	DefaultValue  string        `env:"MISSING_VAR,default=default_value"`
 	RequiredValue string        `env:"IM_REQUIRED,required=true"`
+	CustomError   string        `env:"CUSTOM_ERROR,required=true,error=custom_message"`
 }
 
 func main() {

env.go

@@ -36,11 +36,22 @@ var (
 
 	// ErrUnexportedField returned when a field with tag "env" is not exported.
 	ErrUnexportedField = errors.New("field must be exported")
-
-	// ErrMissingRequiredValue returned when a field with required=true contains no value or default
-	ErrMissingRequiredValue = errors.New("value for this field is required")
 )
 
+// ErrMissingRequiredValue returned when a field with required=true contains no value or default
+type ErrMissingRequiredValue struct {
+	VariableName string
+}
+
+func (e ErrMissingRequiredValue) Error() string {
+	return fmt.Sprintf("value for '%s' is required", e.VariableName)
+}
+
+func IsErrMissingRequiredValue(err error) bool {
+	_, ok := err.(ErrMissingRequiredValue)
+	return ok
+}
+
 // Unmarshal parses an EnvSet and stores the result in the value pointed to by
 // v. Fields that are matched in v will be deleted from EnvSet, resulting in
 // an EnvSet with the remaining environment variables. If v is nil or not a
@@ -93,20 +104,26 @@ func Unmarshal(es EnvSet, v interface{}) error {
 
 		var (
 			envValue string
-			ok bool
+			ok       bool
+			lastKey  string
 		)
 		for _, envKey := range envTag.Keys {
 			envValue, ok = es[envKey]
 			if ok {
 				break
 			}
+			lastKey = envKey
 		}
 
 		if !ok {
 			if envTag.Default != "" {
 				envValue = envTag.Default
 			} else if envTag.Required {
-				return ErrMissingRequiredValue
+				if envTag.ErrorName != "" {
+					return ErrMissingRequiredValue{envTag.ErrorName}
+				} else {
+					return ErrMissingRequiredValue{lastKey}
+				}
 			} else {
 				continue
 			}
@@ -248,9 +265,10 @@ func Marshal(v interface{}) (EnvSet, error) {
 }
 
 type tag struct {
-	Keys []string
-	Default string
-	Required bool
+	Keys      []string
+	Default   string
+	Required  bool
+	ErrorName string
 }
 
 func parseTag(tagString string) tag {
@@ -264,6 +282,8 @@ func parseTag(tagString string) tag {
 				t.Default = keyData[1]
 			case "required":
 				t.Required = strings.ToLower(keyData[1]) == "true"
+			case "error":
+				t.ErrorName = keyData[1]
 			default:
 				// just ignoring unsupported keys
 				continue

env_test.go

@@ -81,6 +81,12 @@ type RequiredValueStruct struct {
 	InvalidExtra        string `env:"INVALID,invalid=invalid"`
 }
 
+type ErrorNameStruct struct {
+	Required         string `env:"REQUIRED_VAL,required=true"`
+	RequiredWithName string `env:"REQUIRED_MISSING,required=true,error=NAME"`
+	NotRequired      string `env:"NOT_REQUIRED,required=false,error=NAME"`
+}
+
 func TestUnmarshal(t *testing.T) {
 	environ := map[string]string{
 		"HOME":             "/home/test",
@@ -251,7 +257,7 @@ func TestUnmarshalUnexported(t *testing.T) {
 }
 
 func TestUnmarshalDefaultValues(t *testing.T) {
-	environ := map[string]string {
+	environ := map[string]string{
 		"PRESENT": "youFoundMe",
 	}
 	var defaultValueStruct DefaultValueStruct
@@ -279,7 +285,7 @@ func TestUnmarshalRequiredValues(t *testing.T) {
 	environ := map[string]string{}
 	var requiredValuesStruct RequiredValueStruct
 	err := Unmarshal(environ, &requiredValuesStruct)
-	if err != ErrMissingRequiredValue {
+	if !IsErrMissingRequiredValue(err) {
 		t.Errorf("Expected error 'ErrMissingRequiredValue' but go '%s'", err)
 	}
 	environ["REQUIRED_VAL"] = "required"
@@ -295,6 +301,22 @@ func TestUnmarshalRequiredValues(t *testing.T) {
 	}
 }
 
+func TestUnmarshalRequiredErrorName(t *testing.T) {
+	var err error
+	environ := map[string]string{}
+	var errorNameStruct ErrorNameStruct
+	err = Unmarshal(environ, &errorNameStruct)
+	if err.Error() != "value for 'REQUIRED_VAL' is required" {
+		t.Errorf("Expected response \"value for 'REQUIRED_VAL' is required\" but go '%s'", err)
+	}
+
+	environ["REQUIRED_VAL"] = "required"
+	err = Unmarshal(environ, &errorNameStruct)
+	if err.Error() != "value for 'NAME' is required" {
+		t.Errorf("Expected response \"value for 'NAME' is required\" but go '%s'", err)
+	}
+}
+
 func TestMarshal(t *testing.T) {
 	validStruct := ValidStruct{
 		Home: "/home/test",