[Proposal] add struct tag for fallback value in case of error

envconfig.go

@@ -212,7 +212,10 @@ func Process(prefix string, spec interface{}) error {
 			continue
 		}
 
-		err = processField(value, info.Field)
+		if err = processField(value, info.Field); err != nil && info.Tags.Get("on_err") != "" {
+			err = processField(info.Tags.Get("on_err"), info.Field)
+		}
+
 		if err != nil {
 			return &ParseError{
 				KeyName:   info.Key,

envconfig_test.go

@@ -60,6 +60,7 @@ type Specification struct {
 	NoPrefixDefault              string  `envconfig:"BROKER" default:"127.0.0.1"`
 	RequiredDefault              string  `required:"true" default:"foo2bar"`
 	Ignored                      string  `ignored:"true"`
+	OnErr                        *bool   `on_err:"false"`
 	NestedSpecification          struct {
 		Property            string `envconfig:"inner"`
 		PropertyWithDefault string `default:"fuzzybydefault"`
@@ -103,6 +104,7 @@ func TestProcess(t *testing.T) {
 	os.Setenv("ENV_CONFIG_TTL", "30")
 	os.Setenv("ENV_CONFIG_REQUIREDVAR", "foo")
 	os.Setenv("ENV_CONFIG_IGNORED", "was-not-ignored")
+	os.Setenv("ENV_CONFIG_ONERR", "invalid-value")
 	os.Setenv("ENV_CONFIG_OUTER_INNER", "iamnested")
 	os.Setenv("ENV_CONFIG_AFTERNESTED", "after")
 	os.Setenv("ENV_CONFIG_HONOR", "honor")
@@ -177,6 +179,10 @@ func TestProcess(t *testing.T) {
 		)
 	}
 
+	if *s.OnErr != false {
+		t.Errorf("expected '%v' bool, got %v", false, *s.OnErr)
+	}
+
 	if s.NestedSpecification.Property != "iamnested" {
 		t.Errorf("expected '%s' string, got %#v", "iamnested", s.NestedSpecification.Property)
 	}
@@ -794,7 +800,7 @@ func TestCheckDisallowedIgnored(t *testing.T) {
 
 func TestErrorMessageForRequiredAltVar(t *testing.T) {
 	var s struct {
-		Foo    string `envconfig:"BAR" required:"true"`
+		Foo string `envconfig:"BAR" required:"true"`
 	}
 
 	os.Clearenv()

testdata/custom.txt

@@ -26,6 +26,7 @@ ENV_CONFIG_DEFAULTVAR=
 ENV_CONFIG_REQUIREDVAR=
 ENV_CONFIG_BROKER=
 ENV_CONFIG_REQUIREDDEFAULT=
+ENV_CONFIG_ONERR=
 ENV_CONFIG_OUTER_INNER=
 ENV_CONFIG_OUTER_PROPERTYWITHDEFAULT=
 ENV_CONFIG_AFTERNESTED=

testdata/default_list.txt

@@ -141,6 +141,11 @@ ENV_CONFIG_REQUIREDDEFAULT
 ..[type]........String
 ..[default].....foo2bar
 ..[required]....true
+ENV_CONFIG_ONERR
+..[description].
+..[type]........True.or.False
+..[default].....
+..[required]....
 ENV_CONFIG_OUTER_INNER
 ..[description].
 ..[type]........String

testdata/default_table.txt

@@ -30,6 +30,7 @@ ENV_CONFIG_DEFAULTVAR............................String.........................
 ENV_CONFIG_REQUIREDVAR...........................String................................................................true........
 ENV_CONFIG_BROKER................................String..........................................127.0.0.1.........................
 ENV_CONFIG_REQUIREDDEFAULT.......................String..........................................foo2bar...............true........
+ENV_CONFIG_ONERR.................................True.or.False.....................................................................
 ENV_CONFIG_OUTER_INNER...........................String............................................................................
 ENV_CONFIG_OUTER_PROPERTYWITHDEFAULT.............String..........................................fuzzybydefault....................
 ENV_CONFIG_AFTERNESTED...........................String............................................................................

testdata/fault.txt

@@ -34,3 +34,4 @@
 {.Key}
 {.Key}
 {.Key}
+{.Key}