restored the error message template

causes.go

@@ -8,9 +8,13 @@ import (
 
 type causes2 struct {
 	Code
-	Causers     []error
-	msg         string
+
+	Causers []error
+	msg     string
+
 	unwrapIndex int
+
+	liveArgs []interface{} // error message template ?
 }
 
 // WithCode for error interface
@@ -73,11 +77,22 @@ func (w *causes2) Attach(errs ...error) {
 	_ = w.WithErrors(errs...)
 }
 
+// FormatWith _
+func (w *WithStackInfo) FormatWith(args ...interface{}) {
+	w.liveArgs = args
+}
+
 func (w *causes2) Error() string {
 	var buf bytes.Buffer
 	if w.msg != "" {
-		buf.WriteString(w.msg)
+		if len(w.liveArgs) > 0 {
+			msg := fmt.Sprintf(w.msg, w.liveArgs...)
+			buf.WriteString(msg)
+		} else {
+			buf.WriteString(w.msg)
+		}
 	}
+
 	var needclose, needsep bool
 	if w.Code != OK {
 		if buf.Len() > 0 {

def.go

@@ -101,6 +101,22 @@ type Buildable interface {
 
 	// Container _
 	Container
+
+	// FormatWith adds live args for formatting the message template.
+	//
+	// While you New an Error with format template without supplying
+	// the args at same time, you'll create an error message template.
+	// You could feed the live args later.
+	// A sample is:
+	//
+	//    err := errors.New("template here: %v")
+	//    // ...
+	//    err.FormatWith("good day")
+	//    println(err)   // got: template here: good day
+	//    err.FormatWith("bye")
+	//    println(err)   // got: template here: bye
+	//
+	FormatWith(args ...interface{})
 }
 
 type causer interface {

errors_test.go

@@ -77,6 +77,7 @@ func TestUnwrap(t *testing.T) {
 		t.Logf("failed: %v", e)
 	}
 }
+
 func TestWithStackInfo_New(t *testing.T) {
 
 	err := New("hello %v", "world")
@@ -103,3 +104,14 @@ func TestWithStackInfo_New(t *testing.T) {
 	t.Logf("Data: %v", err2.Data())
 	t.Logf("TaggedData: %v", err2.TaggedData())
 }
+
+func TestTemplateFormat(t *testing.T) {
+	err := New("cannot set: %v (%v) -> %v (%v)")
+
+	err.FormatWith("a", "b", "c", "d")
+	t.Logf("Error: %v", err)
+	t.Logf("Error: %+v", err)
+
+	err.FormatWith("1", "2", "3", "4")
+	t.Logf("Error: %v", err)
+}