Destructure New/Errorf

Destructure New/Errorf into two components, a call to the stdlib
errors.New or fmt.Errorf to generate a _fundamental_ error, and then a
call to withStack to attach a stack trace to the message.

errors.go

@@ -87,37 +87,16 @@
 package errors
 
 import (
+	"errors"
 	"fmt"
 	"io"
 )
 
-// _error is an error implementation returned by New and Errorf
-// that implements its own fmt.Formatter.
-type _error struct {
-	msg string
-	*stack
-}
-
-func (e _error) Error() string { return e.msg }
-
-func (e _error) Format(s fmt.State, verb rune) {
-	switch verb {
-	case 'v':
-		if s.Flag('+') {
-			io.WriteString(s, e.msg)
-			fmt.Fprintf(s, "%+v", e.StackTrace())
-			return
-		}
-		fallthrough
-	case 's':
-		io.WriteString(s, e.msg)
-	}
-}
-
 // New returns an error with the supplied message.
 func New(message string) error {
-	return _error{
-		message,
+	err := errors.New(message)
+	return &withStack{
+		err,
 		callers(),
 	}
 }
@@ -125,12 +104,32 @@ func New(message string) error {
 // Errorf formats according to a format specifier and returns the string
 // as a value that satisfies error.
 func Errorf(format string, args ...interface{}) error {
-	return _error{
-		fmt.Sprintf(format, args...),
+	err := fmt.Errorf(format, args...)
+	return &withStack{
+		err,
 		callers(),
 	}
 }
 
+type withStack struct {
+	error
+	*stack
+}
+
+func (w *withStack) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			io.WriteString(s, w.Error())
+			w.stack.Format(s, verb)
+			return
+		}
+		fallthrough
+	case 's':
+		io.WriteString(s, w.Error())
+	}
+}
+
 type cause struct {
 	cause error
 	msg   string

stack.go

@@ -100,6 +100,19 @@ func (st StackTrace) Format(s fmt.State, verb rune) {
 // stack represents a stack of program counters.
 type stack []uintptr
 
+func (s *stack) Format(st fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		switch {
+		case st.Flag('+'):
+			for _, pc := range *s {
+				f := Frame(pc)
+				fmt.Fprintf(st, "\n%+v", f)
+			}
+		}
+	}
+}
+
 func (s *stack) StackTrace() StackTrace {
 	f := make([]Frame, len(*s))
 	for i := 0; i < len(f); i++ {