AddingPowerSupport_CI/Testing (#234)

* Update .travis.yml

Adding Power & Updating the go versions to 1.13/1.14/1.15 as lower versions are not supported.

* Update .travis.yml

Removing go 1.13 as desired in the comment section,

* Update .travis.yml

As desired taking out the power(ppc64le) related support.,

.travis.yml

@@ -1,10 +1,12 @@
+arch:
+  - amd64
+  
 language: go
 go_import_path: github.com/pkg/errors
 go:
-  - 1.4.3
-  - 1.5.4
-  - 1.6.2
+  - 1.14
+  - 1.15
   - tip
 
 script:
-  - go test -v ./...
+  - make check

LICENSE

@@ -21,4 +21,3 @@ SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-

Makefile

@@ -0,0 +1,44 @@
+PKGS := github.com/pkg/errors
+SRCDIRS := $(shell go list -f '{{.Dir}}' $(PKGS))
+GO := go
+
+check: test vet gofmt misspell unconvert staticcheck ineffassign unparam
+
+test: 
+	$(GO) test $(PKGS)
+
+vet: | test
+	$(GO) vet $(PKGS)
+
+staticcheck:
+	$(GO) get honnef.co/go/tools/cmd/staticcheck
+	staticcheck -checks all $(PKGS)
+
+misspell:
+	$(GO) get github.com/client9/misspell/cmd/misspell
+	misspell \
+		-locale GB \
+		-error \
+		*.md *.go
+
+unconvert:
+	$(GO) get github.com/mdempsky/unconvert
+	unconvert -v $(PKGS)
+
+ineffassign:
+	$(GO) get github.com/gordonklaus/ineffassign
+	find $(SRCDIRS) -name '*.go' | xargs ineffassign
+
+pedantic: check errcheck
+
+unparam:
+	$(GO) get mvdan.cc/unparam
+	unparam ./...
+
+errcheck:
+	$(GO) get github.com/kisielk/errcheck
+	errcheck $(PKGS)
+
+gofmt:  
+	@echo Checking code is gofmted
+	@test -z "$(shell gofmt -s -l -d -e $(SRCDIRS) | tee /dev/stderr)"

README.md

@@ -1,7 +1,9 @@
-# errors [![Travis-CI](https://travis-ci.org/pkg/errors.svg)](https://travis-ci.org/pkg/errors) [![GoDoc](https://godoc.org/github.com/pkg/errors?status.svg)](http://godoc.org/github.com/pkg/errors) [![Report card](https://goreportcard.com/badge/github.com/pkg/errors)](https://goreportcard.com/report/github.com/pkg/errors)
+# errors [![Travis-CI](https://travis-ci.org/pkg/errors.svg)](https://travis-ci.org/pkg/errors) [![AppVeyor](https://ci.appveyor.com/api/projects/status/b98mptawhudj53ep/branch/master?svg=true)](https://ci.appveyor.com/project/davecheney/errors/branch/master) [![GoDoc](https://godoc.org/github.com/pkg/errors?status.svg)](http://godoc.org/github.com/pkg/errors) [![Report card](https://goreportcard.com/badge/github.com/pkg/errors)](https://goreportcard.com/report/github.com/pkg/errors) [![Sourcegraph](https://sourcegraph.com/github.com/pkg/errors/-/badge.svg)](https://sourcegraph.com/github.com/pkg/errors?badge)
 
 Package errors provides simple error handling primitives.
 
+`go get github.com/pkg/errors`
+
 The traditional error handling idiom in Go is roughly akin to
 ```go
 if err != nil {
@@ -19,18 +21,9 @@ if err != nil {
         return errors.Wrap(err, "read failed")
 }
 ```
-## Retrieving the stack trace of an error or wrapper
-
-`New`, `Errorf`, `Wrap`, and `Wrapf` record a stack trace at the point they are invoked.
-This information can be retrieved with the following interface.
-```go
-type Stack interface {
-        Stack() []uintptr
-}
-```
 ## Retrieving the cause of an error
 
-Using `errors.Wrap` constructs a stack of errors, adding context to the preceding error. Depending on the nature of the error it may be necessary to recurse the operation of errors.Wrap to retrieve the original error for inspection. Any error value which implements this interface can be inspected by `errors.Cause`.
+Using `errors.Wrap` constructs a stack of errors, adding context to the preceding error. Depending on the nature of the error it may be necessary to reverse the operation of errors.Wrap to retrieve the original error for inspection. Any error value which implements this interface can be inspected by `errors.Cause`.
 ```go
 type causer interface {
         Cause() error
@@ -46,14 +39,21 @@ default:
 }
 ```
 
-Would you like to know more? Read the [blog post](http://dave.cheney.net/2016/04/27/dont-just-check-errors-handle-them-gracefully).
+[Read the package documentation for more information](https://godoc.org/github.com/pkg/errors).
+
+## Roadmap
+
+With the upcoming [Go2 error proposals](https://go.googlesource.com/proposal/+/master/design/go2draft.md) this package is moving into maintenance mode. The roadmap for a 1.0 release is as follows:
+
+- 0.9. Remove pre Go 1.9 and Go 1.10 support, address outstanding pull requests (if possible)
+- 1.0. Final release.
 
 ## Contributing
 
-We welcome pull requests, bug fixes and issue reports. With that said, the bar for adding new symbols to this package is intentionally set high.
+Because of the Go2 errors changes, this package is not accepting proposals for new functionality. With that said, we welcome pull requests, bug fixes and issue reports. 
 
-Before proposing a change, please discuss your change by raising an issue.
+Before sending a PR, please discuss your change by raising an issue.
 
-## Licence
+## License
 
-MIT
+BSD-2-Clause

appveyor.yml

@@ -0,0 +1,32 @@
+version: build-{build}.{branch}
+
+clone_folder: C:\gopath\src\github.com\pkg\errors
+shallow_clone: true # for startup speed
+
+environment:
+  GOPATH: C:\gopath
+
+platform:
+  - x64
+
+# http://www.appveyor.com/docs/installed-software
+install:
+  # some helpful output for debugging builds
+  - go version
+  - go env
+  # pre-installed MinGW at C:\MinGW is 32bit only
+  # but MSYS2 at C:\msys64 has mingw64
+  - set PATH=C:\msys64\mingw64\bin;%PATH%
+  - gcc --version
+  - g++ --version
+
+build_script:
+  - go install -v ./...
+
+test_script:
+  - set PATH=C:\gopath\bin;%PATH%
+  - go test -v ./...
+
+#artifacts:
+#  - path: '%GOPATH%\bin\*.exe'
+deploy: off

bench_test.go

@@ -0,0 +1,110 @@
+// +build go1.7
+
+package errors
+
+import (
+	"fmt"
+	"testing"
+
+	stderrors "errors"
+)
+
+func noErrors(at, depth int) error {
+	if at >= depth {
+		return stderrors.New("no error")
+	}
+	return noErrors(at+1, depth)
+}
+
+func yesErrors(at, depth int) error {
+	if at >= depth {
+		return New("ye error")
+	}
+	return yesErrors(at+1, depth)
+}
+
+// GlobalE is an exported global to store the result of benchmark results,
+// preventing the compiler from optimising the benchmark functions away.
+var GlobalE interface{}
+
+func BenchmarkErrors(b *testing.B) {
+	type run struct {
+		stack int
+		std   bool
+	}
+	runs := []run{
+		{10, false},
+		{10, true},
+		{100, false},
+		{100, true},
+		{1000, false},
+		{1000, true},
+	}
+	for _, r := range runs {
+		part := "pkg/errors"
+		if r.std {
+			part = "errors"
+		}
+		name := fmt.Sprintf("%s-stack-%d", part, r.stack)
+		b.Run(name, func(b *testing.B) {
+			var err error
+			f := yesErrors
+			if r.std {
+				f = noErrors
+			}
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				err = f(0, r.stack)
+			}
+			b.StopTimer()
+			GlobalE = err
+		})
+	}
+}
+
+func BenchmarkStackFormatting(b *testing.B) {
+	type run struct {
+		stack  int
+		format string
+	}
+	runs := []run{
+		{10, "%s"},
+		{10, "%v"},
+		{10, "%+v"},
+		{30, "%s"},
+		{30, "%v"},
+		{30, "%+v"},
+		{60, "%s"},
+		{60, "%v"},
+		{60, "%+v"},
+	}
+
+	var stackStr string
+	for _, r := range runs {
+		name := fmt.Sprintf("%s-stack-%d", r.format, r.stack)
+		b.Run(name, func(b *testing.B) {
+			err := yesErrors(0, r.stack)
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				stackStr = fmt.Sprintf(r.format, err)
+			}
+			b.StopTimer()
+		})
+	}
+
+	for _, r := range runs {
+		name := fmt.Sprintf("%s-stacktrace-%d", r.format, r.stack)
+		b.Run(name, func(b *testing.B) {
+			err := yesErrors(0, r.stack)
+			st := err.(*fundamental).stack.StackTrace()
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				stackStr = fmt.Sprintf(r.format, st)
+			}
+			b.StopTimer()
+		})
+	}
+	GlobalE = stackStr
+}

errors.go

@@ -6,7 +6,7 @@
 //             return err
 //     }
 //
-// which applied recursively up the call stack results in error reports
+// which when applied recursively up the call stack results in error reports
 // without context or debugging information. The errors package allows
 // programmers to add context to the failure path in their code in a way
 // that does not destroy the original value of the error.
@@ -14,21 +14,18 @@
 // Adding context to an error
 //
 // The errors.Wrap function returns a new error that adds context to the
-// original error. For example
+// original error by recording a stack trace at the point Wrap is called,
+// together with the supplied message. For example
 //
 //     _, err := ioutil.ReadAll(r)
 //     if err != nil {
 //             return errors.Wrap(err, "read failed")
 //     }
 //
-// Retrieving the stack trace of an error or wrapper
-//
-// New, Errorf, Wrap, and Wrapf record a stack trace at the point they are
-// invoked. This information can be retrieved with the following interface.
-//
-//     type Stack interface {
-//             Stack() []uintptr
-//     }
+// If additional control is required, the errors.WithStack and
+// errors.WithMessage functions destructure errors.Wrap into its component
+// operations: annotating an error with a stack trace and with a message,
+// respectively.
 //
 // Retrieving the cause of an error
 //
@@ -37,12 +34,12 @@
 // to reverse the operation of errors.Wrap to retrieve the original error
 // for inspection. Any error value which implements this interface
 //
-//     type Causer interface {
+//     type causer interface {
 //             Cause() error
 //     }
 //
 // can be inspected by errors.Cause. errors.Cause will recursively retrieve
-// the topmost error which does not implement causer, which is assumed to be
+// the topmost error that does not implement causer, which is assumed to be
 // the original cause. For example:
 //
 //     switch err := errors.Cause(err).(type) {
@@ -51,97 +48,224 @@
 //     default:
 //             // unknown error
 //     }
+//
+// Although the causer interface is not exported by this package, it is
+// considered a part of its stable public interface.
+//
+// Formatted printing of errors
+//
+// All error values returned from this package implement fmt.Formatter and can
+// be formatted by the fmt package. The following verbs are supported:
+//
+//     %s    print the error. If the error has a Cause it will be
+//           printed recursively.
+//     %v    see %s
+//     %+v   extended format. Each Frame of the error's StackTrace will
+//           be printed in detail.
+//
+// Retrieving the stack trace of an error or wrapper
+//
+// New, Errorf, Wrap, and Wrapf record a stack trace at the point they are
+// invoked. This information can be retrieved with the following interface:
+//
+//     type stackTracer interface {
+//             StackTrace() errors.StackTrace
+//     }
+//
+// The returned errors.StackTrace type is defined as
+//
+//     type StackTrace []Frame
+//
+// The Frame type represents a call site in the stack trace. Frame supports
+// the fmt.Formatter interface that can be used for printing information about
+// the stack trace of this error. For example:
+//
+//     if err, ok := err.(stackTracer); ok {
+//             for _, f := range err.StackTrace() {
+//                     fmt.Printf("%+s:%d\n", f, f)
+//             }
+//     }
+//
+// Although the stackTracer interface is not exported by this package, it is
+// considered a part of its stable public interface.
+//
+// See the documentation for Frame.Format for more details.
 package errors
 
 import (
-	"errors"
 	"fmt"
 	"io"
-	"runtime"
-	"strings"
 )
 
-// stack represents a stack of programm counters.
-type stack []uintptr
-
-func (s *stack) Stack() []uintptr { return *s }
-
-func (s *stack) Location() (string, int) {
-	return location((*s)[0] - 1)
-}
-
-// New returns an error that formats as the given text.
-func New(text string) error {
-	return struct {
-		error
-		*stack
-	}{
-		errors.New(text),
-		callers(),
+// New returns an error with the supplied message.
+// New also records the stack trace at the point it was called.
+func New(message string) error {
+	return &fundamental{
+		msg:   message,
+		stack: callers(),
 	}
 }
 
-type cause struct {
-	cause   error
-	message string
-}
-
-func (c cause) Error() string   { return c.Message() + ": " + c.Cause().Error() }
-func (c cause) Cause() error    { return c.cause }
-func (c cause) Message() string { return c.message }
-
 // Errorf formats according to a format specifier and returns the string
 // as a value that satisfies error.
+// Errorf also records the stack trace at the point it was called.
 func Errorf(format string, args ...interface{}) error {
-	return struct {
-		error
+	return &fundamental{
+		msg:   fmt.Sprintf(format, args...),
+		stack: callers(),
+	}
+}
+
+// fundamental is an error that has a message and a stack, but no caller.
+type fundamental struct {
+	msg string
 	*stack
-	}{
-		fmt.Errorf(format, args...),
+}
+
+func (f *fundamental) Error() string { return f.msg }
+
+func (f *fundamental) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			io.WriteString(s, f.msg)
+			f.stack.Format(s, verb)
+			return
+		}
+		fallthrough
+	case 's':
+		io.WriteString(s, f.msg)
+	case 'q':
+		fmt.Fprintf(s, "%q", f.msg)
+	}
+}
+
+// WithStack annotates err with a stack trace at the point WithStack was called.
+// If err is nil, WithStack returns nil.
+func WithStack(err error) error {
+	if err == nil {
+		return nil
+	}
+	return &withStack{
+		err,
 		callers(),
 	}
 }
 
-// Wrap returns an error annotating the cause with message.
-// If cause is nil, Wrap returns nil.
-func Wrap(cause error, message string) error {
-	if cause == nil {
-		return nil
-	}
-	return wrap(cause, message, callers())
-}
-
-// Wrapf returns an error annotating the cause with the format specifier.
-// If cause is nil, Wrapf returns nil.
-func Wrapf(cause error, format string, args ...interface{}) error {
-	if cause == nil {
-		return nil
-	}
-	return wrap(cause, fmt.Sprintf(format, args...), callers())
-}
-
-func wrap(err error, msg string, st *stack) error {
-	return struct {
-		cause
+type withStack struct {
+	error
 	*stack
-	}{
-		cause{
-			cause:   err,
-			message: msg,
-		},
-		st,
+}
+
+func (w *withStack) Cause() error { return w.error }
+
+// Unwrap provides compatibility for Go 1.13 error chains.
+func (w *withStack) Unwrap() error { return w.error }
+
+func (w *withStack) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			fmt.Fprintf(s, "%+v", w.Cause())
+			w.stack.Format(s, verb)
+			return
+		}
+		fallthrough
+	case 's':
+		io.WriteString(s, w.Error())
+	case 'q':
+		fmt.Fprintf(s, "%q", w.Error())
 	}
 }
 
-type causer interface {
-	Cause() error
+// Wrap returns an error annotating err with a stack trace
+// at the point Wrap is called, and the supplied message.
+// If err is nil, Wrap returns nil.
+func Wrap(err error, message string) error {
+	if err == nil {
+		return nil
+	}
+	err = &withMessage{
+		cause: err,
+		msg:   message,
+	}
+	return &withStack{
+		err,
+		callers(),
+	}
+}
+
+// Wrapf returns an error annotating err with a stack trace
+// at the point Wrapf is called, and the format specifier.
+// If err is nil, Wrapf returns nil.
+func Wrapf(err error, format string, args ...interface{}) error {
+	if err == nil {
+		return nil
+	}
+	err = &withMessage{
+		cause: err,
+		msg:   fmt.Sprintf(format, args...),
+	}
+	return &withStack{
+		err,
+		callers(),
+	}
+}
+
+// WithMessage annotates err with a new message.
+// If err is nil, WithMessage returns nil.
+func WithMessage(err error, message string) error {
+	if err == nil {
+		return nil
+	}
+	return &withMessage{
+		cause: err,
+		msg:   message,
+	}
+}
+
+// WithMessagef annotates err with the format specifier.
+// If err is nil, WithMessagef returns nil.
+func WithMessagef(err error, format string, args ...interface{}) error {
+	if err == nil {
+		return nil
+	}
+	return &withMessage{
+		cause: err,
+		msg:   fmt.Sprintf(format, args...),
+	}
+}
+
+type withMessage struct {
+	cause error
+	msg   string
+}
+
+func (w *withMessage) Error() string { return w.msg + ": " + w.cause.Error() }
+func (w *withMessage) Cause() error  { return w.cause }
+
+// Unwrap provides compatibility for Go 1.13 error chains.
+func (w *withMessage) Unwrap() error { return w.cause }
+
+func (w *withMessage) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		if s.Flag('+') {
+			fmt.Fprintf(s, "%+v\n", w.Cause())
+			io.WriteString(s, w.msg)
+			return
+		}
+		fallthrough
+	case 's', 'q':
+		io.WriteString(s, w.Error())
+	}
 }
 
 // Cause returns the underlying cause of the error, if possible.
 // An error value has a cause if it implements the following
 // interface:
 //
-//     type Causer interface {
+//     type causer interface {
 //            Cause() error
 //     }
 //
@@ -149,6 +273,10 @@ type causer interface {
 // be returned. If the error is nil, nil will be returned without further
 // investigation.
 func Cause(err error) error {
+	type causer interface {
+		Cause() error
+	}
+
 	for err != nil {
 		cause, ok := err.(causer)
 		if !ok {
@@ -158,99 +286,3 @@ func Cause(err error) error {
 	}
 	return err
 }
-
-// Fprint prints the error to the supplied writer.
-// If the error implements the Causer interface described in Cause
-// Print will recurse into the error's cause.
-// If the error implements the inteface:
-//
-//     type Location interface {
-//            Location() (file string, line int)
-//     }
-//
-// Print will also print the file and line of the error.
-// If err is nil, nothing is printed.
-func Fprint(w io.Writer, err error) {
-	type location interface {
-		Location() (string, int)
-	}
-	type message interface {
-		Message() string
-	}
-
-	for err != nil {
-		if err, ok := err.(location); ok {
-			file, line := err.Location()
-			fmt.Fprintf(w, "%s:%d: ", file, line)
-		}
-		switch err := err.(type) {
-		case message:
-			fmt.Fprintln(w, err.Message())
-		default:
-			fmt.Fprintln(w, err.Error())
-		}
-
-		cause, ok := err.(causer)
-		if !ok {
-			break
-		}
-		err = cause.Cause()
-	}
-}
-
-func callers() *stack {
-	const depth = 32
-	var pcs [depth]uintptr
-	n := runtime.Callers(3, pcs[:])
-	var st stack = pcs[0:n]
-	return &st
-}
-
-// location returns the source file and line matching pc.
-func location(pc uintptr) (string, int) {
-	fn := runtime.FuncForPC(pc)
-	if fn == nil {
-		return "unknown", 0
-	}
-
-	file, line := fn.FileLine(pc)
-
-	// Here we want to get the source file path relative to the compile time
-	// GOPATH. As of Go 1.6.x there is no direct way to know the compiled
-	// GOPATH at runtime, but we can infer the number of path segments in the
-	// GOPATH. We note that fn.Name() returns the function name qualified by
-	// the import path, which does not include the GOPATH. Thus we can trim
-	// segments from the beginning of the file path until the number of path
-	// separators remaining is one more than the number of path separators in
-	// the function name. For example, given:
-	//
-	//    GOPATH     /home/user
-	//    file       /home/user/src/pkg/sub/file.go
-	//    fn.Name()  pkg/sub.Type.Method
-	//
-	// We want to produce:
-	//
-	//    pkg/sub/file.go
-	//
-	// From this we can easily see that fn.Name() has one less path separator
-	// than our desired output. We count separators from the end of the file
-	// path until it finds two more than in the function name and then move
-	// one character forward to preserve the initial path segment without a
-	// leading separator.
-	const sep = "/"
-	goal := strings.Count(fn.Name(), sep) + 2
-	i := len(file)
-	for n := 0; n < goal; n++ {
-		i = strings.LastIndex(file[:i], sep)
-		if i == -1 {
-			// not enough separators found, set i so that the slice expression
-			// below leaves file unmodified
-			i = -len(sep)
-			break
-		}
-	}
-	// get back to 0 or trim the leading separator
-	file = file[i+len(sep):]
-
-	return file, line
-}

errors_test.go

@@ -1,7 +1,6 @@
 package errors
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -57,13 +56,6 @@ type nilError struct{}
 
 func (nilError) Error() string { return "nil error" }
 
-type causeError struct {
-	cause error
-}
-
-func (e *causeError) Error() string { return "cause error" }
-func (e *causeError) Cause() error  { return e.cause }
-
 func TestCause(t *testing.T) {
 	x := New("error")
 	tests := []struct {
@@ -87,11 +79,23 @@ func TestCause(t *testing.T) {
 		want: io.EOF,
 	}, {
 		// caused error returns cause
-		err:  &causeError{cause: io.EOF},
+		err:  Wrap(io.EOF, "ignored"),
 		want: io.EOF,
 	}, {
 		err:  x, // return from errors.New
 		want: x,
+	}, {
+		WithMessage(nil, "whoops"),
+		nil,
+	}, {
+		WithMessage(io.EOF, "whoops"),
+		io.EOF,
+	}, {
+		WithStack(nil),
+		nil,
+	}, {
+		WithStack(io.EOF),
+		io.EOF,
 	}}
 
 	for i, tt := range tests {
@@ -102,49 +106,6 @@ func TestCause(t *testing.T) {
 	}
 }
 
-func TestFprint(t *testing.T) {
-	x := New("error")
-	tests := []struct {
-		err  error
-		want string
-	}{{
-		// nil error is nil
-		err: nil,
-	}, {
-		// explicit nil error is nil
-		err: (error)(nil),
-	}, {
-		// uncaused error is unaffected
-		err:  io.EOF,
-		want: "EOF\n",
-	}, {
-		// caused error returns cause
-		err:  &causeError{cause: io.EOF},
-		want: "cause error\nEOF\n",
-	}, {
-		err:  x, // return from errors.New
-		want: "github.com/pkg/errors/errors_test.go:106: error\n",
-	}, {
-		err:  Wrap(x, "message"),
-		want: "github.com/pkg/errors/errors_test.go:128: message\ngithub.com/pkg/errors/errors_test.go:106: error\n",
-	}, {
-		err:  Wrap(Wrap(x, "message"), "another message"),
-		want: "github.com/pkg/errors/errors_test.go:131: another message\ngithub.com/pkg/errors/errors_test.go:131: message\ngithub.com/pkg/errors/errors_test.go:106: error\n",
-	}, {
-		err:  Wrapf(x, "message"),
-		want: "github.com/pkg/errors/errors_test.go:134: message\ngithub.com/pkg/errors/errors_test.go:106: error\n",
-	}}
-
-	for i, tt := range tests {
-		var w bytes.Buffer
-		Fprint(&w, tt.err)
-		got := w.String()
-		if got != tt.want {
-			t.Errorf("test %d: Fprint(w, %q): got %q, want %q", i+1, tt.err, got, tt.want)
-		}
-	}
-}
-
 func TestWrapfNil(t *testing.T) {
 	got := Wrapf(nil, "no error")
 	if got != nil {
@@ -188,57 +149,78 @@ func TestErrorf(t *testing.T) {
 	}
 }
 
-func TestStack(t *testing.T) {
-	type fileline struct {
-		file string
-		line int
+func TestWithStackNil(t *testing.T) {
+	got := WithStack(nil)
+	if got != nil {
+		t.Errorf("WithStack(nil): got %#v, expected nil", got)
 	}
+}
+
+func TestWithStack(t *testing.T) {
 	tests := []struct {
 		err  error
-		want []fileline
-	}{{
-		New("ooh"), []fileline{
-			{"github.com/pkg/errors/errors_test.go", 200},
-		},
-	}, {
-		Wrap(New("ooh"), "ahh"), []fileline{
-			{"github.com/pkg/errors/errors_test.go", 204}, // this is the stack of Wrap, not New
-		},
-	}, {
-		Cause(Wrap(New("ooh"), "ahh")), []fileline{
-			{"github.com/pkg/errors/errors_test.go", 208}, // this is the stack of New
-		},
-	}, {
-		func() error { return New("ooh") }(), []fileline{
-			{"github.com/pkg/errors/errors_test.go", 212}, // this is the stack of New
-			{"github.com/pkg/errors/errors_test.go", 212}, // this is the stack of New's caller
-		},
-	}, {
-		Cause(func() error {
-			return func() error {
-				return Errorf("hello %s", fmt.Sprintf("world"))
-			}()
-		}()), []fileline{
-			{"github.com/pkg/errors/errors_test.go", 219}, // this is the stack of Errorf
-			{"github.com/pkg/errors/errors_test.go", 220}, // this is the stack of Errorf's caller
-			{"github.com/pkg/errors/errors_test.go", 221}, // this is the stack of Errorf's caller's caller
-		},
-	}}
+		want string
+	}{
+		{io.EOF, "EOF"},
+		{WithStack(io.EOF), "EOF"},
+	}
+
 	for _, tt := range tests {
-		x, ok := tt.err.(interface {
-			Stack() []uintptr
-		})
-		if !ok {
-			t.Errorf("expected %#v to implement Stack()", tt.err)
-			continue
+		got := WithStack(tt.err).Error()
+		if got != tt.want {
+			t.Errorf("WithStack(%v): got: %v, want %v", tt.err, got, tt.want)
 		}
-		st := x.Stack()
-		for i, want := range tt.want {
-			file, line := location(st[i] - 1)
-			if file != want.file || line != want.line {
-				t.Errorf("frame %d: expected %s:%d, got %s:%d", i, want.file, want.line, file, line)
 	}
 }
+
+func TestWithMessageNil(t *testing.T) {
+	got := WithMessage(nil, "no error")
+	if got != nil {
+		t.Errorf("WithMessage(nil, \"no error\"): got %#v, expected nil", got)
+	}
+}
+
+func TestWithMessage(t *testing.T) {
+	tests := []struct {
+		err     error
+		message string
+		want    string
+	}{
+		{io.EOF, "read error", "read error: EOF"},
+		{WithMessage(io.EOF, "read error"), "client error", "client error: read error: EOF"},
+	}
+
+	for _, tt := range tests {
+		got := WithMessage(tt.err, tt.message).Error()
+		if got != tt.want {
+			t.Errorf("WithMessage(%v, %q): got: %q, want %q", tt.err, tt.message, got, tt.want)
+		}
+	}
+}
+
+func TestWithMessagefNil(t *testing.T) {
+	got := WithMessagef(nil, "no error")
+	if got != nil {
+		t.Errorf("WithMessage(nil, \"no error\"): got %#v, expected nil", got)
+	}
+}
+
+func TestWithMessagef(t *testing.T) {
+	tests := []struct {
+		err     error
+		message string
+		want    string
+	}{
+		{io.EOF, "read error", "read error: EOF"},
+		{WithMessagef(io.EOF, "read error without format specifier"), "client error", "client error: read error without format specifier: EOF"},
+		{WithMessagef(io.EOF, "read error with %d format specifier", 1), "client error", "client error: read error with 1 format specifier: EOF"},
+	}
+
+	for _, tt := range tests {
+		got := WithMessagef(tt.err, tt.message).Error()
+		if got != tt.want {
+			t.Errorf("WithMessage(%v, %q): got: %q, want %q", tt.err, tt.message, got, tt.want)
+		}
 	}
 }
 
@@ -247,18 +229,23 @@ func TestStack(t *testing.T) {
 // various kinds of errors have a functional equality operator, even
 // if the result of that equality is always false.
 func TestErrorEquality(t *testing.T) {
-	tests := []struct {
-		err1, err2 error
-	}{
-		{io.EOF, io.EOF},
-		{io.EOF, nil},
-		{io.EOF, errors.New("EOF")},
-		{io.EOF, New("EOF")},
-		{New("EOF"), New("EOF")},
-		{New("EOF"), Errorf("EOF")},
-		{New("EOF"), Wrap(io.EOF, "EOF")},
+	vals := []error{
+		nil,
+		io.EOF,
+		errors.New("EOF"),
+		New("EOF"),
+		Errorf("EOF"),
+		Wrap(io.EOF, "EOF"),
+		Wrapf(io.EOF, "EOF%d", 2),
+		WithMessage(nil, "whoops"),
+		WithMessage(io.EOF, "whoops"),
+		WithStack(io.EOF),
+		WithStack(nil),
+	}
+
+	for i := range vals {
+		for j := range vals {
+			_ = vals[i] == vals[j] // mustn't panic
 		}
-	for _, tt := range tests {
-		_ = tt.err1 == tt.err2 // mustn't panic
 	}
 }

example_test.go

@@ -2,7 +2,6 @@ package errors_test
 
 import (
 	"fmt"
-	"os"
 
 	"github.com/pkg/errors"
 )
@@ -14,11 +13,79 @@ func ExampleNew() {
 	// Output: whoops
 }
 
-func ExampleNew_fprint() {
+func ExampleNew_printf() {
 	err := errors.New("whoops")
-	errors.Fprint(os.Stdout, err)
+	fmt.Printf("%+v", err)
 
-	// Output: github.com/pkg/errors/example_test.go:18: whoops
+	// Example output:
+	// whoops
+	// github.com/pkg/errors_test.ExampleNew_printf
+	//         /home/dfc/src/github.com/pkg/errors/example_test.go:17
+	// testing.runExample
+	//         /home/dfc/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /home/dfc/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /home/dfc/go/src/testing/testing.go:744
+	// main.main
+	//         /github.com/pkg/errors/_test/_testmain.go:106
+	// runtime.main
+	//         /home/dfc/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /home/dfc/go/src/runtime/asm_amd64.s:2059
+}
+
+func ExampleWithMessage() {
+	cause := errors.New("whoops")
+	err := errors.WithMessage(cause, "oh noes")
+	fmt.Println(err)
+
+	// Output: oh noes: whoops
+}
+
+func ExampleWithStack() {
+	cause := errors.New("whoops")
+	err := errors.WithStack(cause)
+	fmt.Println(err)
+
+	// Output: whoops
+}
+
+func ExampleWithStack_printf() {
+	cause := errors.New("whoops")
+	err := errors.WithStack(cause)
+	fmt.Printf("%+v", err)
+
+	// Example Output:
+	// whoops
+	// github.com/pkg/errors_test.ExampleWithStack_printf
+	//         /home/fabstu/go/src/github.com/pkg/errors/example_test.go:55
+	// testing.runExample
+	//         /usr/lib/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /usr/lib/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /usr/lib/go/src/testing/testing.go:744
+	// main.main
+	//         github.com/pkg/errors/_test/_testmain.go:106
+	// runtime.main
+	//         /usr/lib/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /usr/lib/go/src/runtime/asm_amd64.s:2086
+	// github.com/pkg/errors_test.ExampleWithStack_printf
+	//         /home/fabstu/go/src/github.com/pkg/errors/example_test.go:56
+	// testing.runExample
+	//         /usr/lib/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /usr/lib/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /usr/lib/go/src/testing/testing.go:744
+	// main.main
+	//         github.com/pkg/errors/_test/_testmain.go:106
+	// runtime.main
+	//         /usr/lib/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /usr/lib/go/src/runtime/asm_amd64.s:2086
 }
 
 func ExampleWrap() {
@@ -45,14 +112,34 @@ func ExampleCause() {
 	// error
 }
 
-func ExampleFprint() {
+func ExampleWrap_extended() {
 	err := fn()
-	errors.Fprint(os.Stdout, err)
+	fmt.Printf("%+v\n", err)
 
-	// Output: github.com/pkg/errors/example_test.go:36: outer
-	// github.com/pkg/errors/example_test.go:35: middle
-	// github.com/pkg/errors/example_test.go:34: inner
-	// github.com/pkg/errors/example_test.go:33: error
+	// Example output:
+	// error
+	// github.com/pkg/errors_test.fn
+	//         /home/dfc/src/github.com/pkg/errors/example_test.go:47
+	// github.com/pkg/errors_test.ExampleCause_printf
+	//         /home/dfc/src/github.com/pkg/errors/example_test.go:63
+	// testing.runExample
+	//         /home/dfc/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /home/dfc/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /home/dfc/go/src/testing/testing.go:744
+	// main.main
+	//         /github.com/pkg/errors/_test/_testmain.go:104
+	// runtime.main
+	//         /home/dfc/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /home/dfc/go/src/runtime/asm_amd64.s:2059
+	// github.com/pkg/errors_test.fn
+	// 	  /home/dfc/src/github.com/pkg/errors/example_test.go:48: inner
+	// github.com/pkg/errors_test.fn
+	//        /home/dfc/src/github.com/pkg/errors/example_test.go:49: middle
+	// github.com/pkg/errors_test.fn
+	//      /home/dfc/src/github.com/pkg/errors/example_test.go:50: outer
 }
 
 func ExampleWrapf() {
@@ -63,9 +150,56 @@ func ExampleWrapf() {
 	// Output: oh noes #2: whoops
 }
 
-func ExampleErrorf() {
+func ExampleErrorf_extended() {
 	err := errors.Errorf("whoops: %s", "foo")
-	errors.Fprint(os.Stdout, err)
+	fmt.Printf("%+v", err)
 
-	// Output: github.com/pkg/errors/example_test.go:67: whoops: foo
+	// Example output:
+	// whoops: foo
+	// github.com/pkg/errors_test.ExampleErrorf
+	//         /home/dfc/src/github.com/pkg/errors/example_test.go:101
+	// testing.runExample
+	//         /home/dfc/go/src/testing/example.go:114
+	// testing.RunExamples
+	//         /home/dfc/go/src/testing/example.go:38
+	// testing.(*M).Run
+	//         /home/dfc/go/src/testing/testing.go:744
+	// main.main
+	//         /github.com/pkg/errors/_test/_testmain.go:102
+	// runtime.main
+	//         /home/dfc/go/src/runtime/proc.go:183
+	// runtime.goexit
+	//         /home/dfc/go/src/runtime/asm_amd64.s:2059
+}
+
+func Example_stackTrace() {
+	type stackTracer interface {
+		StackTrace() errors.StackTrace
+	}
+
+	err, ok := errors.Cause(fn()).(stackTracer)
+	if !ok {
+		panic("oops, err does not implement stackTracer")
+	}
+
+	st := err.StackTrace()
+	fmt.Printf("%+v", st[0:2]) // top two frames
+
+	// Example output:
+	// github.com/pkg/errors_test.fn
+	//	/home/dfc/src/github.com/pkg/errors/example_test.go:47
+	// github.com/pkg/errors_test.Example_stackTrace
+	//	/home/dfc/src/github.com/pkg/errors/example_test.go:127
+}
+
+func ExampleCause_printf() {
+	err := errors.Wrap(func() error {
+		return func() error {
+			return errors.New("hello world")
+		}()
+	}(), "failed")
+
+	fmt.Printf("%v", err)
+
+	// Output: failed: hello world
 }

format_test.go

@@ -0,0 +1,560 @@
+package errors
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+func TestFormatNew(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   string
+	}{{
+		New("error"),
+		"%s",
+		"error",
+	}, {
+		New("error"),
+		"%v",
+		"error",
+	}, {
+		New("error"),
+		"%+v",
+		"error\n" +
+			"github.com/pkg/errors.TestFormatNew\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:26",
+	}, {
+		New("error"),
+		"%q",
+		`"error"`,
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.error, tt.format, tt.want)
+	}
+}
+
+func TestFormatErrorf(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   string
+	}{{
+		Errorf("%s", "error"),
+		"%s",
+		"error",
+	}, {
+		Errorf("%s", "error"),
+		"%v",
+		"error",
+	}, {
+		Errorf("%s", "error"),
+		"%+v",
+		"error\n" +
+			"github.com/pkg/errors.TestFormatErrorf\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:56",
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.error, tt.format, tt.want)
+	}
+}
+
+func TestFormatWrap(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   string
+	}{{
+		Wrap(New("error"), "error2"),
+		"%s",
+		"error2: error",
+	}, {
+		Wrap(New("error"), "error2"),
+		"%v",
+		"error2: error",
+	}, {
+		Wrap(New("error"), "error2"),
+		"%+v",
+		"error\n" +
+			"github.com/pkg/errors.TestFormatWrap\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:82",
+	}, {
+		Wrap(io.EOF, "error"),
+		"%s",
+		"error: EOF",
+	}, {
+		Wrap(io.EOF, "error"),
+		"%v",
+		"error: EOF",
+	}, {
+		Wrap(io.EOF, "error"),
+		"%+v",
+		"EOF\n" +
+			"error\n" +
+			"github.com/pkg/errors.TestFormatWrap\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:96",
+	}, {
+		Wrap(Wrap(io.EOF, "error1"), "error2"),
+		"%+v",
+		"EOF\n" +
+			"error1\n" +
+			"github.com/pkg/errors.TestFormatWrap\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:103\n",
+	}, {
+		Wrap(New("error with space"), "context"),
+		"%q",
+		`"context: error with space"`,
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.error, tt.format, tt.want)
+	}
+}
+
+func TestFormatWrapf(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   string
+	}{{
+		Wrapf(io.EOF, "error%d", 2),
+		"%s",
+		"error2: EOF",
+	}, {
+		Wrapf(io.EOF, "error%d", 2),
+		"%v",
+		"error2: EOF",
+	}, {
+		Wrapf(io.EOF, "error%d", 2),
+		"%+v",
+		"EOF\n" +
+			"error2\n" +
+			"github.com/pkg/errors.TestFormatWrapf\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:134",
+	}, {
+		Wrapf(New("error"), "error%d", 2),
+		"%s",
+		"error2: error",
+	}, {
+		Wrapf(New("error"), "error%d", 2),
+		"%v",
+		"error2: error",
+	}, {
+		Wrapf(New("error"), "error%d", 2),
+		"%+v",
+		"error\n" +
+			"github.com/pkg/errors.TestFormatWrapf\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:149",
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.error, tt.format, tt.want)
+	}
+}
+
+func TestFormatWithStack(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   []string
+	}{{
+		WithStack(io.EOF),
+		"%s",
+		[]string{"EOF"},
+	}, {
+		WithStack(io.EOF),
+		"%v",
+		[]string{"EOF"},
+	}, {
+		WithStack(io.EOF),
+		"%+v",
+		[]string{"EOF",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:175"},
+	}, {
+		WithStack(New("error")),
+		"%s",
+		[]string{"error"},
+	}, {
+		WithStack(New("error")),
+		"%v",
+		[]string{"error"},
+	}, {
+		WithStack(New("error")),
+		"%+v",
+		[]string{"error",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:189",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:189"},
+	}, {
+		WithStack(WithStack(io.EOF)),
+		"%+v",
+		[]string{"EOF",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:197",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:197"},
+	}, {
+		WithStack(WithStack(Wrapf(io.EOF, "message"))),
+		"%+v",
+		[]string{"EOF",
+			"message",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:205",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:205",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:205"},
+	}, {
+		WithStack(Errorf("error%d", 1)),
+		"%+v",
+		[]string{"error1",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:216",
+			"github.com/pkg/errors.TestFormatWithStack\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:216"},
+	}}
+
+	for i, tt := range tests {
+		testFormatCompleteCompare(t, i, tt.error, tt.format, tt.want, true)
+	}
+}
+
+func TestFormatWithMessage(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   []string
+	}{{
+		WithMessage(New("error"), "error2"),
+		"%s",
+		[]string{"error2: error"},
+	}, {
+		WithMessage(New("error"), "error2"),
+		"%v",
+		[]string{"error2: error"},
+	}, {
+		WithMessage(New("error"), "error2"),
+		"%+v",
+		[]string{
+			"error",
+			"github.com/pkg/errors.TestFormatWithMessage\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:244",
+			"error2"},
+	}, {
+		WithMessage(io.EOF, "addition1"),
+		"%s",
+		[]string{"addition1: EOF"},
+	}, {
+		WithMessage(io.EOF, "addition1"),
+		"%v",
+		[]string{"addition1: EOF"},
+	}, {
+		WithMessage(io.EOF, "addition1"),
+		"%+v",
+		[]string{"EOF", "addition1"},
+	}, {
+		WithMessage(WithMessage(io.EOF, "addition1"), "addition2"),
+		"%v",
+		[]string{"addition2: addition1: EOF"},
+	}, {
+		WithMessage(WithMessage(io.EOF, "addition1"), "addition2"),
+		"%+v",
+		[]string{"EOF", "addition1", "addition2"},
+	}, {
+		Wrap(WithMessage(io.EOF, "error1"), "error2"),
+		"%+v",
+		[]string{"EOF", "error1", "error2",
+			"github.com/pkg/errors.TestFormatWithMessage\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:272"},
+	}, {
+		WithMessage(Errorf("error%d", 1), "error2"),
+		"%+v",
+		[]string{"error1",
+			"github.com/pkg/errors.TestFormatWithMessage\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:278",
+			"error2"},
+	}, {
+		WithMessage(WithStack(io.EOF), "error"),
+		"%+v",
+		[]string{
+			"EOF",
+			"github.com/pkg/errors.TestFormatWithMessage\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:285",
+			"error"},
+	}, {
+		WithMessage(Wrap(WithStack(io.EOF), "inside-error"), "outside-error"),
+		"%+v",
+		[]string{
+			"EOF",
+			"github.com/pkg/errors.TestFormatWithMessage\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:293",
+			"inside-error",
+			"github.com/pkg/errors.TestFormatWithMessage\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:293",
+			"outside-error"},
+	}}
+
+	for i, tt := range tests {
+		testFormatCompleteCompare(t, i, tt.error, tt.format, tt.want, true)
+	}
+}
+
+func TestFormatGeneric(t *testing.T) {
+	starts := []struct {
+		err  error
+		want []string
+	}{
+		{New("new-error"), []string{
+			"new-error",
+			"github.com/pkg/errors.TestFormatGeneric\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:315"},
+		}, {Errorf("errorf-error"), []string{
+			"errorf-error",
+			"github.com/pkg/errors.TestFormatGeneric\n" +
+				"\t.+/github.com/pkg/errors/format_test.go:319"},
+		}, {errors.New("errors-new-error"), []string{
+			"errors-new-error"},
+		},
+	}
+
+	wrappers := []wrapper{
+		{
+			func(err error) error { return WithMessage(err, "with-message") },
+			[]string{"with-message"},
+		}, {
+			func(err error) error { return WithStack(err) },
+			[]string{
+				"github.com/pkg/errors.(func·002|TestFormatGeneric.func2)\n\t" +
+					".+/github.com/pkg/errors/format_test.go:333",
+			},
+		}, {
+			func(err error) error { return Wrap(err, "wrap-error") },
+			[]string{
+				"wrap-error",
+				"github.com/pkg/errors.(func·003|TestFormatGeneric.func3)\n\t" +
+					".+/github.com/pkg/errors/format_test.go:339",
+			},
+		}, {
+			func(err error) error { return Wrapf(err, "wrapf-error%d", 1) },
+			[]string{
+				"wrapf-error1",
+				"github.com/pkg/errors.(func·004|TestFormatGeneric.func4)\n\t" +
+					".+/github.com/pkg/errors/format_test.go:346",
+			},
+		},
+	}
+
+	for s := range starts {
+		err := starts[s].err
+		want := starts[s].want
+		testFormatCompleteCompare(t, s, err, "%+v", want, false)
+		testGenericRecursive(t, err, want, wrappers, 3)
+	}
+}
+
+func wrappedNew(message string) error { // This function will be mid-stack inlined in go 1.12+
+	return New(message)
+}
+
+func TestFormatWrappedNew(t *testing.T) {
+	tests := []struct {
+		error
+		format string
+		want   string
+	}{{
+		wrappedNew("error"),
+		"%+v",
+		"error\n" +
+			"github.com/pkg/errors.wrappedNew\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:364\n" +
+			"github.com/pkg/errors.TestFormatWrappedNew\n" +
+			"\t.+/github.com/pkg/errors/format_test.go:373",
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.error, tt.format, tt.want)
+	}
+}
+
+func testFormatRegexp(t *testing.T, n int, arg interface{}, format, want string) {
+	t.Helper()
+	got := fmt.Sprintf(format, arg)
+	gotLines := strings.SplitN(got, "\n", -1)
+	wantLines := strings.SplitN(want, "\n", -1)
+
+	if len(wantLines) > len(gotLines) {
+		t.Errorf("test %d: wantLines(%d) > gotLines(%d):\n got: %q\nwant: %q", n+1, len(wantLines), len(gotLines), got, want)
+		return
+	}
+
+	for i, w := range wantLines {
+		match, err := regexp.MatchString(w, gotLines[i])
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !match {
+			t.Errorf("test %d: line %d: fmt.Sprintf(%q, err):\n got: %q\nwant: %q", n+1, i+1, format, got, want)
+		}
+	}
+}
+
+var stackLineR = regexp.MustCompile(`\.`)
+
+// parseBlocks parses input into a slice, where:
+//  - incase entry contains a newline, its a stacktrace
+//  - incase entry contains no newline, its a solo line.
+//
+// Detecting stack boundaries only works incase the WithStack-calls are
+// to be found on the same line, thats why it is optionally here.
+//
+// Example use:
+//
+// for _, e := range blocks {
+//   if strings.ContainsAny(e, "\n") {
+//     // Match as stack
+//   } else {
+//     // Match as line
+//   }
+// }
+//
+func parseBlocks(input string, detectStackboundaries bool) ([]string, error) {
+	var blocks []string
+
+	stack := ""
+	wasStack := false
+	lines := map[string]bool{} // already found lines
+
+	for _, l := range strings.Split(input, "\n") {
+		isStackLine := stackLineR.MatchString(l)
+
+		switch {
+		case !isStackLine && wasStack:
+			blocks = append(blocks, stack, l)
+			stack = ""
+			lines = map[string]bool{}
+		case isStackLine:
+			if wasStack {
+				// Detecting two stacks after another, possible cause lines match in
+				// our tests due to WithStack(WithStack(io.EOF)) on same line.
+				if detectStackboundaries {
+					if lines[l] {
+						if len(stack) == 0 {
+							return nil, errors.New("len of block must not be zero here")
+						}
+
+						blocks = append(blocks, stack)
+						stack = l
+						lines = map[string]bool{l: true}
+						continue
+					}
+				}
+
+				stack = stack + "\n" + l
+			} else {
+				stack = l
+			}
+			lines[l] = true
+		case !isStackLine && !wasStack:
+			blocks = append(blocks, l)
+		default:
+			return nil, errors.New("must not happen")
+		}
+
+		wasStack = isStackLine
+	}
+
+	// Use up stack
+	if stack != "" {
+		blocks = append(blocks, stack)
+	}
+	return blocks, nil
+}
+
+func testFormatCompleteCompare(t *testing.T, n int, arg interface{}, format string, want []string, detectStackBoundaries bool) {
+	gotStr := fmt.Sprintf(format, arg)
+
+	got, err := parseBlocks(gotStr, detectStackBoundaries)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(got) != len(want) {
+		t.Fatalf("test %d: fmt.Sprintf(%s, err) -> wrong number of blocks: got(%d) want(%d)\n got: %s\nwant: %s\ngotStr: %q",
+			n+1, format, len(got), len(want), prettyBlocks(got), prettyBlocks(want), gotStr)
+	}
+
+	for i := range got {
+		if strings.ContainsAny(want[i], "\n") {
+			// Match as stack
+			match, err := regexp.MatchString(want[i], got[i])
+			if err != nil {
+				t.Fatal(err)
+			}
+			if !match {
+				t.Fatalf("test %d: block %d: fmt.Sprintf(%q, err):\ngot:\n%q\nwant:\n%q\nall-got:\n%s\nall-want:\n%s\n",
+					n+1, i+1, format, got[i], want[i], prettyBlocks(got), prettyBlocks(want))
+			}
+		} else {
+			// Match as message
+			if got[i] != want[i] {
+				t.Fatalf("test %d: fmt.Sprintf(%s, err) at block %d got != want:\n got: %q\nwant: %q", n+1, format, i+1, got[i], want[i])
+			}
+		}
+	}
+}
+
+type wrapper struct {
+	wrap func(err error) error
+	want []string
+}
+
+func prettyBlocks(blocks []string) string {
+	var out []string
+
+	for _, b := range blocks {
+		out = append(out, fmt.Sprintf("%v", b))
+	}
+
+	return "   " + strings.Join(out, "\n   ")
+}
+
+func testGenericRecursive(t *testing.T, beforeErr error, beforeWant []string, list []wrapper, maxDepth int) {
+	if len(beforeWant) == 0 {
+		panic("beforeWant must not be empty")
+	}
+	for _, w := range list {
+		if len(w.want) == 0 {
+			panic("want must not be empty")
+		}
+
+		err := w.wrap(beforeErr)
+
+		// Copy required cause append(beforeWant, ..) modified beforeWant subtly.
+		beforeCopy := make([]string, len(beforeWant))
+		copy(beforeCopy, beforeWant)
+
+		beforeWant := beforeCopy
+		last := len(beforeWant) - 1
+		var want []string
+
+		// Merge two stacks behind each other.
+		if strings.ContainsAny(beforeWant[last], "\n") && strings.ContainsAny(w.want[0], "\n") {
+			want = append(beforeWant[:last], append([]string{beforeWant[last] + "((?s).*)" + w.want[0]}, w.want[1:]...)...)
+		} else {
+			want = append(beforeWant, w.want...)
+		}
+
+		testFormatCompleteCompare(t, maxDepth, err, "%+v", want, false)
+		if maxDepth > 0 {
+			testGenericRecursive(t, err, want, list, maxDepth-1)
+		}
+	}
+}

go113.go

@@ -0,0 +1,38 @@
+// +build go1.13
+
+package errors
+
+import (
+	stderrors "errors"
+)
+
+// Is reports whether any error in err's chain matches target.
+//
+// The chain consists of err itself followed by the sequence of errors obtained by
+// repeatedly calling Unwrap.
+//
+// An error is considered to match a target if it is equal to that target or if
+// it implements a method Is(error) bool such that Is(target) returns true.
+func Is(err, target error) bool { return stderrors.Is(err, target) }
+
+// As finds the first error in err's chain that matches target, and if so, sets
+// target to that error value and returns true.
+//
+// The chain consists of err itself followed by the sequence of errors obtained by
+// repeatedly calling Unwrap.
+//
+// An error matches target if the error's concrete value is assignable to the value
+// pointed to by target, or if the error has a method As(interface{}) bool such that
+// As(target) returns true. In the latter case, the As method is responsible for
+// setting target.
+//
+// As will panic if target is not a non-nil pointer to either a type that implements
+// error, or to any interface type. As returns false if err is nil.
+func As(err error, target interface{}) bool { return stderrors.As(err, target) }
+
+// Unwrap returns the result of calling the Unwrap method on err, if err's
+// type contains an Unwrap method returning error.
+// Otherwise, Unwrap returns nil.
+func Unwrap(err error) error {
+	return stderrors.Unwrap(err)
+}

go113_test.go

@@ -0,0 +1,178 @@
+// +build go1.13
+
+package errors
+
+import (
+	stderrors "errors"
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+func TestErrorChainCompat(t *testing.T) {
+	err := stderrors.New("error that gets wrapped")
+	wrapped := Wrap(err, "wrapped up")
+	if !stderrors.Is(wrapped, err) {
+		t.Errorf("Wrap does not support Go 1.13 error chains")
+	}
+}
+
+func TestIs(t *testing.T) {
+	err := New("test")
+
+	type args struct {
+		err    error
+		target error
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			name: "with stack",
+			args: args{
+				err:    WithStack(err),
+				target: err,
+			},
+			want: true,
+		},
+		{
+			name: "with message",
+			args: args{
+				err:    WithMessage(err, "test"),
+				target: err,
+			},
+			want: true,
+		},
+		{
+			name: "with message format",
+			args: args{
+				err:    WithMessagef(err, "%s", "test"),
+				target: err,
+			},
+			want: true,
+		},
+		{
+			name: "std errors compatibility",
+			args: args{
+				err:    fmt.Errorf("wrap it: %w", err),
+				target: err,
+			},
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := Is(tt.args.err, tt.args.target); got != tt.want {
+				t.Errorf("Is() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+type customErr struct {
+	msg string
+}
+
+func (c customErr) Error() string { return c.msg }
+
+func TestAs(t *testing.T) {
+	var err = customErr{msg: "test message"}
+
+	type args struct {
+		err    error
+		target interface{}
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			name: "with stack",
+			args: args{
+				err:    WithStack(err),
+				target: new(customErr),
+			},
+			want: true,
+		},
+		{
+			name: "with message",
+			args: args{
+				err:    WithMessage(err, "test"),
+				target: new(customErr),
+			},
+			want: true,
+		},
+		{
+			name: "with message format",
+			args: args{
+				err:    WithMessagef(err, "%s", "test"),
+				target: new(customErr),
+			},
+			want: true,
+		},
+		{
+			name: "std errors compatibility",
+			args: args{
+				err:    fmt.Errorf("wrap it: %w", err),
+				target: new(customErr),
+			},
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := As(tt.args.err, tt.args.target); got != tt.want {
+				t.Errorf("As() = %v, want %v", got, tt.want)
+			}
+
+			ce := tt.args.target.(*customErr)
+			if !reflect.DeepEqual(err, *ce) {
+				t.Errorf("set target error failed, target error is %v", *ce)
+			}
+		})
+	}
+}
+
+func TestUnwrap(t *testing.T) {
+	err := New("test")
+
+	type args struct {
+		err error
+	}
+	tests := []struct {
+		name string
+		args args
+		want error
+	}{
+		{
+			name: "with stack",
+			args: args{err: WithStack(err)},
+			want: err,
+		},
+		{
+			name: "with message",
+			args: args{err: WithMessage(err, "test")},
+			want: err,
+		},
+		{
+			name: "with message format",
+			args: args{err: WithMessagef(err, "%s", "test")},
+			want: err,
+		},
+		{
+			name: "std errors compatibility",
+			args: args{err: fmt.Errorf("wrap: %w", err)},
+			want: err,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if err := Unwrap(tt.args.err); !reflect.DeepEqual(err, tt.want) {
+				t.Errorf("Unwrap() error = %v, want %v", err, tt.want)
+			}
+		})
+	}
+}

json_test.go

@@ -0,0 +1,51 @@
+package errors
+
+import (
+	"encoding/json"
+	"regexp"
+	"testing"
+)
+
+func TestFrameMarshalText(t *testing.T) {
+	var tests = []struct {
+		Frame
+		want string
+	}{{
+		initpc,
+		`^github.com/pkg/errors\.init(\.ializers)? .+/github\.com/pkg/errors/stack_test.go:\d+$`,
+	}, {
+		0,
+		`^unknown$`,
+	}}
+	for i, tt := range tests {
+		got, err := tt.Frame.MarshalText()
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !regexp.MustCompile(tt.want).Match(got) {
+			t.Errorf("test %d: MarshalJSON:\n got %q\n want %q", i+1, string(got), tt.want)
+		}
+	}
+}
+
+func TestFrameMarshalJSON(t *testing.T) {
+	var tests = []struct {
+		Frame
+		want string
+	}{{
+		initpc,
+		`^"github\.com/pkg/errors\.init(\.ializers)? .+/github\.com/pkg/errors/stack_test.go:\d+"$`,
+	}, {
+		0,
+		`^"unknown"$`,
+	}}
+	for i, tt := range tests {
+		got, err := json.Marshal(tt.Frame)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !regexp.MustCompile(tt.want).Match(got) {
+			t.Errorf("test %d: MarshalJSON:\n got %q\n want %q", i+1, string(got), tt.want)
+		}
+	}
+}

stack.go

@@ -0,0 +1,177 @@
+package errors
+
+import (
+	"fmt"
+	"io"
+	"path"
+	"runtime"
+	"strconv"
+	"strings"
+)
+
+// Frame represents a program counter inside a stack frame.
+// For historical reasons if Frame is interpreted as a uintptr
+// its value represents the program counter + 1.
+type Frame uintptr
+
+// pc returns the program counter for this frame;
+// multiple frames may have the same PC value.
+func (f Frame) pc() uintptr { return uintptr(f) - 1 }
+
+// file returns the full path to the file that contains the
+// function for this Frame's pc.
+func (f Frame) file() string {
+	fn := runtime.FuncForPC(f.pc())
+	if fn == nil {
+		return "unknown"
+	}
+	file, _ := fn.FileLine(f.pc())
+	return file
+}
+
+// line returns the line number of source code of the
+// function for this Frame's pc.
+func (f Frame) line() int {
+	fn := runtime.FuncForPC(f.pc())
+	if fn == nil {
+		return 0
+	}
+	_, line := fn.FileLine(f.pc())
+	return line
+}
+
+// name returns the name of this function, if known.
+func (f Frame) name() string {
+	fn := runtime.FuncForPC(f.pc())
+	if fn == nil {
+		return "unknown"
+	}
+	return fn.Name()
+}
+
+// Format formats the frame according to the fmt.Formatter interface.
+//
+//    %s    source file
+//    %d    source line
+//    %n    function name
+//    %v    equivalent to %s:%d
+//
+// Format accepts flags that alter the printing of some verbs, as follows:
+//
+//    %+s   function name and path of source file relative to the compile time
+//          GOPATH separated by \n\t (<funcname>\n\t<path>)
+//    %+v   equivalent to %+s:%d
+func (f Frame) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 's':
+		switch {
+		case s.Flag('+'):
+			io.WriteString(s, f.name())
+			io.WriteString(s, "\n\t")
+			io.WriteString(s, f.file())
+		default:
+			io.WriteString(s, path.Base(f.file()))
+		}
+	case 'd':
+		io.WriteString(s, strconv.Itoa(f.line()))
+	case 'n':
+		io.WriteString(s, funcname(f.name()))
+	case 'v':
+		f.Format(s, 's')
+		io.WriteString(s, ":")
+		f.Format(s, 'd')
+	}
+}
+
+// MarshalText formats a stacktrace Frame as a text string. The output is the
+// same as that of fmt.Sprintf("%+v", f), but without newlines or tabs.
+func (f Frame) MarshalText() ([]byte, error) {
+	name := f.name()
+	if name == "unknown" {
+		return []byte(name), nil
+	}
+	return []byte(fmt.Sprintf("%s %s:%d", name, f.file(), f.line())), nil
+}
+
+// StackTrace is stack of Frames from innermost (newest) to outermost (oldest).
+type StackTrace []Frame
+
+// Format formats the stack of Frames according to the fmt.Formatter interface.
+//
+//    %s	lists source files for each Frame in the stack
+//    %v	lists the source file and line number for each Frame in the stack
+//
+// Format accepts flags that alter the printing of some verbs, as follows:
+//
+//    %+v   Prints filename, function, and line number for each Frame in the stack.
+func (st StackTrace) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		switch {
+		case s.Flag('+'):
+			for _, f := range st {
+				io.WriteString(s, "\n")
+				f.Format(s, verb)
+			}
+		case s.Flag('#'):
+			fmt.Fprintf(s, "%#v", []Frame(st))
+		default:
+			st.formatSlice(s, verb)
+		}
+	case 's':
+		st.formatSlice(s, verb)
+	}
+}
+
+// formatSlice will format this StackTrace into the given buffer as a slice of
+// Frame, only valid when called with '%s' or '%v'.
+func (st StackTrace) formatSlice(s fmt.State, verb rune) {
+	io.WriteString(s, "[")
+	for i, f := range st {
+		if i > 0 {
+			io.WriteString(s, " ")
+		}
+		f.Format(s, verb)
+	}
+	io.WriteString(s, "]")
+}
+
+// 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++ {
+		f[i] = Frame((*s)[i])
+	}
+	return f
+}
+
+func callers() *stack {
+	const depth = 32
+	var pcs [depth]uintptr
+	n := runtime.Callers(3, pcs[:])
+	var st stack = pcs[0:n]
+	return &st
+}
+
+// funcname removes the path prefix component of a function's name reported by func.Name().
+func funcname(name string) string {
+	i := strings.LastIndex(name, "/")
+	name = name[i+1:]
+	i = strings.Index(name, ".")
+	return name[i+1:]
+}

stack_test.go

@@ -0,0 +1,250 @@
+package errors
+
+import (
+	"fmt"
+	"runtime"
+	"testing"
+)
+
+var initpc = caller()
+
+type X struct{}
+
+// val returns a Frame pointing to itself.
+func (x X) val() Frame {
+	return caller()
+}
+
+// ptr returns a Frame pointing to itself.
+func (x *X) ptr() Frame {
+	return caller()
+}
+
+func TestFrameFormat(t *testing.T) {
+	var tests = []struct {
+		Frame
+		format string
+		want   string
+	}{{
+		initpc,
+		"%s",
+		"stack_test.go",
+	}, {
+		initpc,
+		"%+s",
+		"github.com/pkg/errors.init\n" +
+			"\t.+/github.com/pkg/errors/stack_test.go",
+	}, {
+		0,
+		"%s",
+		"unknown",
+	}, {
+		0,
+		"%+s",
+		"unknown",
+	}, {
+		initpc,
+		"%d",
+		"9",
+	}, {
+		0,
+		"%d",
+		"0",
+	}, {
+		initpc,
+		"%n",
+		"init",
+	}, {
+		func() Frame {
+			var x X
+			return x.ptr()
+		}(),
+		"%n",
+		`\(\*X\).ptr`,
+	}, {
+		func() Frame {
+			var x X
+			return x.val()
+		}(),
+		"%n",
+		"X.val",
+	}, {
+		0,
+		"%n",
+		"",
+	}, {
+		initpc,
+		"%v",
+		"stack_test.go:9",
+	}, {
+		initpc,
+		"%+v",
+		"github.com/pkg/errors.init\n" +
+			"\t.+/github.com/pkg/errors/stack_test.go:9",
+	}, {
+		0,
+		"%v",
+		"unknown:0",
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.Frame, tt.format, tt.want)
+	}
+}
+
+func TestFuncname(t *testing.T) {
+	tests := []struct {
+		name, want string
+	}{
+		{"", ""},
+		{"runtime.main", "main"},
+		{"github.com/pkg/errors.funcname", "funcname"},
+		{"funcname", "funcname"},
+		{"io.copyBuffer", "copyBuffer"},
+		{"main.(*R).Write", "(*R).Write"},
+	}
+
+	for _, tt := range tests {
+		got := funcname(tt.name)
+		want := tt.want
+		if got != want {
+			t.Errorf("funcname(%q): want: %q, got %q", tt.name, want, got)
+		}
+	}
+}
+
+func TestStackTrace(t *testing.T) {
+	tests := []struct {
+		err  error
+		want []string
+	}{{
+		New("ooh"), []string{
+			"github.com/pkg/errors.TestStackTrace\n" +
+				"\t.+/github.com/pkg/errors/stack_test.go:121",
+		},
+	}, {
+		Wrap(New("ooh"), "ahh"), []string{
+			"github.com/pkg/errors.TestStackTrace\n" +
+				"\t.+/github.com/pkg/errors/stack_test.go:126", // this is the stack of Wrap, not New
+		},
+	}, {
+		Cause(Wrap(New("ooh"), "ahh")), []string{
+			"github.com/pkg/errors.TestStackTrace\n" +
+				"\t.+/github.com/pkg/errors/stack_test.go:131", // this is the stack of New
+		},
+	}, {
+		func() error { return New("ooh") }(), []string{
+			`github.com/pkg/errors.TestStackTrace.func1` +
+				"\n\t.+/github.com/pkg/errors/stack_test.go:136", // this is the stack of New
+			"github.com/pkg/errors.TestStackTrace\n" +
+				"\t.+/github.com/pkg/errors/stack_test.go:136", // this is the stack of New's caller
+		},
+	}, {
+		Cause(func() error {
+			return func() error {
+				return Errorf("hello %s", fmt.Sprintf("world: %s", "ooh"))
+			}()
+		}()), []string{
+			`github.com/pkg/errors.TestStackTrace.func2.1` +
+				"\n\t.+/github.com/pkg/errors/stack_test.go:145", // this is the stack of Errorf
+			`github.com/pkg/errors.TestStackTrace.func2` +
+				"\n\t.+/github.com/pkg/errors/stack_test.go:146", // this is the stack of Errorf's caller
+			"github.com/pkg/errors.TestStackTrace\n" +
+				"\t.+/github.com/pkg/errors/stack_test.go:147", // this is the stack of Errorf's caller's caller
+		},
+	}}
+	for i, tt := range tests {
+		x, ok := tt.err.(interface {
+			StackTrace() StackTrace
+		})
+		if !ok {
+			t.Errorf("expected %#v to implement StackTrace() StackTrace", tt.err)
+			continue
+		}
+		st := x.StackTrace()
+		for j, want := range tt.want {
+			testFormatRegexp(t, i, st[j], "%+v", want)
+		}
+	}
+}
+
+func stackTrace() StackTrace {
+	const depth = 8
+	var pcs [depth]uintptr
+	n := runtime.Callers(1, pcs[:])
+	var st stack = pcs[0:n]
+	return st.StackTrace()
+}
+
+func TestStackTraceFormat(t *testing.T) {
+	tests := []struct {
+		StackTrace
+		format string
+		want   string
+	}{{
+		nil,
+		"%s",
+		`\[\]`,
+	}, {
+		nil,
+		"%v",
+		`\[\]`,
+	}, {
+		nil,
+		"%+v",
+		"",
+	}, {
+		nil,
+		"%#v",
+		`\[\]errors.Frame\(nil\)`,
+	}, {
+		make(StackTrace, 0),
+		"%s",
+		`\[\]`,
+	}, {
+		make(StackTrace, 0),
+		"%v",
+		`\[\]`,
+	}, {
+		make(StackTrace, 0),
+		"%+v",
+		"",
+	}, {
+		make(StackTrace, 0),
+		"%#v",
+		`\[\]errors.Frame{}`,
+	}, {
+		stackTrace()[:2],
+		"%s",
+		`\[stack_test.go stack_test.go\]`,
+	}, {
+		stackTrace()[:2],
+		"%v",
+		`\[stack_test.go:174 stack_test.go:221\]`,
+	}, {
+		stackTrace()[:2],
+		"%+v",
+		"\n" +
+			"github.com/pkg/errors.stackTrace\n" +
+			"\t.+/github.com/pkg/errors/stack_test.go:174\n" +
+			"github.com/pkg/errors.TestStackTraceFormat\n" +
+			"\t.+/github.com/pkg/errors/stack_test.go:225",
+	}, {
+		stackTrace()[:2],
+		"%#v",
+		`\[\]errors.Frame{stack_test.go:174, stack_test.go:233}`,
+	}}
+
+	for i, tt := range tests {
+		testFormatRegexp(t, i, tt.StackTrace, tt.format, tt.want)
+	}
+}
+
+// a version of runtime.Caller that returns a Frame, not a uintptr.
+func caller() Frame {
+	var pcs [3]uintptr
+	n := runtime.Callers(2, pcs[:])
+	frames := runtime.CallersFrames(pcs[:n])
+	frame, _ := frames.Next()
+	return Frame(frame.PC)
+}