cue/errors/errors.go - cue - Git at Google

 // Copyright 2018 The CUE Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 // Package errors defines shared types for handling CUE errors.
 //
 // The pivotal error type in CUE packages is the interface type Error.
 // The information available in such errors can be most easily retrieved using
 // the Path, Positions, and Print functions.
 package errors // import "cuelang.org/go/cue/errors"

 import (
 	"bytes"
 	"errors"
 	"fmt"
 	"io"
 	"path/filepath"
 	"sort"
 	"strings"

 	"github.com/mpvl/unique"
 	"golang.org/x/xerrors"

 	"cuelang.org/go/cue/token"
 )

 // New is a convenience wrapper for errors.New in the core library.
 // It does not return a CUE error.
 func New(msg string) error {
 	return errors.New(msg)
 }

 // A Message implements the error interface as well as Message to allow
 // internationalized messages. A Message is typically used as an embedding
 // in a CUE message.
 type Message struct {
 	format string
 	args   []interface{}
 }

 // NewMessage creates an error message for human consumption. The arguments
 // are for later consumption, allowing the message to be localized at a later
 // time. The passed argument list should not be modified.
 func NewMessage(format string, args []interface{}) Message {
 	return Message{format: format, args: args}
 }

 // Msg returns a printf-style format string and its arguments for human
 // consumption.
 func (m *Message) Msg() (format string, args []interface{}) {
 	return m.format, m.args
 }

 func (m *Message) Error() string {
 	return fmt.Sprintf(m.format, m.args...)
 }

 // Error is the common error message.
 type Error interface {
 	// Position returns the primary position of an error. If multiple positions
 	// contribute equally, this reflects one of them.
 	Position() token.Pos

 	// InputPositions reports positions that contributed to an error, including
 	// the expressions resulting in the conflict, as well as values that were
 	// the input to this expression.
 	InputPositions() []token.Pos

 	// Error reports the error message without position information.
 	Error() string

 	// Path returns the path into the data tree where the error occurred.
 	// This path may be nil if the error is not associated with such a location.
 	Path() []string

 	// Msg returns the unformatted error message and its arguments for human
 	// consumption.
 	Msg() (format string, args []interface{})
 }

 // Positions returns all positions returned by an error, sorted
 // by relevance when possible and with duplicates removed.
 func Positions(err error) []token.Pos {
 	e := Error(nil)
 	if !xerrors.As(err, &e) {
 		return nil
 	}

 	a := make([]token.Pos, 0, 3)

 	sortOffset := 0
 	pos := e.Position()
 	if pos.IsValid() {
 		a = append(a, pos)
 		sortOffset = 1
 	}

 	for _, p := range e.InputPositions() {
 		if p.IsValid() && p != pos {
 			a = append(a, p)
 		}
 	}

 	byPos := byPos(a[sortOffset:])
 	sort.Sort(byPos)
 	k := unique.ToFront(byPos)
 	return a[:k+sortOffset]
 }

 type byPos []token.Pos

 func (s *byPos) Truncate(n int)    { (*s) = (*s)[:n] }
 func (s byPos) Len() int           { return len(s) }
 func (s byPos) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
 func (s byPos) Less(i, j int) bool { return comparePos(s[i], s[j]) == -1 }

 // Path returns the path of an Error if err is of that type.
 func Path(err error) []string {
 	if e := Error(nil); xerrors.As(err, &e) {
 		return e.Path()
 	}
 	return nil
 }

 // Newf creates an Error with the associated position and message.
 func Newf(p token.Pos, format string, args ...interface{}) Error {
 	return &posError{
 		pos:     p,
 		Message: NewMessage(format, args),
 	}
 }

 // Wrapf creates an Error with the associated position and message. The provided
 // error is added for inspection context.
 func Wrapf(err error, p token.Pos, format string, args ...interface{}) Error {
 	return &posError{
 		pos:     p,
 		Message: NewMessage(format, args),
 		err:     err,
 	}
 }

 // Promote converts a regular Go error to an Error if it isn't already one.
 func Promote(err error, msg string) Error {
 	switch x := err.(type) {
 	case Error:
 		return x
 	default:
 		return Wrapf(err, token.NoPos, msg)
 	}
 }

 var _ Error = &posError{}

 // In an List, an error is represented by an *posError.
 // The position Pos, if valid, points to the beginning of
 // the offending token, and the error condition is described
 // by Msg.
 type posError struct {
 	pos    token.Pos
 	inputs []token.Pos
 	Message

 	// The underlying error that triggered this one, if any.
 	err error
 }

 func (e *posError) Path() []string              { return Path(e.err) }
 func (e *posError) InputPositions() []token.Pos { return e.inputs }
 func (e *posError) Position() token.Pos         { return e.pos }
 func (e *posError) Unwrap() error               { return e.err }
 func (e *posError) Cause() error                { return e.err }

 // Error implements the error interface.
 func (e *posError) Error() string {
 	if e.err == nil {
 		return e.Message.Error()
 	}
 	if e.Message.format == "" {
 		return e.err.Error()
 	}
 	return fmt.Sprintf("%s: %s", e.Message.Error(), e.err)
 }

 // Append combines two errors, flattening Lists as necessary.
 func Append(a, b Error) Error {
 	switch x := a.(type) {
 	case nil:
 		return b
 	case list:
 		return appendToList(x, b)
 	}
 	// Preserve order of errors.
 	list := appendToList(nil, a)
 	list = appendToList(list, b)
 	return list
 }

 // Errors reports the individual errors associated with an error, which is
 // the error itself if there is only one or, if the underlying type is List,
 // its individual elements. If the given error is not an Error, it will be
 // promoted to one.
 func Errors(err error) []Error {
 	switch x := err.(type) {
 	case nil:
 		return nil
 	case list:
 		return []Error(x)
 	case Error:
 		return []Error{x}
 	default:
 		return []Error{Promote(err, "")}
 	}
 }

 func appendToList(a list, err Error) list {
 	switch x := err.(type) {
 	case nil:
 		return a
 	case list:
 		if a == nil {
 			return x
 		}
 		return append(a, x...)
 	default:
 		return append(a, err)
 	}
 }

 // list is a list of Errors.
 // The zero value for an list is an empty list ready to use.
 type list []Error

 func (p list) Is(err, target error) bool {
 	for _, e := range p {
 		if xerrors.Is(e, target) {
 			return true
 		}
 	}
 	return false
 }

 func (p list) As(err error, target interface{}) bool {
 	for _, e := range p {
 		if xerrors.As(e, target) {
 			return true
 		}
 	}
 	return false
 }

 // AddNewf adds an Error with given position and error message to an List.
 func (p *list) AddNewf(pos token.Pos, msg string, args ...interface{}) {
 	err := &posError{pos: pos, Message: Message{format: msg, args: args}}
 	*p = append(*p, err)
 }

 // Add adds an Error with given position and error message to an List.
 func (p *list) Add(err Error) {
 	*p = appendToList(*p, err)
 }

 // Reset resets an List to no errors.
 func (p *list) Reset() { *p = (*p)[:0] }

 // List implements the sort Interface.
 func (p list) Len() int      { return len(p) }
 func (p list) Swap(i, j int) { p[i], p[j] = p[j], p[i] }

 func (p list) Less(i, j int) bool {
 	if c := comparePos(p[i].Position(), p[j].Position()); c != 0 {
 		return c == -1
 	}
 	// Note that it is not sufficient to simply compare file offsets because
 	// the offsets do not reflect modified line information (through //line
 	// comments).

 	if !equalPath(p[i].Path(), p[j].Path()) {
 		return lessPath(p[i].Path(), p[j].Path())
 	}
 	return p[i].Error() < p[j].Error()
 }

 func lessOrMore(isLess bool) int {
 	if isLess {
 		return -1
 	}
 	return 1
 }

 func comparePos(a, b token.Pos) int {
 	if a.Filename() != b.Filename() {
 		return lessOrMore(a.Filename() < b.Filename())
 	}
 	if a.Line() != b.Line() {
 		return lessOrMore(a.Line() < b.Line())
 	}
 	if a.Column() != b.Column() {
 		return lessOrMore(a.Column() < b.Column())
 	}
 	return 0
 }

 func lessPath(a, b []string) bool {
 	for i, x := range a {
 		if i >= len(b) {
 			return false
 		}
 		if x != b[i] {
 			return x < b[i]
 		}
 	}
 	return len(a) < len(b)
 }

 func equalPath(a, b []string) bool {
 	if len(a) != len(b) {
 		return false
 	}
 	for i, x := range a {
 		if x != b[i] {
 			return false
 		}
 	}
 	return true
 }

 // Sanitize sorts multiple errors and removes duplicates on a best effort basis.
 // If err represents a single or no error, it returns the error as is.
 func Sanitize(err Error) Error {
 	if l, ok := err.(list); ok && err != nil {
 		a := make(list, len(l))
 		copy(a, l)
 		a.Sort()
 		a.RemoveMultiples()
 		return a
 	}
 	return err
 }

 // Sort sorts an List. *posError entries are sorted by position,
 // other errors are sorted by error message, and before any *posError
 // entry.
 //
 func (p list) Sort() {
 	sort.Sort(p)
 }

 // RemoveMultiples sorts an List and removes all but the first error per line.
 func (p *list) RemoveMultiples() {
 	p.Sort()
 	var last Error
 	i := 0
 	for _, e := range *p {
 		pos := e.Position()
 		if last == nil ||
 			pos.Filename() != last.Position().Filename() ||
 			pos.Line() != last.Position().Line() ||
 			!equalPath(e.Path(), last.Path()) {
 			last = e
 			(*p)[i] = e
 			i++
 		}
 	}
 	(*p) = (*p)[0:i]
 }

 // An List implements the error interface.
 func (p list) Error() string {
 	format, args := p.Msg()
 	return fmt.Sprintf(format, args...)
 }

 // Msg reports the unformatted error message for the first error, if any.
 func (p list) Msg() (format string, args []interface{}) {
 	switch len(p) {
 	case 0:
 		return "no errors", nil
 	case 1:
 		return p[0].Msg()
 	}
 	return "%s (and %d more errors)", []interface{}{p[0], len(p) - 1}
 }

 // Position reports the primary position for the first error, if any.
 func (p list) Position() token.Pos {
 	if len(p) == 0 {
 		return token.NoPos
 	}
 	return p[0].Position()
 }

 // InputPositions reports the input positions for the first error, if any.
 func (p list) InputPositions() []token.Pos {
 	if len(p) == 0 {
 		return nil
 	}
 	return p[0].InputPositions()
 }

 // Path reports the path location of the first error, if any.
 func (p list) Path() []string {
 	if len(p) == 0 {
 		return nil
 	}
 	return p[0].Path()
 }

 // Err returns an error equivalent to this error list.
 // If the list is empty, Err returns nil.
 func (p list) Err() error {
 	if len(p) == 0 {
 		return nil
 	}
 	return p
 }

 // A Config defines parameters for printing.
 type Config struct {
 	// Format formats the given string and arguments and writes it to w.
 	// It is used for all printing.
 	Format func(w io.Writer, format string, args ...interface{})

 	// Cwd is the current working directory. Filename positions are taken
 	// relative to this path.
 	Cwd string

 	// ToSlash sets whether to use Unix paths. Mostly used for testing.
 	ToSlash bool
 }

 // Print is a utility function that prints a list of errors to w,
 // one error per line, if the err parameter is an List. Otherwise
 // it prints the err string.
 //
 func Print(w io.Writer, err error, cfg *Config) {
 	if cfg == nil {
 		cfg = &Config{}
 	}
 	for _, e := range Errors(err) {
 		printError(w, e, cfg)
 	}
 }

 // Details is a convenience wrapper for Print to return the error text as a
 // string.
 func Details(err error, cfg *Config) string {
 	w := &bytes.Buffer{}
 	Print(w, err, cfg)
 	return w.String()
 }

 // String generates a short message from a given Error.
 func String(err Error) string {
 	w := &strings.Builder{}
 	writeErr(w, err)
 	return w.String()
 }

 func writeErr(w io.Writer, err Error) {
 	if path := strings.Join(err.Path(), "."); path != "" {
 		_, _ = io.WriteString(w, path)
 		_, _ = io.WriteString(w, ": ")
 	}

 	for {
 		u := xerrors.Unwrap(err)

 		printed := false
 		msg, args := err.Msg()
 		if msg != "" || u == nil { // print at least something
 			fmt.Fprintf(w, msg, args...)
 			printed = true
 		}

 		if u == nil {
 			break
 		}

 		if printed {
 			_, _ = io.WriteString(w, ": ")
 		}
 		err, _ = u.(Error)
 		if err == nil {
 			fmt.Fprint(w, u)
 			break
 		}
 	}
 }

 func defaultFprintf(w io.Writer, format string, args ...interface{}) {
 	fmt.Fprintf(w, format, args...)
 }

 func printError(w io.Writer, err error, cfg *Config) {
 	if err == nil {
 		return
 	}
 	fprintf := cfg.Format
 	if fprintf == nil {
 		fprintf = defaultFprintf
 	}

 	positions := []string{}
 	for _, p := range Positions(err) {
 		pos := p.Position()
 		s := pos.Filename
 		if cfg.Cwd != "" {
 			if p, err := filepath.Rel(cfg.Cwd, s); err == nil {
 				s = p
 				// Some IDEs (e.g. VSCode) only recognize a path if it start
 				// with a dot. This also helps to distinguish between local
 				// files and builtin packages.
 				if !strings.HasPrefix(s, ".") {
 					s = fmt.Sprintf(".%s%s", string(filepath.Separator), s)
 				}
 			}
 		}
 		if cfg.ToSlash {
 			s = filepath.ToSlash(s)
 		}
 		if pos.IsValid() {
 			if s != "" {
 				s += ":"
 			}
 			s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
 		}
 		if s == "" {
 			s = "-"
 		}
 		positions = append(positions, s)
 	}

 	if e, ok := err.(Error); ok {
 		writeErr(w, e)
 	} else {
 		fprintf(w, "%v", err)
 	}

 	if len(positions) == 0 {
 		fprintf(w, "\n")
 		return
 	}

 	fprintf(w, ":\n")
 	for _, pos := range positions {
 		fprintf(w, "    %s\n", pos)
 	}
 }
	// Copyright 2018 The CUE Authors
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	// Package errors defines shared types for handling CUE errors.
	//
	// The pivotal error type in CUE packages is the interface type Error.
	// The information available in such errors can be most easily retrieved using
	// the Path, Positions, and Print functions.
	package errors // import "cuelang.org/go/cue/errors"

	import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"path/filepath"
	"sort"
	"strings"

	"github.com/mpvl/unique"
	"golang.org/x/xerrors"

	"cuelang.org/go/cue/token"
	)

	// New is a convenience wrapper for errors.New in the core library.
	// It does not return a CUE error.
	func New(msg string) error {
	return errors.New(msg)
	}

	// A Message implements the error interface as well as Message to allow
	// internationalized messages. A Message is typically used as an embedding
	// in a CUE message.
	type Message struct {
	format string
	args []interface{}
	}

	// NewMessage creates an error message for human consumption. The arguments
	// are for later consumption, allowing the message to be localized at a later
	// time. The passed argument list should not be modified.
	func NewMessage(format string, args []interface{}) Message {
	return Message{format: format, args: args}
	}

	// Msg returns a printf-style format string and its arguments for human
	// consumption.
	func (m *Message) Msg() (format string, args []interface{}) {
	return m.format, m.args
	}

	func (m *Message) Error() string {
	return fmt.Sprintf(m.format, m.args...)
	}

	// Error is the common error message.
	type Error interface {
	// Position returns the primary position of an error. If multiple positions
	// contribute equally, this reflects one of them.
	Position() token.Pos

	// InputPositions reports positions that contributed to an error, including
	// the expressions resulting in the conflict, as well as values that were
	// the input to this expression.
	InputPositions() []token.Pos

	// Error reports the error message without position information.
	Error() string

	// Path returns the path into the data tree where the error occurred.
	// This path may be nil if the error is not associated with such a location.
	Path() []string

	// Msg returns the unformatted error message and its arguments for human
	// consumption.
	Msg() (format string, args []interface{})
	}

	// Positions returns all positions returned by an error, sorted
	// by relevance when possible and with duplicates removed.
	func Positions(err error) []token.Pos {
	e := Error(nil)
	if !xerrors.As(err, &e) {
	return nil
	}

	a := make([]token.Pos, 0, 3)

	sortOffset := 0
	pos := e.Position()
	if pos.IsValid() {
	a = append(a, pos)
	sortOffset = 1
	}

	for _, p := range e.InputPositions() {
	if p.IsValid() && p != pos {
	a = append(a, p)
	}
	}

	byPos := byPos(a[sortOffset:])
	sort.Sort(byPos)
	k := unique.ToFront(byPos)
	return a[:k+sortOffset]
	}

	type byPos []token.Pos

	func (s byPos) Truncate(n int) { (s) = (*s)[:n] }
	func (s byPos) Len() int { return len(s) }
	func (s byPos) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
	func (s byPos) Less(i, j int) bool { return comparePos(s[i], s[j]) == -1 }

	// Path returns the path of an Error if err is of that type.
	func Path(err error) []string {
	if e := Error(nil); xerrors.As(err, &e) {
	return e.Path()
	}
	return nil
	}

	// Newf creates an Error with the associated position and message.
	func Newf(p token.Pos, format string, args ...interface{}) Error {
	return &posError{
	pos: p,
	Message: NewMessage(format, args),
	}
	}

	// Wrapf creates an Error with the associated position and message. The provided
	// error is added for inspection context.
	func Wrapf(err error, p token.Pos, format string, args ...interface{}) Error {
	return &posError{
	pos: p,
	Message: NewMessage(format, args),
	err: err,
	}
	}

	// Promote converts a regular Go error to an Error if it isn't already one.
	func Promote(err error, msg string) Error {
	switch x := err.(type) {
	case Error:
	return x
	default:
	return Wrapf(err, token.NoPos, msg)
	}
	}

	var _ Error = &posError{}

	// In an List, an error is represented by an *posError.
	// The position Pos, if valid, points to the beginning of
	// the offending token, and the error condition is described
	// by Msg.
	type posError struct {
	pos token.Pos
	inputs []token.Pos
	Message

	// The underlying error that triggered this one, if any.
	err error
	}

	func (e *posError) Path() []string { return Path(e.err) }
	func (e *posError) InputPositions() []token.Pos { return e.inputs }
	func (e *posError) Position() token.Pos { return e.pos }
	func (e *posError) Unwrap() error { return e.err }
	func (e *posError) Cause() error { return e.err }

	// Error implements the error interface.
	func (e *posError) Error() string {
	if e.err == nil {
	return e.Message.Error()
	}
	if e.Message.format == "" {
	return e.err.Error()
	}
	return fmt.Sprintf("%s: %s", e.Message.Error(), e.err)
	}

	// Append combines two errors, flattening Lists as necessary.
	func Append(a, b Error) Error {
	switch x := a.(type) {
	case nil:
	return b
	case list:
	return appendToList(x, b)
	}
	// Preserve order of errors.
	list := appendToList(nil, a)
	list = appendToList(list, b)
	return list
	}

	// Errors reports the individual errors associated with an error, which is
	// the error itself if there is only one or, if the underlying type is List,
	// its individual elements. If the given error is not an Error, it will be
	// promoted to one.
	func Errors(err error) []Error {
	switch x := err.(type) {
	case nil:
	return nil
	case list:
	return []Error(x)
	case Error:
	return []Error{x}
	default:
	return []Error{Promote(err, "")}
	}
	}

	func appendToList(a list, err Error) list {
	switch x := err.(type) {
	case nil:
	return a
	case list:
	if a == nil {
	return x
	}
	return append(a, x...)
	default:
	return append(a, err)
	}
	}

	// list is a list of Errors.
	// The zero value for an list is an empty list ready to use.
	type list []Error

	func (p list) Is(err, target error) bool {
	for _, e := range p {
	if xerrors.Is(e, target) {
	return true
	}
	}
	return false
	}

	func (p list) As(err error, target interface{}) bool {
	for _, e := range p {
	if xerrors.As(e, target) {
	return true
	}
	}
	return false
	}

	// AddNewf adds an Error with given position and error message to an List.
	func (p *list) AddNewf(pos token.Pos, msg string, args ...interface{}) {
	err := &posError{pos: pos, Message: Message{format: msg, args: args}}
	p = append(p, err)
	}

	// Add adds an Error with given position and error message to an List.
	func (p *list) Add(err Error) {
	p = appendToList(p, err)
	}

	// Reset resets an List to no errors.
	func (p list) Reset() { p = (*p)[:0] }

	// List implements the sort Interface.
	func (p list) Len() int { return len(p) }
	func (p list) Swap(i, j int) { p[i], p[j] = p[j], p[i] }

	func (p list) Less(i, j int) bool {
	if c := comparePos(p[i].Position(), p[j].Position()); c != 0 {
	return c == -1
	}
	// Note that it is not sufficient to simply compare file offsets because
	// the offsets do not reflect modified line information (through //line
	// comments).

	if !equalPath(p[i].Path(), p[j].Path()) {
	return lessPath(p[i].Path(), p[j].Path())
	}
	return p[i].Error() < p[j].Error()
	}

	func lessOrMore(isLess bool) int {
	if isLess {
	return -1
	}
	return 1
	}

	func comparePos(a, b token.Pos) int {
	if a.Filename() != b.Filename() {
	return lessOrMore(a.Filename() < b.Filename())
	}
	if a.Line() != b.Line() {
	return lessOrMore(a.Line() < b.Line())
	}
	if a.Column() != b.Column() {
	return lessOrMore(a.Column() < b.Column())
	}
	return 0
	}

	func lessPath(a, b []string) bool {
	for i, x := range a {
	if i >= len(b) {
	return false
	}
	if x != b[i] {
	return x < b[i]
	}
	}
	return len(a) < len(b)
	}

	func equalPath(a, b []string) bool {
	if len(a) != len(b) {
	return false
	}
	for i, x := range a {
	if x != b[i] {
	return false
	}
	}
	return true
	}

	// Sanitize sorts multiple errors and removes duplicates on a best effort basis.
	// If err represents a single or no error, it returns the error as is.
	func Sanitize(err Error) Error {
	if l, ok := err.(list); ok && err != nil {
	a := make(list, len(l))
	copy(a, l)
	a.Sort()
	a.RemoveMultiples()
	return a
	}
	return err
	}

	// Sort sorts an List. *posError entries are sorted by position,
	// other errors are sorted by error message, and before any *posError
	// entry.
	//
	func (p list) Sort() {
	sort.Sort(p)
	}

	// RemoveMultiples sorts an List and removes all but the first error per line.
	func (p *list) RemoveMultiples() {
	p.Sort()
	var last Error
	i := 0
	for _, e := range *p {
	pos := e.Position()
	if last == nil \|\|
	pos.Filename() != last.Position().Filename() \|\|
	pos.Line() != last.Position().Line() \|\|
	!equalPath(e.Path(), last.Path()) {
	last = e
	(*p)[i] = e
	i++
	}
	}
	(p) = (p)[0:i]
	}

	// An List implements the error interface.
	func (p list) Error() string {
	format, args := p.Msg()
	return fmt.Sprintf(format, args...)
	}

	// Msg reports the unformatted error message for the first error, if any.
	func (p list) Msg() (format string, args []interface{}) {
	switch len(p) {
	case 0:
	return "no errors", nil
	case 1:
	return p[0].Msg()
	}
	return "%s (and %d more errors)", []interface{}{p[0], len(p) - 1}
	}

	// Position reports the primary position for the first error, if any.
	func (p list) Position() token.Pos {
	if len(p) == 0 {
	return token.NoPos
	}
	return p[0].Position()
	}

	// InputPositions reports the input positions for the first error, if any.
	func (p list) InputPositions() []token.Pos {
	if len(p) == 0 {
	return nil
	}
	return p[0].InputPositions()
	}

	// Path reports the path location of the first error, if any.
	func (p list) Path() []string {
	if len(p) == 0 {
	return nil
	}
	return p[0].Path()
	}

	// Err returns an error equivalent to this error list.
	// If the list is empty, Err returns nil.
	func (p list) Err() error {
	if len(p) == 0 {
	return nil
	}
	return p
	}

	// A Config defines parameters for printing.
	type Config struct {
	// Format formats the given string and arguments and writes it to w.
	// It is used for all printing.
	Format func(w io.Writer, format string, args ...interface{})

	// Cwd is the current working directory. Filename positions are taken
	// relative to this path.
	Cwd string

	// ToSlash sets whether to use Unix paths. Mostly used for testing.
	ToSlash bool
	}

	// Print is a utility function that prints a list of errors to w,
	// one error per line, if the err parameter is an List. Otherwise
	// it prints the err string.
	//
	func Print(w io.Writer, err error, cfg *Config) {
	if cfg == nil {
	cfg = &Config{}
	}
	for _, e := range Errors(err) {
	printError(w, e, cfg)
	}
	}

	// Details is a convenience wrapper for Print to return the error text as a
	// string.
	func Details(err error, cfg *Config) string {
	w := &bytes.Buffer{}
	Print(w, err, cfg)
	return w.String()
	}

	// String generates a short message from a given Error.
	func String(err Error) string {
	w := &strings.Builder{}
	writeErr(w, err)
	return w.String()
	}

	func writeErr(w io.Writer, err Error) {
	if path := strings.Join(err.Path(), "."); path != "" {
	_, _ = io.WriteString(w, path)
	_, _ = io.WriteString(w, ": ")
	}

	for {
	u := xerrors.Unwrap(err)

	printed := false
	msg, args := err.Msg()
	if msg != "" \|\| u == nil { // print at least something
	fmt.Fprintf(w, msg, args...)
	printed = true
	}

	if u == nil {
	break
	}

	if printed {
	_, _ = io.WriteString(w, ": ")
	}
	err, _ = u.(Error)
	if err == nil {
	fmt.Fprint(w, u)
	break
	}
	}
	}

	func defaultFprintf(w io.Writer, format string, args ...interface{}) {
	fmt.Fprintf(w, format, args...)
	}

	func printError(w io.Writer, err error, cfg *Config) {
	if err == nil {
	return
	}
	fprintf := cfg.Format
	if fprintf == nil {
	fprintf = defaultFprintf
	}

	positions := []string{}
	for _, p := range Positions(err) {
	pos := p.Position()
	s := pos.Filename
	if cfg.Cwd != "" {
	if p, err := filepath.Rel(cfg.Cwd, s); err == nil {
	s = p
	// Some IDEs (e.g. VSCode) only recognize a path if it start
	// with a dot. This also helps to distinguish between local
	// files and builtin packages.
	if !strings.HasPrefix(s, ".") {
	s = fmt.Sprintf(".%s%s", string(filepath.Separator), s)
	}
	}
	}
	if cfg.ToSlash {
	s = filepath.ToSlash(s)
	}
	if pos.IsValid() {
	if s != "" {
	s += ":"
	}
	s += fmt.Sprintf("%d:%d", pos.Line, pos.Column)
	}
	if s == "" {
	s = "-"
	}
	positions = append(positions, s)
	}

	if e, ok := err.(Error); ok {
	writeErr(w, e)
	} else {
	fprintf(w, "%v", err)
	}

	if len(positions) == 0 {
	fprintf(w, "\n")
	return
	}

	fprintf(w, ":\n")
	for _, pos := range positions {
	fprintf(w, " %s\n", pos)
	}
	}