From bad9aac9e719a7bba287711f1d2b788ae6acebd2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lovro=20Ma=C5=BEgon?= <lovro.mazgon@gmail.com>
Date: Wed, 30 Aug 2023 18:38:41 +0200
Subject: [PATCH] add functions Join and Unwrap

---
 error_1_13.go     | 34 ++++++++++++++++++++++--
 error_backward.go | 68 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+), 2 deletions(-)

diff --git a/error_1_13.go b/error_1_13.go
index 0af2fc8..91e709c 100644
--- a/error_1_13.go
+++ b/error_1_13.go
@@ -1,3 +1,4 @@
+//go:build go1.13
 // +build go1.13
 
 package errors
@@ -6,14 +7,17 @@ import (
 	baseErrors "errors"
 )
 
-// find error in any wrapped error
+// As finds the first error in err's tree that matches target, and if one is found, sets
+// target to that error value and returns true. Otherwise, it returns false.
+//
+// For more information see stdlib errors.As.
 func As(err error, target interface{}) bool {
 	return baseErrors.As(err, target)
 }
 
 // Is detects whether the error is equal to a given error. Errors
 // are considered equal by this function if they are matched by errors.Is
-// or if their contained errors are matched through errors.Is
+// or if their contained errors are matched through errors.Is.
 func Is(e error, original error) bool {
 	if baseErrors.Is(e, original) {
 		return true
@@ -29,3 +33,29 @@ func Is(e error, original error) bool {
 
 	return false
 }
+
+// Join returns an error that wraps the given errors.
+// Any nil error values are discarded.
+// Join returns nil if every value in errs is nil.
+// The error formats as the concatenation of the strings obtained
+// by calling the Error method of each element of errs, with a newline
+// between each string.
+//
+// A non-nil error returned by Join implements the Unwrap() []error method.
+//
+// For more information see stdlib errors.Join.
+func Join(errs ...error) error {
+	return baseErrors.Join(errs...)
+}
+
+// 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.
+//
+// Unwrap only calls a method of the form "Unwrap() error".
+// In particular Unwrap does not unwrap errors returned by [Join].
+//
+// For more information see stdlib errors.Unwrap.
+func Unwrap(err error) error {
+	return baseErrors.Unwrap(err)
+}
diff --git a/error_backward.go b/error_backward.go
index 80b0695..ff14c4b 100644
--- a/error_backward.go
+++ b/error_backward.go
@@ -1,3 +1,4 @@
+//go:build !go1.13
 // +build !go1.13
 
 package errors
@@ -55,3 +56,70 @@ func Is(e error, original error) bool {
 
 	return false
 }
+
+// Disclaimer: functions Join and Unwrap are copied from the stdlib errors
+// package v1.21.0.
+
+// Join returns an error that wraps the given errors.
+// Any nil error values are discarded.
+// Join returns nil if every value in errs is nil.
+// The error formats as the concatenation of the strings obtained
+// by calling the Error method of each element of errs, with a newline
+// between each string.
+//
+// A non-nil error returned by Join implements the Unwrap() []error method.
+func Join(errs ...error) error {
+	n := 0
+	for _, err := range errs {
+		if err != nil {
+			n++
+		}
+	}
+	if n == 0 {
+		return nil
+	}
+	e := &joinError{
+		errs: make([]error, 0, n),
+	}
+	for _, err := range errs {
+		if err != nil {
+			e.errs = append(e.errs, err)
+		}
+	}
+	return e
+}
+
+type joinError struct {
+	errs []error
+}
+
+func (e *joinError) Error() string {
+	var b []byte
+	for i, err := range e.errs {
+		if i > 0 {
+			b = append(b, '\n')
+		}
+		b = append(b, err.Error()...)
+	}
+	return string(b)
+}
+
+func (e *joinError) Unwrap() []error {
+	return e.errs
+}
+
+// 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.
+//
+// Unwrap only calls a method of the form "Unwrap() error".
+// In particular Unwrap does not unwrap errors returned by [Join].
+func Unwrap(err error) error {
+	u, ok := err.(interface {
+		Unwrap() error
+	})
+	if !ok {
+		return nil
+	}
+	return u.Unwrap()
+}