feat: support join error match for eris.Is and eris.As (#27)

eris.go

@@ -433,8 +433,18 @@ func (e *rootError) Format(s fmt.State, verb rune) {
 	printError(e, s, verb)
 }
 
-// Is returns true if both errors have the same message and code. Ignores additional KV pairs.
+// Is returns true if both errors have the same message and code.
+// In case of a joined error, returns true if at least one of the joined errors is equal to target.
+// Ignores additional KV pairs.
 func (e *rootError) Is(target error) bool {
+	if joinErr, ok := e.ext.(joinError); ok {
+		for _, err := range joinErr.Unwrap() {
+			if Is(err, target) {
+				return true
+			}
+		}
+		return false
+	}
 	if err, ok := target.(*rootError); ok {
 		return e.msg == err.msg && e.code == err.code && reflect.DeepEqual(e.kvs, err.kvs)
 	}
@@ -446,6 +456,14 @@ func (e *rootError) Is(target error) bool {
 
 // As returns true if the error message in the target error is equivalent to the error message in the root error.
 func (e *rootError) As(target any) bool {
+	if joinErr, ok := e.ext.(joinError); ok {
+		for _, err := range joinErr.Unwrap() {
+			if As(err, target) {
+				return true
+			}
+		}
+		return false
+	}
 	t := reflect.Indirect(reflect.ValueOf(target)).Interface()
 	if err, ok := t.(*rootError); ok {
 		if e.msg == err.msg {

eris_test.go

@@ -427,6 +427,7 @@ func TestErrorUnwrap(t *testing.T) {
 }
 
 func TestErrorIs(t *testing.T) {
+	rootErr := eris.New("root error")
 	externalErr := errors.New("external error")
 	customErr := withLayer{
 		msg: "additional context",
@@ -518,6 +519,53 @@ func TestErrorIs(t *testing.T) {
 			compare: nil,
 			output:  true,
 		},
+		"join error (external)": {
+			cause:   eris.Join(externalErr, rootErr),
+			compare: externalErr,
+			output:  true,
+		},
+		"join error (root)": {
+			cause:   eris.Join(externalErr, rootErr),
+			compare: rootErr,
+			output:  true,
+		},
+		"join error (nil)": {
+			cause:   eris.Join(nil, nil),
+			compare: nil,
+			output:  true,
+		},
+		"join error (wrap)": {
+			cause:   eris.Join(externalErr, eris.Wrap(rootErr, "eris wrap error")),
+			compare: eris.New("eris wrap error").WithCode(eris.CodeInternal),
+			output:  true,
+		},
+		"join error not found (code don't match)": {
+			cause:   eris.Join(externalErr, eris.Wrap(rootErr, "eris wrap error")),
+			compare: eris.New("eris wrap error"),
+			output:  false,
+		},
+		"join error not found (message don't match)": {
+			cause:   eris.Join(externalErr, eris.Wrap(rootErr, "eris wrap error")),
+			compare: eris.New("eris root error message wrong"),
+			output:  false,
+		},
+		"join error not found (external don't match)": {
+			cause:   eris.Join(externalErr, rootErr),
+			compare: errors.New("external error not match"),
+			output:  false,
+		},
+		"wrapped join error (match join errors)": {
+			cause:   eris.Join(externalErr, rootErr),
+			input:   []string{"additional context"},
+			compare: rootErr,
+			output:  true,
+		},
+		"wrapped join error (match wrap)": {
+			cause:   eris.Join(externalErr, rootErr),
+			input:   []string{"additional context"},
+			compare: eris.New("additional context").WithCode(eris.CodeUnknown),
+			output:  true,
+		},
 	}
 
 	for desc, tc := range tests {
@@ -535,6 +583,7 @@ func TestErrorIs(t *testing.T) {
 func TestErrorAs(t *testing.T) {
 	externalError := errors.New("external error")
 	rootErr := eris.New("root error").WithCode(eris.CodeUnknown)
+	anotherRootErr := eris.New("another root error").WithCode(eris.CodeUnknown)
 	wrappedErr := eris.WithCode(eris.Wrap(rootErr, "additional context"), eris.CodeUnknown)
 	customErr := withLayer{
 		msg: "additional context",
@@ -544,7 +593,6 @@ func TestErrorAs(t *testing.T) {
 			},
 		},
 	}
-
 	tests := map[string]struct {
 		cause  error // root error
 		target any   // errors for comparison
@@ -641,6 +689,29 @@ func TestErrorAs(t *testing.T) {
 			match:  true,
 			output: customErr,
 		},
+		"join error (external)": {
+			cause:  eris.Join(externalError, rootErr),
+			target: &externalError,
+			match:  true,
+			output: externalError,
+		},
+		"join error (root)": {
+			cause:  eris.Join(externalError, rootErr),
+			target: &rootErr,
+			match:  true,
+			output: rootErr,
+		},
+		"join error (custom)": {
+			cause:  eris.Join(externalError, withMessage{"test"}),
+			target: &withMessage{""},
+			match:  true,
+			output: withMessage{"test"},
+		},
+		"join error not found (message don't match)": {
+			cause:  eris.Join(externalError, rootErr),
+			target: &anotherRootErr,
+			match:  false,
+		},
 	}
 
 	for desc, tc := range tests {