feat: add Join for multierror support

err_impl.go

@@ -1,14 +1,17 @@
 package errors
 
 import (
-	"errors"
+	"cmp"
 )
 
 func newE(opts ...any) error {
 	var err e
 
 	skipFrames := FrameSkips(3)
 	for _, o := range opts {
+		if o == nil {
+			continue
+		}
 		switch arg := o.(type) {
 		case string:
 			err.msg = arg
@@ -83,19 +86,22 @@ func (err *e) Fields() []any {
 		out  []any
 		kind Kind
 	)
-	for err := error(err); err != nil; err = errors.Unwrap(err) {
-		ee, ok := err.(*e)
-		if !ok {
-			continue
-		}
-
-		for _, kv := range ee.kvs {
+	for err := error(err); err != nil; err = Unwrap(err) {
+		em := getErrMeta(err)
+		for _, kv := range em.kvs {
 			out = append(out, kv.K, kv.V)
 		}
-		if kind == "" {
-			kind = ee.kind
+		kind = cmp.Or(kind, em.kind)
+		if ej, ok := err.(*joinE); ok {
+			innerKind, multiErrFields := ej.subErrFields()
+			kind = cmp.Or(kind, innerKind)
+			if len(multiErrFields) > 0 {
+				out = append(out, "multi_err", multiErrFields)
+			}
+			break
 		}
 	}
+
 	if kind != "" {
 		out = append(out, "err_kind", string(kind))
 	}
@@ -110,18 +116,18 @@ func (err *e) Fields() []any {
 	return out
 }
 
+func (err *e) Is(target error) bool {
+	kind, ok := target.(Kind)
+	return ok && err.kind == kind
+}
+
 func (err *e) Unwrap() error {
 	return err.wrappedErr
 }
 
 func (err *e) V(key string) (any, bool) {
-	for err := error(err); err != nil; err = errors.Unwrap(err) {
-		ee, ok := err.(*e)
-		if !ok {
-			continue
-		}
-
-		for _, kv := range ee.kvs {
+	for err := error(err); err != nil; err = Unwrap(err) {
+		for _, kv := range getErrMeta(err).kvs {
 			if kv.K == key {
 				return kv.V, true
 			}
@@ -132,10 +138,47 @@ func (err *e) V(key string) (any, bool) {
 
 func (err *e) stackTrace() StackFrames {
 	var out StackFrames
-	for err := error(err); err != nil; err = errors.Unwrap(err) {
-		if ee, ok := err.(*e); ok && ee.frame.FilePath != "" {
-			out = append(out, ee.frame)
+	for err := error(err); err != nil; err = Unwrap(err) {
+		em := getErrMeta(err)
+		if em.frame.FilePath == "" {
+			continue
+		}
+		out = append(out, em.frame)
+		if em.errType == errTypeJoin {
+			break
 		}
 	}
 	return out
 }
+
+type errMeta struct {
+	kind    Kind
+	frame   Frame
+	kvs     []KV
+	errType string
+}
+
+const (
+	errTypeE    = "e"
+	errTypeJoin = "j"
+)
+
+func getErrMeta(err error) errMeta {
+	var em errMeta
+	switch err := err.(type) {
+	case *e:
+		em.kind, em.frame, em.kvs, em.errType = err.kind, err.frame, err.kvs, errTypeE
+	case *joinE:
+		em.kind, em.frame, em.kvs, em.errType = err.kind, err.frame, err.kvs, errTypeJoin
+	}
+	return em
+}
+
+func getKind(err error) Kind {
+	for ; err != nil; err = Unwrap(err) {
+		if em := getErrMeta(err); em.kind != "" {
+			return em.kind
+		}
+	}
+	return ""
+}

errors.go

@@ -23,6 +23,16 @@ func Wrap(err error, opts ...any) error {
 	return newE(passedOpts...)
 }
 
+// Join returns a new multi error.
+//
+// TODO:
+//   - play with Join(opts ...any) and Join(errs []error, opts ...any) sigs
+//     and ask for feedback regarding tradeoffs with type safety of first arg.
+//     As of writing some tests, I kind of dig the loose Join(opts ...any).
+func Join(opts ...any) error {
+	return newJoinE(opts...)
+}
+
 // Fields returns logging fields for a given error.
 func Fields(err error) []any {
 	if err == nil {
@@ -41,8 +51,10 @@ func Fields(err error) []any {
 // TODO:
 //  1. make this more robust with Is
 //  2. determine if its even worth exposing an accessor for this private method
+//  3. allow for StackTraces() to accommodate joined errors, perhaps returning a map[string]StackFrames
+//     or some graph representation would be awesome.
 func StackTrace(err error) StackFrames {
-	ee, ok := err.(*e)
+	ee, ok := err.(interface{ stackTrace() StackFrames })
 	if !ok {
 		return nil
 	}

errors_stack_traces_test.go

@@ -181,6 +181,43 @@ func Test_Errors(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "with wrapped joined  errors error with inner kind",
+			input: errors.Wrap(
+				errors.Wrap(
+					errors.Join(
+						errors.Wrap(
+							errors.New("first error", errors.Kind("inner")),
+						),
+						errors.New("second error"),
+					),
+				),
+			),
+			want: wants{
+				msg: `2 errors occurred:
+	* first error
+	* second error
+`,
+				fields: []any{
+					"multi_err", []any{
+						"err_0", []any{
+							"err_kind", "inner",
+							"stack_trace", []string{
+								"github.com/jsteenb2/errors/errors_stack_traces_test.go:189[Test_Errors]",
+								"github.com/jsteenb2/errors/errors_stack_traces_test.go:190[Test_Errors]",
+							},
+						},
+						"err_1", []any{"stack_trace", []string{"github.com/jsteenb2/errors/errors_stack_traces_test.go:192[Test_Errors]"}},
+					},
+					"err_kind", "inner",
+					"stack_trace", []string{
+						"github.com/jsteenb2/errors/errors_stack_traces_test.go:186[Test_Errors]",
+						"github.com/jsteenb2/errors/errors_stack_traces_test.go:187[Test_Errors]",
+						"github.com/jsteenb2/errors/errors_stack_traces_test.go:188[Test_Errors]",
+					},
+				},
+			},
+		},
 	}
 
 	for _, tt := range tests {

errors_test.go

@@ -1,6 +1,7 @@
 package errors_test
 
 import (
+	"encoding/json"
 	"reflect"
 	"testing"
 
@@ -77,6 +78,13 @@ func eqV[T comparable](t *testing.T, err error, key string, want T) bool {
 func eqFields(t *testing.T, want, got []any) bool {
 	t.Helper()
 
+	defer func() {
+		if t.Failed() {
+			b, _ := json.MarshalIndent(got, "", "  ")
+			t.Logf("got: %s", string(b))
+		}
+	}()
+
 	if matches := eqLen(t, len(want), got); !matches {
 		return matches
 	}