Add Builder feature

README.md

@@ -215,7 +215,39 @@ Output:
 }
 ```
 
+### Builder
 
+`goerr` provides `goerr.NewBuilder()` to create an error with pre-defined contextual variables. It is useful when you want to create an error with the same contextual variables in multiple places.
+
+```go
+type object struct {
+	id    string
+	color string
+}
+
+func (o *object) Validate() error {
+	eb := goerr.NewBuilder().With("id", o.id)
+
+	if o.color == "" {
+		return eb.New("color is empty")
+	}
+
+	return nil
+}
+
+func main() {
+	obj := &object{id: "object-1"}
+
+	if err := obj.Validate(); err != nil {
+		slog.Default().Error("Validation error", "err", err)
+	}
+}
+```
+
+Output:
+```
+2024/10/19 14:19:54 ERROR Validation error err.message="color is empty" err.values.id=object-1 (snip)
+```
 
 ## License
 

builder.go

@@ -0,0 +1,40 @@
+package goerr
+
+import (
+	"fmt"
+)
+
+// Builder keeps a set of key-value pairs and can create a new error and wrap error with the key-value pairs.
+type Builder struct {
+	values values
+}
+
+// NewBuilder creates a new Builder
+func NewBuilder() *Builder {
+	return &Builder{values: make(values)}
+}
+
+// With copies the current Builder and adds a new key-value pair.
+func (x *Builder) With(key string, value any) *Builder {
+	newVS := &Builder{values: x.values.clone()}
+	newVS.values[key] = value
+	return newVS
+}
+
+// New creates a new error with message
+func (x *Builder) New(format string, args ...any) *Error {
+	err := newError()
+	err.msg = fmt.Sprintf(format, args...)
+	err.values = x.values.clone()
+
+	return err
+}
+
+// Wrap creates a new Error with caused error and add message.
+func (x *Builder) Wrap(cause error, msg ...any) *Error {
+	err := newError()
+	err.msg = toWrapMessage(msg)
+	err.cause = cause
+	err.values = x.values.clone()
+	return err
+}

builder_test.go

@@ -0,0 +1,32 @@
+package goerr_test
+
+import (
+	"testing"
+
+	"github.com/m-mizutani/goerr"
+)
+
+func newErrorWithBuilder() *goerr.Error {
+	return goerr.NewBuilder().With("color", "orange").New("error")
+}
+
+func TestBuilderNew(t *testing.T) {
+	err := newErrorWithBuilder()
+
+	if err.Values()["color"] != "orange" {
+		t.Errorf("Unexpected value: %v", err.Values())
+	}
+}
+
+func TestBuilderWrap(t *testing.T) {
+	cause := goerr.New("cause")
+	err := goerr.NewBuilder().With("color", "blue").Wrap(cause, "error")
+
+	if err.Values()["color"] != "blue" {
+		t.Errorf("Unexpected value: %v", err.Values())
+	}
+
+	if err.Unwrap().Error() != "cause" {
+		t.Errorf("Unexpected cause: %v", err.Unwrap().Error())
+	}
+}

errors.go

@@ -18,18 +18,18 @@ func New(format string, args ...any) *Error {
 	return err
 }
 
+func toWrapMessage(msgs []any) string {
+	var newMsgs []string
+	for _, m := range msgs {
+		newMsgs = append(newMsgs, fmt.Sprintf("%v", m))
+	}
+	return strings.Join(newMsgs, " ")
+}
+
 // Wrap creates a new Error and add message.
 func Wrap(cause error, msg ...any) *Error {
 	err := newError()
-
-	if len(msg) > 0 {
-		var newMsgs []string
-		for _, m := range msg {
-			newMsgs = append(newMsgs, fmt.Sprintf("%v", m))
-		}
-		err.msg = strings.Join(newMsgs, " ")
-	}
-
+	err.msg = toWrapMessage(msg)
 	err.cause = cause
 
 	return err
@@ -56,19 +56,29 @@ func Unwrap(err error) *Error {
 	return nil
 }
 
+type values map[string]any
+
+func (x values) clone() values {
+	newValues := make(values)
+	for key, value := range x {
+		newValues[key] = value
+	}
+	return newValues
+}
+
 // Error is error interface for deepalert to handle related variables
 type Error struct {
 	msg    string
 	id     string
 	st     *stack
 	cause  error
-	values map[string]any
+	values values
 }
 
 func newError() *Error {
 	return &Error{
 		st:     callers(),
-		values: make(map[string]any),
+		values: make(values),
 		id:     uuid.New().String(),
 	}
 }

examples/builder/main.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"log/slog"
+
+	"github.com/m-mizutani/goerr"
+)
+
+type object struct {
+	id    string
+	color string
+}
+
+func (o *object) Validate() error {
+	eb := goerr.NewBuilder().With("id", o.id)
+
+	if o.color == "" {
+		return eb.New("color is empty")
+	}
+
+	return nil
+}
+
+func main() {
+	obj := &object{id: "object-1"}
+
+	if err := obj.Validate(); err != nil {
+		slog.Default().Error("Validation error", "err", err)
+	}
+}