From 59a30b4a6d9be7446136db9477963c12971c4fd8 Mon Sep 17 00:00:00 2001
From: Masayoshi Mizutani <masayoshi.mizutani@dr-ubie.com>
Date: Sat, 19 Oct 2024 14:21:40 +0900
Subject: [PATCH] Add example code for error creation using Builder pattern

---
 README.md                | 32 ++++++++++++++++++++++++++++++++
 examples/builder/main.go | 30 ++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 examples/builder/main.go

diff --git a/README.md b/README.md
index 642ae79..b3e738c 100644
--- a/README.md
+++ b/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
 
diff --git a/examples/builder/main.go b/examples/builder/main.go
new file mode 100644
index 0000000..b289a10
--- /dev/null
+++ b/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)
+	}
+}