src/blog/post-6.md

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+---
+title: 'The Difference Between %v and %w in Go’s fmt.Errorf'
+pubDate: 2026-01-12
+author: 'Nakahara Daisuke'
+tags: ["Go"]
+---
+
+I learned the difference between `%v` and `%w` through reviewing Go code with the help of an AI generative model.
+
+## `%v`: String representation of a value
+
+```go
+err := someFunction()
+return fmt.Errorf("failed to do something: %v", err)
+```
+
+This approach simply embeds the error as a string.
+As a result, information about the original error is lost, which means it may not be usable with `errors.Is()` or `errors.As()` for error inspection.
+
+## `%w`: Wrapping an error
+
+```go
+err := someFunction()
+return fmt.Errorf("failed to do something: %w", err)
+```
+`%w` is a special format specifier used with `fmt.Errorf()` to wrap an error. This allows the original error to be retrieved using `errors.Unwrap()`.
+
+The %w verb for wrapping errors was introduced in [Go 1.13](https://go.dev/doc/go1.13).
+
+```go
+orig := errors.Unwrap(err)
+```
+In addition, wrapped errors can be examined using `errors.Is()` and `errors.As()`.
+
+```go
+if errors.Is(err, io.EOF) {
+    // Handle EOF
+}
+
+var pathErr *os.PathError
+if errors.As(err, &pathErr) {
+    // Type assertion succeeded
+}
+```
+
+The official documentation is available [here](https://pkg.go.dev/fmt#Errorf).
+
+---
+
+> **Note**:Note The review and translation were assisted by an AI generative model. The author is responsible for the final content.