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--- go:errors:error_wrapping [2026/01/05 23:17] – created phong2018
+++ go:errors:error_wrapping [2026/01/05 23:22] (current) – phong2018
@@ Line 24: / Line 24: @@
   * Use `%v` if you only want formatting without wrapping.
   * Wrapping enables `errors.Is` and `errors.As` to work through layers.
+===== Error wrapping (%w) vs formatting (%v) =====
+==== What is it? ====
+In `fmt.Errorf`, you can use:
+  * `%w` to **wrap** an error (keeps the original error inside)
+  * `%v` to **format** an error (string formatting only; does NOT wrap)
+==== Why it matters ====
+  * `%w` enables `errors.Is` / `errors.As` to find the root cause through wrapped layers.
+  * `%v` only prints the error message; `errors.Is` will not match the original error.
+==== Example code ====
+<code go>
+package main
+import (
+    "errors"
+    "fmt"
+)
+var ErrNotFound = errors.New("not found")
+func repo() error {
+    return ErrNotFound
+}
+// Wrap with %w (keeps original error)
+func usecaseWrap() error {
+    if err := repo(); err != nil {
+        return fmt.Errorf("usecase (wrap): %w", err)
+    }
+    return nil
+}
+// Format only with %v (does NOT wrap)
+func usecaseFormatOnly() error {
+    if err := repo(); err != nil {
+        return fmt.Errorf("usecase (format only): %v", err)
+    }
+    return nil
+}
+func main() {
+    errW := usecaseWrap()
+    errV := usecaseFormatOnly()
+    fmt.Println("== %w ==")
+    fmt.Println("Error:", errW)
+    fmt.Println("errors.Is(errW, ErrNotFound):", errors.Is(errW, ErrNotFound))
+    fmt.Println("\n== %v ==")
+    fmt.Println("Error:", errV)
+    fmt.Println("errors.Is(errV, ErrNotFound):", errors.Is(errV, ErrNotFound))
+}
+</code>
+==== Example output ====
+<code>
+== %w ==
+Error: usecase (wrap): not found
+errors.Is(errW, ErrNotFound): true
+== %v ==
+Error: usecase (format only): not found
+errors.Is(errV, ErrNotFound): false
+</code>
+==== Rule of thumb ====
+  * Use `%w` when you want callers to detect the original error using `errors.Is/As`.
+  * Use `%v` when you only want a formatted message and do not need error matching.
+==== Related pages ====
+  * [[go:errors:error_wrapping|Error wrapping (%w)]]
+  * [[go:errors:errors_is_as|errors.Is / errors.As]]
+  * [[go:errors:sentinel_errors|Sentinel errors]]
 ==== Hard words (English) ====
   * **wrap** /ræp/: bọc
-  * **preserve** /prɪˈzɝːv/: giữ lại
+  * **format** /ˈfɔːrmæt/: định dạng
   * **root cause** /ruːt kɔːz/: nguyên nhân gốc
+  * **detect** /dɪˈtekt/: phát hiện/nhận biết
+  * **matching** /ˈmætʃɪŋ/: so khớp
+  * **preserve** /prɪˈzɝːv/: giữ lại
   * **layer** /ˈleɪər/: tầng