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ch9/2/ch9_2.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+)
+
+type CustomError struct {
+	fieldName string
+}
+
+type CustomError2 struct {
+	fieldName string
+}
+
+func (c CustomError) Error() string {
+	return fmt.Sprintf("missing field: %s", c.fieldName)
+}
+
+func (c CustomError) Is(target error) bool {
+	return c.Error() == target.Error()
+}
+
+func (c CustomError2) Error() string {
+	return fmt.Sprintf("missing field: %s", c.fieldName)
+}
+
+func (c CustomError2) Is(target error) bool {
+	return c.Error() == target.Error()
+}
+
+func main() {
+
+	test1 := CustomError{"Test1"}
+	test2 := CustomError2{"Test2"}
+	test3 := fmt.Errorf("Test3 %w", test1)
+	test4 := fmt.Errorf("Test4: %w", test3)
+
+	if errors.As(test3, &test1) {
+		fmt.Println("Test3 contains Test1")
+		fmt.Printf("Test3 contains Test1 field: %s", test3.Error())
+	} else {
+		fmt.Println("Test3 doesn't contain Test1")
+	}
+	if errors.As(test3, &test2) {
+		fmt.Println("Test3 contains Test2")
+	} else {
+		fmt.Println("Test3 doesn't contain Test1")
+	}
+	if errors.As(test4, &test1) {
+		fmt.Println("Test4 contains Test1")
+	} else {
+		fmt.Println("Test3 doesn't contain Test1")
+	}
+	if errors.As(test4, &test2) {
+		fmt.Println("Test4 contains Test2")
+	} else {
+		fmt.Println("Test3 doesn't contain Test1")
+	}
+	if errors.As(test4, &test3) {
+		fmt.Println("Test4 contains Test3")
+	} else {
+		fmt.Println("Test3 doesn't contain Test1")
+	}
+}

ch9/2/go.mod

@@ -0,0 +1,3 @@
+module ch9_2
+
+go 1.25.0