add test

04-errors-semantics/20-nil-interface-gotcha/myerror_test.go

@@ -97,3 +97,31 @@ func TestErrorsAsExtractionMatrix(t *testing.T) {
 		})
 	}
 }
+
+func TestErrorMethodOutput(t *testing.T) {
+	tests := []struct {
+		name        string
+		err         *MyError
+		expectedMsg string
+	}{
+		{
+			name:        "nil receiver",
+			err:         nil,
+			expectedMsg: "<nil MyError>",
+		},
+		{
+			name:        "normal error",
+			err:         &MyError{Op: "test-operation"},
+			expectedMsg: "operation failed: test-operation",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			msg := tt.err.Error()
+			if msg != tt.expectedMsg {
+				t.Errorf("expected message: %q, got: %q", tt.expectedMsg, msg)
+			}
+		})
+	}
+}