Program execution monitoring consists of checking whole executions for given properties, and collecting global run-time information. Monitoring gives valuable insights and helps programmers maintain their programs. However, application developers face the following dilemma: either they use existing monitoring tools which never exactly fit their needs, or they invest a lot of effort to implement relevant monitoring code. In this paper, we argue that when an event-oriented tracer exists, the compiler developers can enable the application developers to easily code their own monitors. We propose a high-level primitive called foldt which operates on execution traces. One of the key advantages of our approach is that it allows a clean separation of concerns; the definition of monitors is totally distinct from both the user source code and the language compiler. We give a number of applications of the use of foldt to define monitors for Mercury program executions: execution profiles, graphical abstract views, and two test coverage measurements. Each example is implemented by a few simple lines of Mercury.