The World model is an existing functional input-output mechanism for
event-driven programming. It is used in numerous popular textbooks and
curricular settings. The World model conflates two different tasks -- the
definition of an event processor and its execution -- into one. This conflation
imposes a significant (even unacceptable) burden on student users in several
educational settings where we have tried to use it, e.g., for teaching physics.
While it was tempting to pile on features to address these issues, we instead
used the Scheme language design dictum of removing weaknesses that made them
seem necessary. By separating the two tasks above, we arrived at a slightly
different primitive, the reactor, as our basis. This only defines the event
processor, and a variety of execution operators dictate how it runs. The new
design enables programmatic control over event-driven programs. This simplifies
reflecting on program behavior, and eliminates many unnecessary curricular
dependencies imposed by the old design. This work has been implemented in the
Pyret programming language. The separation of concerns has enabled new
curricula, such as the Bootstrap:Physics curriculum, to take flight. Thousands
of students use this new mechanism every year. We believe that reducing
impedance mismatches improves their educational experience
