Improving legacy Om code (II): Using effects and coeffects to isolate effectful code from pure code

Introduction.

In a previous post, we applied the humble object pattern idea to avoid having to write end-to-end tests for the interesting logic of a hard to test legacy Om code, and managed to write cheaper unit tests instead. Then, we saw how those unit tests were far from ideal because they were highly coupled to implementation details, and how these problems were caused by a lack of separation of concerns in the code design.

In this post we’ll show a solution to those design problems using effects and coeffects that will make the interesting logic pure and, as such, really easy to test and reason about.

Refactoring to isolate side-effects and side-causes using effects and coeffects.

We refactored the code to isolate side-effects and side-causes from pure logic. This way, not only testing the logic got much easier (the logic would be in pure functions), but also, it made tests less coupled to implementation details. To achieve this we introduced the concepts of coeffects and effects.

The basic idea of the new design was:

1. Extracting all the needed data from globals (using coeffects for getting application state, getting component state, getting DOM state, etc).
2. Using pure functions to compute the description of the side effects to be performed (returning effects for updating application state, sending messages, etc) given what was extracted in the previous step (the coeffects).
3. Performing the side effects described by the effects returned by the called pure functions.

The main difference that the code of horizon.controls.widgets.tree.hierarchy presented after this refactoring was that the event handler functions were moved back into it again, and that they were using the process-all! and extract-all! functions that were used to perform the side-effects described by effects, and extract the values of the side-causes tracked by coeffects, respectively. The event handler functions are shown in the next snippet (to see the whole code click here):