[hinkley]

It can but it’s very hard to catch. It’s like running your database at the wrong isolation level. By the time the bug happens it’s under heavy load and the system is too noisy to catch the real problem. So you have glitches nobody can explain and they just deal with cleanup.

For this and other reasons I think that in addition to Functional Core, Imperative Shell, you want a “square” call tree in your code. Avoid functions with no fanout, and functions with high fanout. Rearrange code that uses the same data to happen as close together as you can, to improve local reasoning. When functions get unwieldy, or deleted code makes them too small, use the b-tree algorithm as inspiration to rebalance the tree.

Refactor when new features change the coupling of the code.