When you look up the same value 10 times, you not only pollute the flame graphs and call counts which makes proving that a better algorithm is necessary or has any effect much harder, but more importantly, you could get 3 different states and try to make a bunch of updates based on what should be mutually exclusive states in the system. That's the global shared state problem.

When you look up a value once and remember it throughout a calculation, it may not be the current state, but at least you have a clean snapshot of the data. Which in situations such as cancelling an account immediately after getting one last discount on a purchase, well, we know which scenario the customer probably meant.

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.