Synchronous Core, Asynchronous Shell

In a software architecture concept called "Synchronous Core, Asynchronous Shell," inspired by Gary Bernhardt's work, the idea is to use functional programming for the core logic and imperative programming for side effects. This approach aims to enhance understanding, testing, and modifying the logic without the complexities of fully functional programs. Recently, discussions have arisen regarding the ergonomic challenges of Rust's async functionality, particularly the difficulties in integrating synchronous and asynchronous parts due to different execution models. The concept of function coloring has been highlighted as a limitation, where asynchronous functions can call synchronous ones but not vice versa easily. The author suggests employing a synchronous core, asynchronous shell pattern in Rust services, aligning with Gary's original idea but with a direct mapping of synchronous and asynchronous code. This pattern helps maintain advantages of the original concept and prevents synchronous code from calling asynchronous code directly. The author also proposes using the command pattern to trigger side effects from the core, emphasizing the benefits of function coloring as a safeguard against unintended side effects.