nimcity/doc/part04/ch46.md

Chapter 46. Final advice

46.1. What to avoid

1. Global variables. Global variables make your routines lack the noSideEffect effect that is required for using either createThread or spawn safely.
2. Don’t use createThread unless you implement your own thread pool.
3. Don’t use ref. Prefer seq as seq lacks the problematic aliasing aspects and can be moved around just as easily as a ref.
4. Avoid channels as these do not compose well with potential concurrency, where the runtime system is free not to exploit the concurrency and to run a task on the same thread instead. 46.2. What to use Malebolgia’s abstractions have been the result of years of research and cover most use cases:
5. Use parMap, parFind, and parReduce which work at a very high level and are easy to use correctly and hard to use incorrectly.
6. Use spawn and awaitAll or alternatives that enforce structured concurrency.
7. Share memory via a Locker[T] wrapper that ensures at compile time that data races cannot happen. In some sense Malebolgia’s abstractions are the most natural extensions that add concurrency to “single-threaded” Nim: • Function call expressions can be run in parallel via spawn. Divergent 281 control flow, as it exists in the single-threaded Nim in the form of case or if statements, eventually converges again via awaitAll. • Spawned function calls can return values. • Global variables or shared memory in the form of var parameters are enabled with Locker[T].