That and that Nim's ARC system extends the concept a bit further to include move semantics and limited ownership rules. Nim had a traditional RC garbage collect

That and that Nim's ARC system extends the concept a bit further to include move semantics and limited ownership rules. Nim had a traditional RC garbage collector before, but ARC is case where "the whole is greater than the sum of parts". Swift and Obj-C's ARC system relies on atomics and their associated issues (explained well here: https://news.ycombinator.com/item?id=15046481 https://news.ycombinator.com/item?id=15046481). Nim's ARC system relies on ownership rules that require a single thread to "own" any ARC-managed memory which removes the need for locks or atomics in the reference counts. That eliminates the non-determinism of locks & atomics when doing the RC accounting, overall improving latency and performance. Move semantics increase performance and enables "giving" ownership of ARC-managed memory to another thread.