One version of the parallel vs concurrency distinction is parallelism is the easy part and concurrency is the hard part. I would say that for concurrency, FRP

One version of the parallel vs concurrency distinction is parallelism is the easy part and concurrency is the hard part. I would say that for concurrency, FRP (or various incremental programming technics) is essential. The problem is the possible control flow / traces space is just too large to characterize by hand. Another problem is control flow can be bidirectional. If you join together two streams and wish to be push driven, when you reach the merge you need to be able to pull from the other side. Virtually all of Futures/Tokio, hit JS thing du hour, etc etc fail at this. Even forks are a bit hard to do manual CPS as you only have one continuation. Blocking calls fail because the inflexibility of linear 1 input 1 output, but this is better known.