I recommend reading "Parallel and Concurrent Programming in Haskell" by Simon Marlow in which he explains that back in 1997 he decided to "build concurrency rig

I recommend reading "Parallel and Concurrent Programming in Haskell" by Simon Marlow in which he explains that back in 1997 he decided to "build concurrency right into the core of the system rather than making it an optional extra or an add-on library".