"This page contains notes and information about how to write concurrent programs in Haskell. If you're more interested in performance than non-determinism, learn about parallelism first."
Here it's not capitalized so it must be the concepts and not the modules...
"If you have parallelism, you have concurrency" - not according to Rob Pike, who called Sawzall parallel but not concurrent.
The main point I was making though was that computational code can be easily checked even if it's not pure - and the fact that you can express it as pure is a result of the same thing that let you easily check an impure spelling as well (basically it's determinism, and avoidance of constructs where determinism is hard to prove.) For event handling systems you can't easily check them, for much the same reasons that you can't express them as pure code.