I disagree. In force, the Haskell RTS is just a "model" of the IO effects as well. In particular, there are at least two such models, the GHCi interpreter and t

I disagree. In force, the Haskell RTS is just a "model" of the IO effects as well. In particular, there are at least two such models, the GHCi interpreter and the GHC compiler! As another example, it'd be completely possible to write your own IO and interpret it in another language. You can read about this on Edward Kmett's blog where he talks about implementing IO for Ermine [0] http://www.tuicool.com/articles/I3EJVb http://www.tuicool.com/articles/I3EJVb Of course, there's something important that makes us want to differentiate "real world" side effects from internal "model effects". Ultimately, from a correctness and reasoning POV, there ought to be no difference. From a practical point of view, some models are more interesting or important than others. But as a compiler writer you're put right back into the same hot seat. [0] Unfortunately, his blog appears to be OOC right now, so here's a weird chinese mirror!