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So, speaking as someone who loves functional programming but finds a lot of the flap surrounding the recent popularity of Haskell to be tiresome (just the flap, the language is great), I see it this way: Purity makes it easier to reason about the semantics of your code. This isn't about parallelism, it's about concurrency, including single-threaded concurrency. Case in point, I recently spent quite a while scratching my head over a bug that happened because someone else had written some code to mutate a piece of shared state when I wasn't expecting it. But the pure functional programming model is a very high level of abstraction (deep down, every interesting thing in computing is a state machine), and it has a tendency to leak like mad. One such case where it does so is I/O. In fact, you can't even do I/O in a 100% pure language - and that's what the I/O monad is really about; it's punching a hole in the language in order to let the big bad ephemeral outside world in. But in a controlled manner, so that the language's fundamental ethos of purity can be maintained, which in turn makes its laziness manageable. In short, the deeper downer reason why Haskell loves its I/O monad so much is because without it the language would be fairly useless. Anyone who tells you the I/O monad's really about making I/O concurrency headaches less of a hassle has been doing more blog-reading than programming. So why preserve the illusion? Well, ghettoizing all things stateful lets you take advantage of pure semantics everywhere else in your code, which theoretically makes it easier to reason about and maintain. As for monads themselves, IMHO they're kind of overblown. It's just another design pattern, akin to Visitor or Strategy or Decorator, only functional instead of object-oriented. Super-useful, applicable in all sorts of circumstances, and easily worth knowing. And, just like object-oriented design patterns, easy to make sound way more complicated than it is if you try to explain the idea to someone else without having fully digested it yourself first. |