Cafe,
I was watching a panel on languages[0] recently and Martin Odersky (the creator of Scala) said something about Monads:
"What's wrong with Monads is that if you go into a Monad you have to change your whole syntax from scratch. Every single line of your program changes if you get it in or out of a Monad. They're not polymorphic so it's really the old days of Pascal. A monomorphic type system that says 'well that's all I do' ...
Jonathan Geddes
Thoughts?
In my mind, his first sentence could be paraphrased as follows: "What's wrong with having a strongly typed mechanism for separating pure and effectful code is that pure code and effectful code are actually separate." Of course every line of your program that uses a Foo will change if you switch to IO Foo instead. If the two were indistinguishable, then you'd have an impure language instead of a pure one. Furthermore, this isn't a characteristic specific to monads, it's the same for any type constructor whether it's a monad or not. To Martin's credit, earlier in the talk (around 45:15) he makes the following statement: "What you need in the language is the ability to essentially reduce state. And potentially what Scala doesn't do, but for instance Haskell does is to control state and purity, and I think that might be the next frontier." Then at 50:15 he says, "So right now, I think...in the world of effect checking we're about where we were with Pascal in terms of static typing." It seems that his heart is in the right place, but he isn't aware of where we actually are right now, and probably doesn't fully appreciate the consequences of strongly typed purity.