On Dec 4, 2007 11:39 AM, Jules Bean
Ben Franksen wrote:
I don't buy this. As has been noted by others before, IO is a very special case, in that it can't be defined in Haskell itself, and there is no evaluation function runIO :: IO a -> a.
This is a straw man. Most monads will not have such a function:
When I first learned monads, I heard that "once you get into IO, you can never get out". The point here was that that doesn't generalize, so a student might start thinking that a monad is like a taint flag or something. Not to say that statement that it's a complete falsity when generalized to other monads -- it's reflective of the algebra of monads -- you just have to define "never" a little differently. :-) In any case, I don't think that's a big issue. While it is important to eliminate things that monads aren't from students' possible models, it's better just to build a good model in the first place. FWIW, the list monad was how I made the leap from "monads do IO" to "monads do nifty stuff". Luke
There is no function (State s a) -> a.
There is no function (r -> a) -> a.
There is no function (Random a) -> a. [assuming some random monad, often discussed]
There is no function (Supply s a) -> a. [Another useful monad although not one of the standard ones]
There are no (total) functions Maybe a -> a, [a] -> a, Either e a -> a.
As to the topic of the thread: I agree IO is an unusual monad. I'm not sure if I agree that it shouldn't be used as a teaching basis. I think there are all kinds of ways to teach haskell; I'd be inclined to want to start with some IO, without explaining the plumbing in detail, and then come back to it later with better perspective when discussing general monads.
Jules
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