Jeff Newbern wrote (on Tue, 12 Aug 2003 at 17:20): (proposed revisions)
> In the section "No Way Out": > ---------- > The IO monad is a familiar example of a one-way monad in Haskell. > Because you can't escape from the IO monad, it is impossible to write a > function that does a computation in the IO monad but returns a > non-monadic value. I wouldn't say that, as it is inaccurate. Of course you can return a value of _some_ monadic type eg. (Maybe ...). > Not only are functions of the type IO a -> a > impossible to create, You can quite easily write a function of type IO (IO a) -> IO a, which is a special case of that type. > but any function whose result type does not > contain the IO type constructor is guaranteed not to use the IO monad. That's rather vague: what does it mean for a function to use a monad? > Other monads, such as List and Maybe, do allow values out of the monad. > So it is possible to write non-monadic functions that internally do > computations in those monads. The one-way nature of the IO monad also > has consequences when combining monads, a topic that is discussed in > part III. > ---------- In summary, I've only a vague idea of what you are trying to say. If you can't reformulate it more precisely, don't add the above stuff. > and a little farther down: > ---------- > Some people argue that using monads to introduce non-pure features into > Haskell disqualifies it from claiming to be a pure functional language. > This subtle question not particularly relevant to the practical > programmer is revisited in the context of the I/O monad later in the > tutorial. > ---------- That's fair enough. I don't think the question is so much subtle as religious, as we might expect from the terminology of "purity". > Later, in the section on the I/O monad: > ---------- > In Haskell, the top-level main function must have type IO (), so that > programs are typically structured at the top level as an > imperative-style sequence of I/O actions and calls to functional-style > code. Revisiting the debate about the purity of Haskell (in a functional > sense), it is important to note that the IO monad only simulates > imperative-style I/O. That (about simulation) seems weak. A simulation isn't a vague syntactic resemblance. > The functions exported from the IO module do not > perform I/O themselves. They return I/O actions, which describe an I/O > operation to be performed. The I/O actions are combined within the IO > monad (in a purely functional manner) to create more complex I/O > actions, resulting in the final I/O action that is the main value of the > program. The result of the Haskell compiler is an executable function > incorporating the main I/O action. Executing the program "applies" this > ultimate I/O action to the outside world to produce a new state of the > world. That seems to me the wrong thing to say. There is no application. Whether or not the word is put in quotes, it is something involving a function and an argument. An IO action is not a function. > This occurs only once per execution of the program, and since the > state of the world changes for each execution, the issue of purity is > neatly side-stepped. > ---------- By "the program", I think you mean the IO action. I think it is right to speak of the action as something that is executed. Execution may involve (side-effect free) calculation; but execution is something essentially different from calculation, not an impure form of it. I'm sorry to sound negative -- it's just that you invited criticism. Your pages seem generally of a very high quality to me. Sorry not to be more constructive too. Peter Hancock