Yet Another Monad Tutorial
Jeff Newbern
jnewbern@nomaware.com
12 Aug 2003 19:13:58 +0100
Peter,
Thank you for criticism. This is exactly the kind of feedback I need.
The overall message I have taken from your post is that I need to
be more precise to convey the correct information and avoid confusion
(or worse, misinformation).
I should have said that a function which performs a computation
in the I/O monad must have IO as the outermost constructor
in its return type. I think that is more precise (and correct :-).
As for the later text concerning "application" of the top-level I/O
action, I will need to think for a while to see how I can word this
more clearly. If you (or anyone else) have ideas, I'd like to hear
them.
Thanks again,
Jeff
On Tue, 2003-08-12 at 18:24, Peter G. Hancock wrote:
> >>>>> 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
--
Jeff Newbern <jnewbern@nomaware.com>
Nomaware, Inc.