Michael Hobbs wrote:
> > Consider this:
> > > type IO a = StateOfUniverse -> (a, StateOfUniverse)
> > > -- Not syntactically correct, but you know what I mean.
> >
> > So anything that is declared, say `IO Int', means that it is actually a
> > function that reads in the state of the universe, potentially modifies
> > it, and then returns an Int value along with the new state of the
> > universe. The interesting thing to note is that the state of the
> > universe never changes between calls that are strung together using the
> > `>>=' operator. That is, the StateOfUniverse that is returned by the
> > first monad is exactly same state that is fed into the second. Whether
> > or not you want to call this "referentially transparent", well I guess
> > that's up to your own philosophic bias.
>
> I rescind the statement that "the state of the universe never changes
> between calls that are strung together using the `>>=' operator". After
> further consideration, I believe that that's incorrect.
Unless, of course, you consider StateOfUniverse to encapsulate all past,
present, and future events (a 4-dimensional value). In which case, you
don't need to return a new StateOfUniverse, since it will be exactly the
same as the one given, except maybe with different `currentTime' value.
But I'm getting way too deep here.
The problem is a function like `getChar' that is declared `IO Char'. If
the user has not typed a character when this monad is invoked, it will
sit and wait for the event. That is, the current StateOfUniverse that is
passed to getChar has absolutely nothing in it to indicate what
character will be returned, unless it also contains future events.
However, if we define `getChar' like this, we might get around the nasty
issue of future events:
> getChar = do
> c <- peekKbdBuffer :: IO [Char] -- length of 0 or 1
> if null c then getChar else return head c
In this case, `getChar' will continue looping until StateOfUniverse
changes such that the keyboard buffer actually has a value in it. Of
course, this means that the StateOfUniverse must be able to alter itself
somehow between the function calls.
Philosophy Thursday,
- Michael Hobbs
