[Haskell-beginners] Re: clarification on IO
dev at mobileink.com
Mon Mar 2 11:26:38 EST 2009
I can tell I'm talking to a kindred spirit - we oughta be able to describe
all this stuff in plain, simple, clear English. It's a great challenge for
a prose writer.
On Mon, Mar 2, 2009 at 9:21 AM, Will Ness <will_n48 at yahoo.com> wrote:
> >Maybe antilog or prelog or future trace or the like. In any case, I think
> that's useful for explaining lazy evaluation, but it's not directly
> by monad semantics. IOW monad semantics and evaluation strategy are
> I don't follow. Monad semantics in general is to chain together its action-
> functions (:: a -> M b). IO monad's semantics is that it promises to
> the recorded requests, if called upon. Not only is it directly implicated
> its semantics, it IS its semantics. It is what IO-bind is. Other monad's
> will mean something else.
Right, but this is //Haskell// monad semantics. It's an artifact of lazy
evaluation. Referring back to the mathematical definition of monad, there's
no evaluation process or promise, only denotation.
> To recapture, the only difference IO monad has, is that it refers
> EXPLICITLY to
> a compiler, and its runtime execution operations. But from inside Haskell
> just anther monad (I've said that already haven't I? :) ).
Mmm, I wouldn't say explicitly. Again, this is because of lazy evaluation,
not because of the nature of monads. They just put stuff in order,
regardless of evaluation strategy.
> That's why I propose to call these bind-chainable functions action
> not just actions, and actual I/O acivities to call just that, activity.
> notice I write I/O there where it belongs to the actual world action (...
> terminology really MUST be refined here!). I think anything else is
Agreed, that's the key distinction.
> I'm open for another suggestion for how to name these "action-functions",
> it's time the definite name is finalized; it's very confusing to see these
> action-functions referred to, in all these tutorials, as performing real
> I/O actions. They don't, of course.
Alas, this is where taste enters into it. You can come up with the perfect
set of names and somebody won't like it. ;) My own preference is to
dispense with the action/function idiom and just say that IO //terms// are
essentially unbound (but typed) variables, that will be bound to values when
interpreted at runtime. So I wouldn't even call "getChar" an action; I'd
just say it's a term denoting an IO Char value to be provided later. In
contrast with a constant term like '3'.
> Better to avoid "evaluation" altogether (that seems to be your another idea
> from that log, is it?).
Yep. Hate evaluation. ;)
> I think the KEY is to always keep a clear separation of what is inside the
> Haskell world, and what is executed by its run-time, as an imperative
> iving inside the real volatile world (capable of calling back into
Yeah, I don't recall many tutorials zeroing in on this. I think it's very
> And when inside the pure Haskell world, IO monad is ABSOLUTELY in NO
> RESPECT no
> different than any other. The operational log metaphor help keep this part
> its semantics clear, from the other part - the fact that its operational
> will actually get executed by run-time.
> >> > Correction: special name for IO "functions" (actually "IO terms"
> would be
> >> > better).
> >> Why? They are just fuctions, of type (Monad m => a -> m b). What I'm
> >> they are of special type, chainable by the M monad, so it seems logical
> >> have
> >> a special name for such M-chainable functions, e.g. "M-action functions"
> >> (whatever the M).
> > Technically they cannot be functions - there's no "same input, same
> (at least not for input operations).
> Yes there is. There's the whole point I'm driving at. We are not performing
> computation with our code. We describe the computation that will be
> Our values are functions. The usage of actual input is deferred to the
What's the domain of "getChar"? We can't mess with the mathematical
definition of function, so we really can't use it for IO stuff (or any
non-deterministic value, e.g. random).
To quibble yet more: there's no computation involved in IO, strictly
speaking, since it's analog. A Turing machine (as he originally described
it) can't do IO.
> It's just like Show functions that (will) add their output onto a hidden
> parameter, the string-being-built (when called). Same here, with the
> built. Assentially, we're dealing here with the delayed application of
> functions, that's all.
> Here's again a simple outline of how an IO monad might look like, inside
> Haskell. It helped clarify things for me (dealing with output only, but
> data IO a = IORec -> (a,IORec)
> -- building the record of I/O activities to be performed
> instance Monad IO where
> return a rec = (a,rec) -- return :: a -> IO a
> (m »= g) rec = uncurry g $ m rec -- g :: a -> IO b
> putStrLn :: a -> IO ()
> putStrLn a rec = ((),rec ++ [("putStrLn", a)])
> > No referential transparency. That's the problem.
> Everything is referentially transparent. There are no side effects in
> You know that. :) You wrote as much yourself (assuming you're the author of
> that blog).
> IO value describes the computation that
> WILL BE performed OUTSIDE of Haskell.
> That is a statement that is easy to understand, and is not at all
> confusing. I
But also logically inconsistent: how can an expression "inside" of Haskell
refer to something outside of Haskell? More specifically, Haskell
expressions can only denote values in the Haskell semantic universe. IO
processes (not computations) lie outside of that universe, so Haskell cannot
say anything about them. But the //result// of an IO process is a value
within the semantic universe, so it can be referenced.
>> > This was a big problem for me; I find terms
>> like "action", "computation", "function" completely misleading for IO
>> Why? A function of type (a -> M b) is a function that returns a value,
>> b), tagged with some monadic hidden data. In case of IO, it is a promise
>> perform some actual I/O that's passed around, hidden. But the M-action
>> itself is just a regular Haskell function. It can be defined elsewhere,
> But the "promise to perform" is a matter of evaluation semantics, not
denotational semantics. Denotationally these things cannot be functions.
But they are. They describe future computation to be performed outside of
> Haskell. Their values - inside Haskell - are one and the same - it's (:: IO
> entities. Which encapsulate the record, the
> _sceleton_of_future_computation_to_be_performed, which is ONE and only.
> just that it has holes in it, where the actual values will go into. It's
> back into Prolog with its yet-unassigned variables. Or to any imperative
> language with set-once.
The whole future/promise thing comes from lazy evaluation. With strict
evaluation, there would be no such promise; expressions would be evaluated
(reduced) on the spot, so there would be no log of promised execution.
Language semantics (denotational) and evaluation strategy (operational?) are
orthogonal. Evaluation strategy doesn't change the meaning (denotation) of
the program, but it does affect its execution profile - memory consumption,
etc. - so programmers have to think about it. Except of course it does
change the behavior of the program where IO is concerned. In a lazy
language you can write IO expressions that will never get
evaluate/performed, but not so in a strict language. But behavior and
meaning are different things.
Take another example: the strict application operator '$!'. It doesn't
change the denotation of a program but it does change its behavior, by which
I mean the interpretational process. Such operators don't denote, really;
they're more like meta-syntax or pragmas than Haskell syntax.
> >Denotationally, all a monad does is ensure sequencing, which is necessary
> properly order the (non-deterministic) IO values.
> No it does more than that. It ascribes actual meaning to what its M-action-
> functions mean, and it defines what it means for them to be combined in a
> chain. They are of course kept in sequence, in that chain.
Ok, then for the IO monad all it does is ensure sequencing. The behavior of
getChar comes from its implementation, not from the monad it is wrapped in.
Remember GHC's implementation of IO as a state transformer is not the only
> >With lazy eval this gets translated into the building of a "future log"
> Right, only better not to use "eval" - ever. Haskell has expressions which
> reduced; values belong to its runtime system. They are OUTSIDE of Haskell
> We do not "evaluate" anything. It would be an imperative. :)
We're probably stuck with it, practically speaking, but where extra clarity
is needed I suggest "reduction" instead of "evaluation", from the lambda
> > Thanks,-gregg
> Thank you, for a great and enlightening discussion.
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Beginners