[Haskell-cafe] what is inverse of mzero and return?

[Daniel Fischer]

Am Dienstag, 25. Januar 2005 10:17 schrieben Sie:
> Or rather:
>          getChar :: (?x :: RealWorld) -> (Char,RealWorld)
>
> Which is the whole point of IO, no?  So yes, that's the essence.
>
> > if you insist on getChar being of
> > pure type IO Char, I still have a problem.
>
> Not if "pure type IO" means RealWorld -> (_,RealWorld)
>
Okay, so everything in an IO type has an implicit RealWorld parameter, right? 
In the above formulation, I meant 'pure' to exclude the RealWorld. 
If IO always includes RealWorld, we are d'accord.
> > Now, I'd say two values of type IO a are the same if (on execution)
> > 1. they return the same result,
> > 2. they have the same relevant side effects.
> >
> > I think, 1. should be acceptable to everybody, and 2. as a principle
> > too, only the question of which effects are relevant needs to be
> > answered.
>
> Well, why not use the same definition as for functions - quoted
> previously in this thread.  They must have the same domain and range,
> and for each value in the domain, they must give the same result.
>
> So for (f,g :: IO a), for these to be equal, they must produce the
> same value of type a if the state of the RealWorld is equal.
                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Quite my point.
Jules:
In your example, the state is explicitely mentioned, no reason for confusion 
there.
>
> "Relevance" isn't necessary for this definition, but I guess you could
> consider a dimension of the domain irrelevant if the resulting values
> of f and g is independent of it.
>
> > It's plain that not all measurable effects are relevant.
>
> I'm not sure.  If I can tell the difference from within my program, I
> would say that they were different -- even if the difference was
> "irrelevant" (e.g. one function taking more time than another).
>
> But as Eq isn't defined for IO actions, it's a metaphysical question;
> you can happily call two IO actions the same, even if you can measure
> a different number of reductions or whatever - for getChar you usually
> care about input and output, and not about pedantic resource use, so
> this is not "relevant" in that particular case.  (In a real-time
> setting, you would perhaps ha different criteria for relevance.)
>
> Just my opinion, anyway.

And I can agree with it.
>
> -kzm
D.