OK, just to prevent this getting side-tracked: I'm absolutely uninterested in the results of performActionA before determining if performActionB is permitted/possible/whatever. Think more in terms of security permissions or resource availability/claiming than in terms of chaining results. I want to know before I begin to collect the results of performAction* that I will actually stand a chance at getting results at all.
On Mon, 2007-25-06 at 10:58 +0200, peterv wrote: > I'm baffled. So using the Arrow abstraction (which I don't know yet) would > solve this problem? How can (perfectActionB x) be checked with without ever > executing performActionA which evaluates to x? This can only be done when x > is a constant expression no? > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Tomasz Zielonka > Sent: Monday, June 25, 2007 10:43 AM > To: Henning Thielemann > Cc: haskell-cafe@haskell.org > Subject: Re: [Haskell-cafe] Practical Haskell question. > > On Mon, Jun 25, 2007 at 10:29:14AM +0200, Henning Thielemann wrote: > > Imagine all performActions contain their checks somehow. Let > > performActionB take an argument. > > > > > do > > > x <- performActionA > > > y <- performActionB x > > > z <- performActionC > > > return $ calculateStuff x y z > > > > Now performActionB and its included check depend on x. That is, the check > > relies formally on the result of performActionA and thus check B must be > > performed after performActionA. > > IIUC, this limitation of Monads was one of the reasons why John Hughes > introduced the new Arrow abstraction. > > Best regards > Tomek > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -- Michael T. Richter <[EMAIL PROTECTED]> (GoogleTalk: [EMAIL PROTECTED]) I'm not schooled in the science of human factors, but I suspect surprise is not an element of a robust user interface. (Chip Rosenthal)
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