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
