There is a related discussion, with a lot of pointers, in a recent D.Piponi blog post: http://sigfpe.blogspot.com/2007/04/homeland-security-threat-level- monad.html On 25/06/2007, at 10:58, 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: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] 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
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