Joel Reymont
I don't think it will work for me either way as I'm returning m (Maybe Int) where m is my own monad. It seems that folks assumed that m itself was the maybe monad. Unless I'm mistaken the code below won't work otherwise.
There are two monads involved. The outer return injects into your m monad. That's all there is for your m. Then there is the inner stuff. Because the constructor of the inner expressions, your Maybes, is an instance of MonadPlus, you can use all the nice stuff there is for MonadPlus. I'd usually write it like this ... return $ do guard (cmdType cmd /= CmdSitError Serv) return seat_num In case the guard fails, you'll get back mzero (Nothing in your case). And then there is also mplus to handle alternatives ... -Matthias
On Nov 22, 2005, at 8:50 PM, Tomasz Zielonka wrote:
On Tue, Nov 22, 2005 at 10:15:15PM +0300, Bulat Ziganshin wrote:
return $ when (cmdType cmd /= CmdSitError Serv) (return seat_num)
must also work :)
But it won't. I have made this mistake too in the past ;-)
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
-- Matthias Neubauer | Universität Freiburg, Institut für Informatik | tel +49 761 203 8060 Georges-Köhler-Allee 79, 79110 Freiburg i. Br., Germany | fax +49 761 203 8052