On Tue, 22 Nov 2005, Matthias Neubauer wrote:

Arjan van IJzendoorn writes:

...

...

Is there a shorter way to write the if-then-else part below? if (cmdType cmd) /= (CmdSitError Server) then return $ Just seat_num else return Nothing

return $ if cmdType cmd /= CmdSitError Serv then Just seat_num else Nothing

return $ guard (cmdType cmd /= CmdSitError Serv) >> return seat_num

Because I often need it, I'm used to use my private function 'toMaybe' return $ toMaybe (cmdType cmd /= CmdSitError Serv) seat_num

On Tue, Nov 22, 2005 at 10:15:15PM +0300, Bulat Ziganshin wrote:

Tuesday, November 22, 2005, 9:17:57 PM, you wrote:

MN> return $ guard (cmdType cmd /= CmdSitError Serv) >> return seat_num

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 ;-) Best regards Tomasz

Joel Reymont writes:

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 ;-)

-- http://wagerlabs.com/

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-- 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

Bulat Ziganshin writes:

Hello Matthias,

Tuesday, November 22, 2005, 9:17:57 PM, you wrote:

MN> return $ guard (cmdType cmd /= CmdSitError Serv) >> return seat_num

return $ when (cmdType cmd /= CmdSitError Serv) (return seat_num)

must also work :)

Only if seat_num is of type () ... :-) -Matthias -- 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