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Today's Topics:
1. Re: Monad transformers and MonadPlus (Daniel Fischer)
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Message: 1
Date: Fri, 30 Jul 2010 14:48:17 +0200
From: Daniel Fischer <daniel.is.fisc...@web.de>
Subject: Re: [Haskell-beginners] Monad transformers and MonadPlus
To: beginners@haskell.org
Message-ID: <201007301448.17738.daniel.is.fisc...@web.de>
Content-Type: text/plain; charset="utf-8"
On Friday 30 July 2010 13:56:44, Antoine Latter wrote:
> You could always add a MondPlus instance to IO directly (or a newtype
> wrapper).
>
> 'mplus' would set up exception handling and 'mzero' would through an
> exception. You might want to limit it to a subset of exceptions, though.
Once upon a time, there was a MonadPlus instance for IO.
However, one of the laws a MonadPlus instance should satisfy is
m >> mzero === mzero
You can't have that in IO, since there's no unPutStrLn and such.
Therefore, the MonadPlus instance for IO was removed.
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