Hi, thanks for your interesting ideas an inspiring answers. However I meant something a bit different. My point wasn't how to implement this problem with the current set of tools. Instead, I was wondering if it'd be worth expanding the current Reader/Writer library. Best regards, Petr 2012/10/18 Strake :

On 17/10/2012, Petr P wrote:

...

Hi,

(this is a literate Haskell post.)

lately I was playing with the Writer monad and it seems to me that it is too tightly coupled with monoids. Currently, MonadWriter makes the following assumptions:

(1) The written value can be read again later. (2) For that to be possible it has to be monoid so that multiple (or zero) values can be combined.

I fell say that this is a bit restricting. Sometimes, the written value can be lost - either used to compute something else or for example sent out using some IO action to a file, network etc. For example, I'd like to create an IO-based writer monad whose `tell` logs its argument somewhere - prints it, stores to a file etc.

No need:

newtype SequenceM m a = SequenceM (m a);

instance (Monad m, Monoid a) => Monoid (SequenceM m a) where { mempty = SequenceM (return mempty); SequenceM mx `mappend` SequenceM my = SequenceM (liftM2 mappend mx my); }

whatever :: (MonadWriter (SequenceM IO ()) m) => m (); whatever = tell (SequenceM (someIO :: IO ()));

Cheers, Strake

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