Re: [Haskell-cafe] mtl: Why there is "Monoid w" constraint in the definition of class MonadWriter?

Am 08.12.2012 um 23:18 schrieb Edward Z. Yang:

Excerpts from Roman Cheplyaka's message of Sat Dec 08 14:00:52 -0800 2012:

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* Edward Z. Yang [2012-12-08 11:19:01-0800]

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The monoid instance is necessary to ensure adherence to the monad laws.

This doesn't make any sense to me. Are you sure you're talking about the MonadWriter class and not about the Writer monad?

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Now, it's possible what GP is actually asking about is more a question of encapsulation. Well, one answer is, "Well, just give the user specialized functions which do the appropriate wrapping/unwrapping"; another answer is, "if you let the user run a writer action and extract the resulting written value, then he can always reverse engineer the monoid instance out of it".

For deriving a monoid instance of w from monad (Writer w), you will need function execWriter:: Writer w a -> w, but in case of a general instance of (MonadWriter w m) you would have to use function listen :: m a -> m (a, w) that will only provide you a value of type (m w), but not of type w. Therefore, I'm not yet convinced that every instance of (MonadWriter w m) gives rise to a monoid instance of w. Regards, Holger