On Sat, Dec 3, 2011 at 10:55 AM, edgar klerks
Hi list,
I am using MonadSplit (from http://www.haskell.org/haskellwiki/New_monads/MonadSplit ) for a project and now I want to make a library out of it. This seems to be straightforward, but I got stuck when I tried to move miszero out of the class:
miszero :: m a -> Bool
It tests if the provided monad instance is empty. My naive attempt was:
You can write: miszero :: MonadPlus m => m a -> m Bool miszero m = (m >> return False) <|> return True but that will invoke any monadic effects as well as determining the nature of the value, which may not be what you want. Antoine
miszero :: (Eq (m a), MonadPlus m) => m a -> Bool miszero = ( == mzero )
This works, but not correctly. It adds an Eq constraint that is unneeded. I would prefer to have something like:
miszero :: MonadPlus m => m a -> Bool
Because I am not comparing the contents of the monad. I don't even touch it. Is this possible to write?
with kind regards,
Edgar
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