Re: [Haskell-cafe] ismzero operator possible without equal constraint

On Sat, Dec 3, 2011 at 3:55 PM, Antoine Latter wrote:

On Sat, Dec 3, 2011 at 10:55 AM, edgar klerks wrote:

...

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.

It's almost certainly not what you want for the list monad. -- Dave Menendez http://www.eyrie.org/~zednenem/