Of course it is not possible! Take a simple composition of reader and Maybe functors for an example:
miszero :: (b -> Maybe a) -> Bool I'm pretty sure (b -> Maybe a) for a is a MonadPlus, but you can't implement miszero for it. Arseniy. On 3 December 2011 16:55, edgar klerks <edgar.kle...@gmail.com> 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: > > 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 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe