On Sat, Dec 28, 2013 at 04:13:52PM +0000, Tom Ellis wrote:
On Sat, Dec 28, 2013 at 03:58:10PM +0000, Tom Ellis wrote:
On Sat, Dec 28, 2013 at 04:47:40PM +0100, Hans Höglund wrote:
Recently I have found myself using these two combinators a lot: http://lpaste.net/97643
FYI it looks a lot like these give rise to a monad transformer.
Specifically, the following. Careful though: I haven't made any effort to check this satisfies the monad or transformer laws.
In fact, since is is 'n' that requires the 'Monad' constraint, I suspect you'll need to swap the order of the type arguments to get a monad transformer: data Wrap n m a = Wrap (m (n a)) Anyway, my main point remains: your first check should be whether what you have can be captured as a genuine monad.
import Control.Monad.Trans (MonadTrans, lift) import Control.Monad (join) import qualified Data.Traversable as T import Data.Traversable (Traversable)
data Wrap m n a = Wrap (m (n a))
unwrap :: Wrap m n a -> m (n a) unwrap (Wrap m) = m
mbind :: (Monad m, Monad n, Functor m, Traversable n) => (a -> m (n b)) -> m (n a) -> m (n b) mbind = (join .) . fmap . (fmap join .) . T.mapM
instance (Functor m, Traversable n, Monad m, Monad n) => Monad (Wrap m n) where return = Wrap . return . return m >>= f = Wrap (mbind (unwrap . f) (unwrap m))
instance Monad m => MonadTrans (Wrap m) where lift = Wrap . return _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe