On Wed, Mar 31, 2004 at 08:48:35AM +0200, Wolfgang Jeltsch wrote:
Now, as i think a little more about it, i believe what you want to do makes no sense. The monad operation '>>=' works on monads over *different* 'element' (i.e. argument) types (look at the type of '>>='). Your implementation only works if argument types are the same. I can't see how this can be generalized to different argument types even if both are instances of class Ord.
I disagree. AFAICS, his implementation also works with different element types. Am I overlooking something?
I think the real issue is that you can't restrict the types on which monad operates without modifying the Monad class. Think about this code: f :: Monad m => a -> m a f x = do return id return putStrLn return x It shouldn't be used in a Set monad, because it internally operates on uncomparable values, but the type signature doesn't reflect this fact. You can try to define a different version of Monad using multiparameter type classes, something like: class M m a b where (>>>=) :: m a -> (a -> m b) -> m b ... but it would complicate type signature contexts a lot, for example you would have (\a b c d -> a >>>= b >>>= c >>>= d) :: forall m a b b1 b2. (M m b1 b2, M m b b1, M m a b) => m a -> (a -> m b) -> (b -> m b1) -> (b1 -> m b2) -> m b2 instead of (\a b c d -> a >>= b >>= c >>= d) :: forall m a b b1 b2. (Monad m) => m a -> (a -> m b) -> (b -> m b1) -> (b1 -> m b2) -> m b2 Best regards, Tom -- .signature: Too many levels of symbolic links