On Sun, Jun 5, 2011 at 12:51 PM, KC wrote:

If new intermediate classes crop up then there would be no point in fixing

class (Applicative m) => Monad m where

since it would have to be changed if new intermediate classes are found.

You might check out a few articles regarding Kleisli arrows [1][2] for possibilities that live between applicative and monad. Applicative itself is also a little on the strong side. I had to reject Applicative for one model of signal transformers because 'pure' was not a legal constructor, even though 'fmap . const' and '<*>' were okay. And even Functor is too strong if you want effective linearity. I've found Adam Megacz's Generalized Arrows [3] to be a suitable chassis for weaker models. [1] http://www.haskell.org/haskellwiki/Arrow_tutorial#Kleisli_Arrows [2] http://lambda-the-ultimate.org/node/4273 [3] http://www.cs.berkeley.edu/~megacz/garrows/