On Tue, Aug 5, 2008 at 2:19 PM, Henning Thielemann wrote:

The Functor instance of ReaderT has the header: Monad m => Functor (ReaderT r m)

I thought (Functor m) must be the appropriate constraint.

Note that there *is* an advantage to having Monad as the context: not every monad implements Functor, even though it could. So having Functor as the context isn't *trivially* superior. Taking this into consideration, and taking into consideration that ReaderT is usually thought of as related to monads (e.g., it's in Control.Monad.Reader), I'm not sure that the preferred way of transforming a functor wouldn't be (r ->) :.: f where :.: is functor composition. But then, functor composition apparently isn't actually in the library, which surprised me (it's in the applicative functor paper, but not in Control.Applicative), so maybe changing ReaderT to be usable with functors is more warranted. Or perhaps adding functor composition is the way to go? All the best, - Benja