On Wed, Aug 14, 2013 at 10:51:01AM -0400, Edward Kmett wrote:
I was trying to fire off one last shot across the bow that in the big 2.0 switch there was a move to make "State s" be "StateT s Identity" that was mostly argued for code reuse and simplification reasons, that it cut code duplication by a factor of 2 in the body of transformers and the mtl and reduced the chance for human error.
The fact that State s = StateT s Identity rather than merely being isomorphic seems to me to be an emergent property of this change, not its purpose.
I think there's still mileage in that argument. For example, the module Control.Error.Safe in the errors package has 13 functions of the form fooErr :: e -> args -> Either e r and another 13 of the form tryFoo :: Monad m => e -> args -> EitherT e m r If the recommended base exception monad were a specialization of the recommended transformer, only one set would be needed.