On 08/15/2013 07:50 AM, Ross Paterson wrote:
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.
Like I said before, you can always define: type Except e = EitherT e Identity That way people who want to ignore the base monad can use `Except`. However, there is no way that I'm getting rid of the `Either` functions from `errors` even if `transformers` has `Except`. `Either` is here to stay, and people need those `Either` functions. Don't get me wrong: I use `transformers` exclusively for effects, to a fault even. However, I think `Either` is more fundamental than `transformers` and that there is no way an `Identity`-specialized `EitherT` will ever supplant the use of `Either`.
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