As an application writer I'm a big fan of Edward's `either` package because his `EitherT` does not require an `Error` constraint. As a library writer, I am wary of heavy dependencies and I would prefer something more light-weight that I can depend on. Normally I would just directly contact Edward about reducing the number of dependencies but there was a previous discussion here about possibly merging his work into `transformers`, so I'm renewing that discussion and bringing up a few approaches for consideration. There are three approaches that I'd like to submit for consideration and receive feedback on: * Approach 1: Remove the `Error` constraint from `transformers` Disadvantage: This silently breaks all existing uses of `fail`. I don't know any way to add a compiler warning to notify downstream libraries of this change. * Approach 2: Add `EitherT` to `transformers` alongside `ErrorT` and have them both implement `MonadError`. Disadvantage: Bloat from having two almost identical monad transformers. However, there is precedent from duplicating `StateT`, `WriterT` and `RWST` for both strict and lazy instances. * Approach 3: Convince Edward to reduce the dependencies of the `either` package and submit the simplified version to the Haskell platform. Disadvantage: One extra import instead of just importing `transformers`. * Approach 4: None of the above. Disadvantage: Packages that depend on `either` have very low prospects of making it into the Haskell platform. If you have time, just take a moment to vote on which approach you favor or propose another alternative if you have another idea that I haven't listed.