On Sun, 16 Jun 2013, Gabriel Gonzalez wrote:
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.
I prefer this one. Additionally one might deprecate ErrorT. Btw. MaybeT also went into transformers although there was already a MaybeT package.
* 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.