Proposal to solve the `EitherT` problem.
R.Paterson at city.ac.uk
Mon Jun 17 02:32:29 CEST 2013
On Sun, Jun 16, 2013 at 02:52:46PM -0700, 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.
I'm in favour of this one. We can catch some of these uses by deprecating
the Error class, as the only predefined instances are for IOException
> * 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.
Those is at least a rationale for two StateT transformers with strict
and lazy instances. (Since the strict WriterT doesn't achieve its aim,
I'm in favour of deprecating it, and the strict RWST.)
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