Re: Proposal: merge either into transformers

On Sat, Apr 26, 2014 at 11:28 AM, Ross Paterson wrote:

On Sat, Apr 26, 2014 at 09:16:57AM +0300, Roman Cheplyaka wrote:

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* Ross Paterson [2014-04-26 01:21:09+0100]

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Instead of EitherT, the next version will deprecate ErrorT in favour of a transformer ExceptT with base monad Except. The idea is to have analogous transformers and monads

ExceptT : Except : Either WriterT : Writer : (,) ReaderT : Reader : (->)

Great! When do you plan to make a release?

Let's say Wednesday, if no-one notices a major problem.

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Pre-release docs are here:

http://code.haskell.org/~ross/transformers/dist/doc/html/transformers/

The docs for Control.Monad.Trans.Writer.Strict say:

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Although the output is built strictly, it is not possible to achieve linear space behaviour with this transformer: for that, use Control.Monad.Trans.State.Strict instead.

I think it should be "constant", not "linear".

Indeed -- thanks for catching that. I wonder whether the strict writer is of any use at all.

I think it's worth resurrecting Gabriel's proposed modification to have the strict writer transformer exposed as an abstract type, built on top of StateT (or using the same implementation as StateT). I've been bitten by the laziness of strict Writer in the past, and thanks to Gabriel's email, I knew how to solve the problem. But I think many people will be misled by the name, documentation improvements notwithstanding. *If* it's exposed as an abstract type, I'd also like to propose that a .Internal module be available for exposing the constructor, as some libraries will have legitimate need of looking at the constructor. Michael