Proposal: merge either into transformers

30 Apr 2014

      Hello, Ross

I discovered mplus of ExceptT doesn't call mappend to accumulate
error states which is different from origitnal EitherT like below.
I suppose this EitherT semantics is more useful than fixed adoption
of last error state.
(For example, Last Monoid is pre-defined in base)

ExceptT
...
instance (Monad m, Monoid e) => MonadPlus (ExceptT e m) where
   mzero = ExceptT $ return (Left mempty)
   ExceptT m `mplus` ExceptT n = ExceptT $ do
       a <- m
       case a of
           Left _ -> n                  -- throw left error away
           Right x -> return (Right x)
EitherT
...
instance (Monad m, Monoid e) => Alternative (EitherT e m) where
 EitherT m <|> EitherT n = EitherT $ m >>= \a -> case a of
   Left l -> liftM (\b -> case b of
     Left l' -> Left (mappend l l')     -- mappend error states
     Right r -> Right r) n
   Right r -> return (Right r)
empty = EitherT $ return (Left mempty)
From: R.Paterson at city.ac.uk (Ross Paterson)
Subject: Proposal: merge either into transformers
Date: Sat, 26 Apr 2014 01:21:09 +0100
...
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 : (->)
Other changes are:
* Added infixr 9 `Compose` to match (.)
        * Added Eq, Ord, Read and Show instances where possible
        * Replaced record syntax for newtypes with separate inverse functions
        * Added delimited continuation functions to ContT
        * Added instance Alternative IO to ErrorT
Pre-release docs are here:
http://code.haskell.org/~ross/transformers/dist/doc/html/transformers/
--
Kei Hibino
ex8k.hibino at gmail.com
https://github.com/khibino/