Inject Either into a MaybeT (Writer e)

CollectLeft2.hs

import Data.Monoid
import Control.Monad
import Control.Applicative
import Control.Error
import Control.Monad.Trans.Class
import Control.Monad.Trans.Writer

collectEither :: (Monoid (es a), Applicative es) => Either a b -> MaybeT (Writer (es a)) b
collectEither (Left e) = lift (tell (pure e)) >> mzero
collectEither (Right a) = return a

runCollectLeft :: MaybeT (Writer es) b -> Either es b
runCollectLeft maybeWriter =
	case runWriter (runMaybeT maybeWriter) of
		(Just x, _) -> Right x
		(Nothing, es) -> Left es