hub.darcs.net :: ross -> transformers -> issue

thumphries

Hi Ross,

I occasionally use Errors to accumulate unrelated errors in an Either-heavy program (where the Monad is required). It feels like hoistErrors is missing from the API; it would be nice to use a type like this without learning about the internals of Lift and Constant.

Curious of your opinions on including this in Control.Applicative.Lift.

I also include sequenceErrors for your consideration; this has been the most useful application of Errors for me.

hoistErrors :: Either e a -> Errors e a
hoistErrors e =
  case e of
    Left es ->
      Other (Constant es)

    Right a ->
      Pure a

-- | Like 'sequence', but accumulating all errors in case of a 'Left'.
sequenceErrors :: (Monoid e, Traversable f) => f (Either e a) -> Either e (f a)
sequenceErrors =
  runErrors . traverse hoistErrors

Thanks for maintaining transformers! I use it every day.

• Tim