sequenceRights.hs

-- Sequence a list of Either-yielding monadic actions.
-- Sequencing stops on the first Left value, or yields a list successful Right values.
sequenceRights :: Monad m => [m (Either e t)] -> m (Either e [t])
sequenceRights [] = pure (pure [])
sequenceRights (x:xs) = handle =<< x
  where
    handle (Left e)  = pure (Left e)
    handle (Right t) = (t:) <<$>> sequenceRights xs


-- This should be in a standard library somewhere.
(<<$>>) :: (Functor f, Functor g) => (a -> b) -> f (g a) -> f (g b)
(<<$>>) = fmap . fmap

Tangentially-related Gist: https://gist.github.com/jml/c308c351d2ccf39d03a421370d7e48df