either combinator?

[chessai]

Just found myself wanting this:

either :: MonadGen m => m e -> m a -> m (Either e a)
either genE genA =
  sized $ \n ->
    frequency [
        (2, Left <$> genE)
      , (1 + fromIntegral n, Right <$> genA)
      ]

[moodmosaic]

Resolved by #382.

Thank you both @dcastro and @chessai.