On Wed, 7 Aug 2013, Erik de Castro Lopo wrote:
In the second link above you wrote:
I have included both links since the discussion in November continued in December.
It would be a good opportunity to check how these packages use isLeft and isRight. E.g. if they use them in connection with fromLeft and fromRight then this would be an argument for me to exclude isLeft and isRight as well.
My main usage of isLeft/isRight is in HSpec tests where I write:
value1 `shouldSatisfy' isLeft value2 `shouldSatisfy' isRight
Yes, from what I have seen in some packages, testing seems to be the most sensible usage of 'isLeft' and 'isRight'. In other contexts 'isLeft' is usually combined with a non-total 'fromLeft', which is certainly a bad idea.
I would also be opposed to a fromLeft defined anything like fromJust:
Data.Maybe.fromJust :: Maybe a -> a
but would not be opposed to a fromLeft defined as John Wiegley suggested:
fromLeft :: a -> Either a b -> a fromLeft _ (Left x) = x fromLeft x _ = x
It looks indeed better than the non-total fromLeft. On the other hand the name would be inconsistent with fromJust, and the total fromLeft could be written as: fromLeft x = either (const x) id or using the proposed maybeLeft as fromLeft x = fromMaybe x . maybeLeft I don't know how often you need fromLeft.