isLeft/isRight (Was: leftToMaybe/rightToMaybe)

Henning Thielemann lemming at
Wed Aug 7 10:26:03 CEST 2013

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 

>> 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 

> 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.

More information about the Libraries mailing list