isLeft/isRight (Was: leftToMaybe/rightToMaybe)

[amindfv at gmail.com]

El Aug 7, 2013, a las 4:26, Henning Thielemann <lemming at henning-thielemann.de> escribió:

> 
> 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.
> 

Another use, where I've defined isLeft in the past, is checking that a set of computations was successful, e.g. "if all isRight foo"

+1 to all of these (although I could take or leave mapEither)

-Tom

> 
>> 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.
> 
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