leftToMaybe/rightToMaybe

[Henning Thielemann]

On Thu, 8 Aug 2013, David Luposchainsky wrote:

> On 2013-08-07 19:13, Erik Hesselink wrote:
>>> mapEither :: (l -> l') -> (r -> r') -> Either l r -> Either l' r'
>>> mapEither fl fr = either (Left . fl) (Right . fr)
>>
>> This function is hidden in Control.Arrow as (+++).
>
> I didn't know about those functions, and it seems everything I wanted is
> in there somewhere. However, I'm not sure what to think of
> Control.Arrow, I've always (ab)used it as an extended Data.Tuple, and as
> of today probably for its Either API functions.

For me using the arrow functions as Data.Tuple and Data.Either utility 
functions is abuse. Control.Arrow is meant to generalize (->). If you call 
maybeLeft and MaybeRight this is certainly not your intention.