Proposal: add fromRight and fromLeft to Data.Either

[Andreas Abel]

Ah, yes, we want fromLeft and fromRight, but with types

   fromLeft  :: a -> Either a b -> a
   fromRight :: b -> Either a b -> b

where the first argument is the "default", like

   fromLeft a (Right b) = a

Then you can write

   fromLeft impossible e

where `impossible` does a controlled crash of your program.  (See 
fromMaybe.)

--Andreas

On 21.05.2016 15:16, Edward Kmett wrote:
> Sorry, I meant fromJust.
>
> On Sat, May 21, 2016 at 8:58 AM, David Feuer <david.feuer at gmail.com
> <mailto:david.feuer at gmail.com>> wrote:
>
>     No one wants to remove fromMaybe. In fact, it could serve as a
>     template for total fromLeft and fromRight:
>
>     fromLeft :: b -> (a -> b) -> Either a x -> b
>     fromRight :: b -> (a -> b) -> Either x a -> b
>
>     On May 21, 2016 8:55 AM, "Edward Kmett" <ekmett at gmail.com
>     <mailto:ekmett at gmail.com>> wrote:
>
>         I'm personally -1 on this.
>
>         A large portion of the community is vehemently against adding
>         new partial functions, and they have to share base with the rest
>         of us. I'd say more people want to remove fromMaybe, head, tail,
>         etc. than want to double down on the pattern at this time.
>
>         -Edward
>
>         On Fri, May 20, 2016 at 4:08 PM, Anton Felix Lorenzen
>         <anfelor at posteo.de <mailto:anfelor at posteo.de>> wrote:
>
>             When working with Either,
>             I am often missing two simple functions:
>             fromRight :: Either a b -> b
>             fromLeft  :: Either a b -> a
>
>             It has been implemented a couple of times:
>             http://hayoo.fh-wedel.de/?query=fromRight
>
>             But I don't want to depend on yet another library
>             for such a basic function.
>
>             Could it be added?
>
>             Anton
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-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www2.tcs.ifi.lmu.de/~abel/