Re: Proposal: add fromRight and fromLeft to Data.Either

21 May 2016

      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 mailto:david.feuer@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" mailto:ekmett@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
        mailto:anfelor@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
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