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

I would add (as I probably did last time something like this came up) that it's difficult to get excited about all these functions named by a systematic grammar, when we know _today_ how to just implement that grammar as a library of composable pieces. And it has been implemented in lens (and possibly other libraries that are subsets of lens). Then you can have `is`, `over` (like map), `from` (take your pick on whether it's partial or with-default), `foldOf`, `traverseOf`, ..., and `_Left`, `_Right`, `_Just`, `_Nothing` (not all of these correspond to lens; just you could define them). These building blocks make all these functions definable, with almost exactly the names they'd be given as one-off functions in several modules. But of course, they also compose in more interesting ways, too. You can have `from (_Left . _Just)`, but we will never be adding a `fromLeftAndJust` function to base. I don't really expect (or hope for) base to start pulling in big chunks of lens. And maybe it makes sense for all of these to be in base just for uniformity. But it also seems like people could just be using something better. -- Dan On Sat, May 21, 2016 at 8:55 AM, Edward Kmett 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 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 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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