Re: Pre-Proposal: Add Validation somewhere easily accessible

Joseph, your idiom seems indeed fundamental to some extend and seems to be rediscovered time and again. (My PhD student showed a version of it to me very recently.) I also found that it generalizes the existing "Failing a" in Control.Applicative.Error http://hackage.haskell.org/package/applicative-extras-0.1.3/docs/Control-App... Control.Applicative.Error is the place where it should go, whether you generalize the existing package or roll your own... Bikeshedding: "Failing" sounds more fundamental and generic than "Validation", so I'd prefer the former. However, "Failing" is too negative, as if the computation must fail, "CanFail" is less definite. UPDATE: I found that "Errors e" on https://hackage.haskell.org/package/transformers-0.4.1.0/docs/Control-Applic... seems to be doing the job of "Validation" already (no nice names of the operations, though). Cheers, Andreas On 22.09.2014 17:11, Chris Allen wrote:

I was already working on adding it to errors because of EitherT - should it go into either instead?

Sent from my iPhone

On Sep 22, 2014, at 9:49 AM, Joseph Abrahamson mailto:me@jspha.com> wrote:

...

I was just thinking about this opportunity last night. I think it is a fairly appropriate place for Validation and lets `errors` include the type naturally. Further, Michael is right in that so long as the package dependency is above `semigroups` then it’d be more accurate to state the instance using Semigroup e. I’d personally be in favor of that as well.

I’d be interested in entertaining a small amount of bikeshedding over the name as well. I wrote this up using Validation to mime the `validation` package, but have personally used a number of names in the past. Other potential options include: Result, Collect, and Combine.

I propose sticking with Validation (and the implementation in the head of this thread, potentially modulo [Monoid\Semigroup]) by default but would like to hear if anyone has strong justification for another name.

Joseph

On Mon, Sep 22, 2014 at 10:41 AM, Michael Snoyman mailto:michael@snoyman.com> wrote:

FWIW, my neither package[1] (deprecated in favor of your either package) does in fact define this Applicative Either datatype (though under a different name). IMO, either is the right place for this.

One minor bikeshedding question: it seems like the Applicative instance doesn't require a Monoid constraint, but rather only a Semigroup constraint. Since semigroup is already a dependency of either, would it make sense to define the instance as such?

[1] http://hackage.haskell.org/package/neither-0.3.1.1/docs/Data-Neither.html

On Mon, Sep 22, 2014 at 5:38 PM, Edward Kmett mailto:ekmett@gmail.com> wrote:

Another option is to consider sliding it up into the 'either' package which is a dependency of errors. I'd be willing to help out there.

-Edward

On Sun, Sep 21, 2014 at 4:56 PM, Joseph Abrahamson mailto:me@jspha.com> wrote:

I think errors is a good place to begin a process like what Ed suggests. I do think that, ultimately, it’s a more pervasively useful type than that—just to leave that on the record.

Joseph

On Sun, Sep 21, 2014 at 2:05 PM, Greg Weber mailto:greg@gregweber.info> wrote:

The errors package seems like a good location for it.

On Sun, Sep 21, 2014 at 10:51 AM, Joseph Abrahamson mailto:me@jspha.com> wrote:

I think that's my feeling about the risk of placing it in such a prominent place as transformers as well. I feel that such a core location is not necessarily needed, but the functionality provided would not be difficult to standardize relatively quickly.

The benefit is to make the technique more widely known and more easily included. Typically its use tosh involves the addition of either an extra one-off package which provides little extra and more deps (this is the case today with `validation` especially given the lens/profunctors dep but even without) or nestled deep inside another package with a different focus.

So I feel the proper place for such a simple type is somewhere inside a toolkit of other commonly used types of its brand. If there were a good `applicatives` package that would be ideal. I mentioned transformers as nothing more or less than the nearest approximation to that which I know of.

Joseph

On Sun, Sep 21, 2014 at 1:17 PM, Edward Kmett mailto:ekmett@gmail.com> wrote:

I suppose ultimately the question I'd have is what is gained from standardization here per se?

I can see the merit of the authors of those libraries coming together to agree on a common API and type if they can, but I think I'd rather see that sort of consolidation out in package space than in a package that falls under the purview of the libraries@ process -- and if they can't come together that too says something.

Tying it to transformers in particular would tie changes to a couple year long dev cycle. It is a ghc build package, and hence not easily upgraded for users. That point in the design space strikes me as even worse than the status quo.

Once such a package exists and it sees wide adoption, then the discussion of whether it belongs in the platform seems to make sense.

-Edward

> On Sep 21, 2014, at 1:05 PM, "Joseph Abrahamson" mailto:me@jspha.com> wrote: > > The “purely applicative Either” is Either with a new Applicative interface which can no longer take a corresponding Monad interface. I have personally reinvented it over and over although it is perhaps commonly known as Validation: > > data Validation e a = Invalid e | Valid a deriving ( Show, Functor, … ) > > instance Monoid e => Applicative (Validation e) where > pure = Valid > Invalid e1 <*> Invalid e2 = Invalid (e1 <> e2) > Invalid e1 <*> _ = Invalid e1 > <*> Invalid e2 = Invalid e2 > Valid f <*> Valid a = Valid (f a) > > -- No corresponding Monad > > It could be perhaps better implemented as a newtype wrapper over Either to facilitate rapid conversion to the more common type. > > Validation appears in its own package developed by Tony Morris and Nick Partridge. It also exists in Jonathan Sterling’s Vinyl package under the name Data.Vinyl.Idiom.Validation.Result. Gabriel Gonzalez has also told me that he’d be happy to add it to his `errors` package if a PR was made. > > The common use for Validation is to traverse over a structure and accumulate errors throughout it. In other words, failures should not halt the process and we can think of each operation as occurring in parallel. > > Not only is Validation a good example of an “Applicative without a Monad” its also a very useful practical type. > > I would like to build a proposal to include Validation in some form in a common package. Despite the misalignment with the name, `transformers` seems like a plausible target, though I fear changes to that package are rarely worth the pain. Suggestions on implementation and target package are welcome. Pending informative feedback, I will try to write a true proposal within 2 weeks. > > Thanks everyone, > > Joseph > > Joseph > _______________________________________________ > Libraries mailing list > Libraries@haskell.org mailto:Libraries@haskell.org > http://www.haskell.org/mailman/listinfo/libraries

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