Re: data-default WAS: [containers] Proposal: Change to the Data.Map Monoid

On 21.05.2014 18:11, Adam Bergmark wrote:

* I think it's useful to be explicit about the "default" value you are using. I'm unlikely to use `def' over `Set.empty'. Code is read more often than it's written.

I think Empty sounds like a better name. Some things have a clear "empty" value that will never change.

+1 "default list" does not immediately resolve to "empty list" in my mind. I understand "default value" as "resonable value in this situation" or "don't care", rather than the specific "zero" or "empty". On 21.05.2014 18:11, Adam Bergmark wrote:

As is, I don't find data-default to be useful, and decided to move away from it in fay for a number of reasons:

* You cannot add arguments to it because of the existing Default http://hackage.haskell.org/package/data-default-0.5.3/docs/Data-Default.html... r => Default http://hackage.haskell.org/package/data-default-0.5.3/docs/Data-Default.html... (e -> r) instance or run it in e.g. IO (via Chris Done) * The previous point means it's likely you'll have to scrap your instance at some point instead of just changing a type * You need to either re-export def or users need to depend on data-default themselves. Even if it was in base (but not in Prelude) they would need to add an additional import * The semantics needs to be defined for each instance * I don't see any use case for abstracting over different instances of Default * I think it's useful to be explicit about the "default" value you are using. I'm unlikely to use `def' over `Set.empty'. Code is read more often than it's written.

I think Empty sounds like a better name. Some things have a clear "empty" value that will never change. I don't object strongly to using Empty/Default for these.

On Wed, May 21, 2014 at 5:37 PM, João Cristóvão mailto:jmacristovao@gmail.com> wrote:

I stand corrected, he actually posted something on github this month.

Lucas, what would you think about merging generics support into Data.Default?

João

2014-05-21 16:31 GMT+01:00 João Cristóvão mailto:jmacristovao@gmail.com>:

I definitely see your point, and have suffered that myself, of course.

I just guess I did not want to get with the trouble of taking ownership of a package, after trying to contact Lucas Mai on February this year, with no response. His github repository shows no activity since October last year. https://github.com/mauke

But I do guess it is the right thing to do: if anyone knows Lucas and can contact him, to see if he stills wishes to maintain the package, I would very much appreciate that (I've also included the .cabal listed email in this conversation). If he is not interested or does not respond within a reasonable time frame, I propose myself as maintainer of the package (and will gladly accept suggestions).

Cheers João

2014-05-21 16:18 GMT+01:00 Michael Snoyman mailto:michael@snoyman.com>:

Starting a new thread to continuing discussing this (I should have done so in the first place).

Here's the problem: a class like Default is most useful if everyone's using the same class. The reason is that multiple libraries are all free to reexport the `def` symbol, and they'll all refer to the same identifier. If multiple `def`s exist, then this doesn't work, you need to be careful about *which* `def` you're using, and you may as well just export a default value under its own unique name.

I think data-default-generics is overall an improvement on data-default, but the very nature of a fork here makes both packages less useful. I'd much rather that data-default simply merged in data-default-generics.

On Wed, May 21, 2014 at 6:12 PM, João Cristóvão mailto:jmacristovao@gmail.com> wrote:

Sorry for continuing the off-topic, and for promoting a package of mine:

If you only need the class definition, http://hackage.haskell.org/package/data-default-class is a better choice, with no dependencies.

If you prefer a single package with all the dependencies (and with additional generics support), my own fork might be useful: http://hackage.haskell.org/package/data-default-generics

Although I mirrored the original package dependencies, many of which might actually be unnecessary... (given the generics implementation).

Cheers

2014-05-21 15:59 GMT+01:00 Michael Snoyman mailto:michael@snoyman.com>:

At the risk of veering terribly off-topic... the splitting of data-default into all of those packages caused me a bunch of dependency headaches, most of which I still don't fully comprehend. I miss the good ol' days of a single package.

On Wed, May 21, 2014 at 5:53 PM, Edward Kmett mailto:ekmett@gmail.com> wrote:

I find myself rather hesitant to recommend that instantiation of the idea ever since it exploded into a half-dozen packages full of orphan instances, but yes.

-Edward

On Wed, May 21, 2014 at 6:38 AM, Herbert Valerio Riedel mailto:hvr@gnu.org> wrote:

On 2014-05-21 at 00:23:44 +0200, Andreas Abel wrote: >>> _Lots_ of users initialize empty maps with mempty > > Well, this is another issue, empty should be overloaded via > > class Empty a where > empty :: a

Btw, isn't this what

http://hackage.haskell.org/package/data-default

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