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

Wed May 21 18:46:09 UTC 2014

I honestly don't feel very strongly about usage of Default. What I've found
it useful for is a very narrow category: a function which takes a
configuration parameter that users likely never want to modify. For
example: xml-conduit has various parsing and rendering settings, but most
use cases are good with the default. There's a cognitive overhead for a
user to remember the default parameter name (is it defaultXMLSettings?
defaultXmlSettings? defParseSettings?).

On Wed, May 21, 2014 at 7:11 PM, Adam Bergmark <adam at bergmark.nl> 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#t:Default> r
> => Default<http://hackage.haskell.org/package/data-default-0.5.3/docs/Data-Default.html#t:Default> (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 <jmacristovao at 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 <jmacristovao at 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 <michael at 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 <jmacristovao at 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 <michael at 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 <ekmett at 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 <hvr at 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
>>>>>>>>
>>>>>>>> provides?
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>>>>>>>
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>>>>>>
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>>>>>
>>>>
>>>
>>
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>
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