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

Markus Läll markus.l2ll at
Sat May 24 21:11:25 UTC 2014

For me, the default makes sense, too, in the very specialised cases (in the
world of things that have types) like settings of a web server or cvs
parser, where the application author defined it for to quickly launch some
program or function. The 'default' there would mean 'something sensible',
the meaning of which I don't yet care about. Probably the author of the
package is also a frequent user of the default to test the thing he is

So I would be +1 for adding a class to the base (as many would otherwise
re-implement it anyway), but with no instances. The module could also state
the policy behind the class, i.e "don't write defaults for widely used
types/types that you didn't create yourself" or whatever else people agree

On Thu, May 22, 2014 at 10:08 PM, John Meacham <john at> wrote:

> The issue i have is not the concept, I have actually written similar
> classes in a few situations, the issue is that there is little value
> in a shared implementation of it. There is no underlying meaning to
> 'default' that crosses project boundries. seeing 'Default a => blah '
> in a type signature doesn't convey any useful information about a
> other than at some point, someone somewhere added a to a list based on
> their current whim.
> That said, I have written the class a few times, and that is fine,
> just because the code looks the same it doesn't necesarily mean it is
> conceptually reusable  because they had a specific meaning. looking at
> my codebase I find
> PerlDefault  - what the value would default to if 'undef' in perl.
> used when generating perl code.
> HsSynDefault  - haskell syntax default, bits of haskell syntax (HsExp,
> HsDecl, etc..) with the annotations set to default leaving only the
> source bits to fill in.
> But both have defined meanings and when I see them in a type
> signature, it conveys useful information.
> --
> John Meacham -
> _______________________________________________
> Libraries mailing list
> Libraries at

Markus Läll
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