How does this proposal compare to https://github.com/abarbu/ghc-proposals/blob/master/proposals/0000-defaulting-plugins.rst ? That alternative proposal was ruled "out of scope", because it describes a plugin interface, which is out of scope of this committee. But I think it's a viable alternative to Mario's proposal here, and appears to be more powerful (although more difficult to use). I'm worried that the "out of scope" label derailed the plugins proposal, as I do not believe that further work on that idea (even though it's already implemented!) has been pursued.

Richard

On May 18, 2021, at 10:11 PM, Eric Seidel <eric@seidel.io> wrote:

Hi all,

Mario has proposed a handful of language extensions around type defaulting.

1. NamedDefaults: this extension simply allows specifying the class to default, instead of it always being Num (or a handful of other hardcoded classes if one enables ExtendedDefaultRules). The rule only applies to the current module, as usual.

2. ExportedDefaults: this extension allows exporting defaulting rules.

3. ImportedDefaults: this extension makes import declarations pull in defaulting rules implicitly, like class instances.

Extensions (2) and (3) work together to provide a mechanism for sharing sets of defaulting rules across modules. It is possible to import conflicting sets of defaulting rules from different modules, in that case the conflict must be resolved manually by the importing module, with a new defaulting rule.

My recommendation is that we

* Accept extension (1), as it is a clear improvement over the status quo and can stand on its own.

* Reject (without prejudice) extensions (2) and (3). These extensions bring considerable extra complexity and another orphan-like mechanism. There's an open question here of whether defaulting rules should be globally coherent like type classes, or if they're something different; the discussion has arguments for both sides. I'm not sure, and so I recommend we don't commit ourselves one way or the other for now.

Please take a look at the proposal.

Discussion: https://github.com/ghc-proposals/ghc-proposals/pull/409
Proposal: https://github.com/blamario/ghc-proposals/blob/exportable-named-default/proposals/0000-exportable-named-default.rst

Eric

On Sun, Apr 4, 2021, at 06:34, Joachim Breitner wrote:
Dear Committe,

Exportable named defaults
has been proposed by Mario
https://github.com/ghc-proposals/ghc-proposals/pull/409
https://github.com/blamario/ghc-proposals/blob/exportable-named-default/proposals/0000-exportable-named-default.rst

I propose Eric as the Shepherd.

This did not gather a lot of attention on Github, or rather none, so
Eric, maybe also consider whether this needs to be advertised more, or
maybe who should be pointed to it.

Please guide us to a conclusion as outlined in
https://github.com/ghc-proposals/ghc-proposals#committee-process

Thanks,
Joachim
--
--
Joachim Breitner
 mail@joachim-breitner.de
 http://www.joachim-breitner.de/


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