[ghc-steering-committee] #409: Exportable named defaults, Recommendation: Partial Accept

Eric Seidel eric at seidel.io
Wed May 19 02:11:48 UTC 2021

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


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 at joachim-breitner.de
>   http://www.joachim-breitner.de/
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