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

[Eric Seidel]

The response to the NamedDefaults extension has been uniformly positive, so I think we can consider that part accepted.

However, we still need to make a decision about the ExportedDefaults extension. Since my original recommendation to reject this part of the proposal, I've come around to the argument that defaulting rules don't need global coherence like class instances, so explicit exports are fine. Simon PJ and Richard have also voiced support for explicit exports as suggested in the proposal.

So I would like to revise my recommendation for ExportedDefaults to *accept*.

That leaves the question of how defaulting rules should be imported. The two options are

1. *implicit*: all defaulting rules exported by a module M are automatically imported by *any* import of M, just like class instances. The proposal suggests doing this, and hiding it behind an `ImportedDefaults` extension, which feels unnecessary to me.

2. *explicit*: defaulting rules must be explicitly imported, using a syntax like `import M (default C)`. If we go this route, we will also need to decide whether a plain `import M` should import defaulting rules. Richard argues on GitHub that it should not, but I think that veers too far from the existing behavior of imports.

Between the two, I lean towards (2) for the symmetry between explicit exports and imports, with the `import M` syntax pulling in defaulting rules.

Eric


On Tue, May 25, 2021, at 16:01, Richard Eisenberg wrote:
> I have commented on GitHub with my thoughts here: 
> https://github.com/ghc-proposals/ghc-proposals/pull/409#issuecomment-848221001
> 
> Thanks,
> Richard
> 
> > On May 20, 2021, at 11:24 AM, Alejandro Serrano Mena <trupill at gmail.com> wrote:
> > 
> > Hi all,
> > 
> > I agree with the recommendations of accepting (1) and rejecting (2) and (3). The Report here (https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-790004.3.4) mentions that defaults are local to a module, and I think this is the right move, even more so since we can think of other ways of importing/exporting defaults, like plug-ins.
> > 
> > Alejandro
> > 
> > El 19 may 2021 4:11:48, Eric Seidel <eric at seidel.io> escribió:
> >> 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 at joachim-breitner.de
> >>>   http://www.joachim-breitner.de/
> >>> 
> >>> 
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> >>> 
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