Proposal: Remove HFunctor's fmap' with GHCs supporting QuantifiedConstraints. #153
Conversation
As was mentioned in the `HFunctor` documentation, we can eliminate the vestigial `fmap'` method with QuantifiedConstraints. This implementation is provided by Bottu et al.'s _Quantified Class Constraints_. The advantage of this patch is that it expresses the nature of `HFunctor` in a more direct manner, rather than with the dummy-function workaround. The disadvantage is that only GHC 8.6 supports QuantifiedConstraints, so we have to use CPP to define the polyfill (as it were) to handle it. This changes the `HFunctor` interface but should maintain compatibility, as `fmap'` was never implemented by hand outside of `Sum`.
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If CPP+QuantifiedConstraints is on the table, perhaps this newtype could make it in as well? |
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@mitchellwrosen I don't see why not. In my industrial experience, most people are pinned to relatively recent GHCs, so I think it behooves us to program for the newest GHC available. @robrix has the final say on it though. |
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We’d be forcing |
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I wonder if we could do something like Simulating Quantified Class Constraints? |
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Upon reflection I think the question I’m left with is: what problem is this solving for users? It complicates the code & build process somewhat, and (AFAICS) doesn’t really improve the usability of the library. On the balance, I think I’d rather not go this route, until such time as we only support 8.6+. |
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I think that’s fair. Closing this for now. |
As was mentioned in the
HFunctordocumentation, we can eliminate thevestigial
fmap'method with QuantifiedConstraints. This implementationis provided by Bottu et al.'s Quantified Class Constraints.
The advantage of this patch is that it expresses the nature of
HFunctorin a more direct manner, rather than with thedummy-function workaround. The disadvantage is that only GHC 8.6
supports QuantifiedConstraints, so we have to use CPP to define the
polyfill (as it were) to handle it.
This changes the
HFunctorinterface but should maintaincompatibility, as
fmap'was never implemented by hand outside ofSum.