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I took a few minutes to bang out a sketch of this, but I don't have the QC branch of GHC built to test with and fix it, so there's probably a bunch of typos/thinkos.
https://gist.github.com/ekmett/af1c460582b1de467c8461abdf134b6f …
QCs completely replace my old approach for finer AsFoo/HasFoo constraints. /cc @dibblego
Looks similar, but different again, to https://github.com/ekmett/lens-constraints/blob/master/Constraints.hs …
The difference is that the explosion of instances isn't necessary with quantified constraints. The type checker will do all the work. Also you don't need two _Foo variants, one with the stronger constraint and one without: one will do, because we can now state entailment directly
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