wip(data/subtype_instances): introduce closed_under predicate typeclasses #7834
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eric-wieser
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Jun 7, 2021
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| /-- Typeclass inference can't find this without a large amount of help :(. -/ | ||
| example [semiring β] [has_measurable_add₂ β] [has_measurable_mul₂ β]: | ||
| semiring (subtype (measurable : (α → β) → Prop)) := | ||
| begin | ||
| /- `id _` lets us see why typeclass inference is failing. -/ | ||
| refine @subtype.closed_under.subtype.semiring _ _ _ (id _) sorry sorry sorry, | ||
| convert measurable.subtype.closed_under_zero, | ||
| -- uh oh: `mul_zero_class.to_has_zero (α → β) = pi.has_zero` | ||
| swap, | ||
| apply_instance, | ||
| refl, | ||
| end |
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Here's where everything goes wrong
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I think you can generalize these to an arbitrary number of inputs with |
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That PR looks good and I was already aware of it, but I think that approach would make the typeclass problem worse. |
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This seemed like it would be a nice generalization, but typeclass inference gets completely stuck in finding these new instances.
This is a more general version of #7833, with the hope of expanding the typeclasses to be used with
submonoidetc in a future PR.