feat: find Decidable instances via unification #2816
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Agreed.
Happy to merge the workaround. We have used it in other places. |
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| @@ -806,6 +806,12 @@ decidability instance instead of the proposition, which has no code). | |||
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| If a proposition `p` is `Decidable`, then `(by decide : p)` will prove it by | |||
| evaluating the decidability instance to `isTrue h` and returning `h`. | |||
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| Because `Decidable` carries data, | |||
| when writing `@[simp]` lemmas which include a `Decidable` instance on the LHS, | |||
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Is the corollary here that you can no longer use the lemma with ← if no Decidable instance is on the RHS? And so in those cases, you need to write two versions of the same lemma?
(Re-posted as this got lost as a non-PR comment)
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Because `Decidable` carries data,
when writing `@[simp]` lemmas which include a `Decidable` instance on the LHS,
it is best to use `{_ : Decidable p}` rather than `[Decidable p]`
so that non-canonical instances can be found via unification rather than
typeclass search.
(Previously this behaviour was often being hidden by the default `decide :=
true` in `simp`.)
This was referenced Jan 4, 2024
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I've been finding situations where these
simplemmas don't fire, because the decidable instance is not the one found by typeclass inference.This perhaps deserves a deeper fix, but I want to see if just changing the binder types here suffices for my purposes.
(Previously this behaviour was being hidden by the default
decide := trueinsimp.)