leanprover-community / leanprover-community/lean
Make `@[trans]`/`calc` work with `homeomorph` etc
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Description
Here is a minimal non-working example. See also homeomorph.trans in mathlib. It seems that calc makes very strict assumptions on the meaning of implicit arguments preceding the explicit arguments of the @[trans] lemma, and we can't satisfy these assumptions with homeomorph, local_homeomorph, monoid_hom etc, see Zulip chat.
structure inhabited_hom (α β : Type*) [inhabited α] [inhabited β] :=
(to_fun : α → β)
(map_default : to_fun (default α) = default β)
notation α `→'`:25 β := inhabited_hom α β
@[trans] def inhabited_hom.trans {α β γ : Type*} [inhabited α] [inhabited β] [inhabited γ]
(f : α →' β) (g : β →' γ) :
α →' γ :=
{ to_fun := λ x, g.to_fun (f.to_fun x),
map_default := by rw [f.map_default, g.map_default] }
example {α β γ : Type*} [inhabited α] [inhabited β] [inhabited γ]
(f : α →' β) (g : β →' γ) : α →' γ :=
calc α →' β : f
... →' γ : g
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the minimal inhabited_hom example and reading how calc resolves @[trans] lemmas. Compare this behavior with homeomorph.trans and the linked Zulip discussion; done means the example and analogous homomorphism cases elaborate successfully without breaking existing calc behavior.
Written by the indexing model from the issue text.
Assessment
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 25/100