RFC: f.eq_unfold lemmas

[nomeata]

Proposal

For a function definition like Option.map we currently (well, for non-recursive only after #4154) create two kinds of rewrite lemmas 👍🏻

• map.eq_<n>: Matches when map is fully applied and there are constructors (map f none = …)
• map.eq_def: Matches when map is fully applied (map f x = match x with …)

Users can use this with simp or rw to control how aggressive the function should be unfolded. For example, rw [Option.map] will leave map x in place, but rw [Option.map.eq_def] will unfold it.

In this “algebra of rewrite specificity” there is a spot that we are missing and that might be useful:

• map.eq_unfold: Matches every map, even under applied or under binders (map = fun f x => match x with …).

One particular use-case for this is to rewrite with rw under binders where that isn't possible with f.eq_def:

theorem Option.map.eq_unfold : @Option.map = fun {α : Type u1} {β : Type u2} (f : α → β) x => match x with | .some x => some (f x) | .none => none := by
  rfl

example : List.map (fun x => Option.map f x) xs = xs := by
  fail_if_success rw [Option.map]
  fail_if_success rw [Option.map.eq_def]
  rw [Option.map.eq_unfold]
  trace_state
  /-
  ⊢ List.map
    (fun x =>
      (fun {α β} f x =>
          match x with
          | some x => some (f x)
          | none => none)
        f x)
    xs =
  xs
  -/
  sorry

This would probably also have helped with #5026.

Naming

As always, naming is hard. Some options:

• f.unfold -- clashes with too many existing theorems
• f.const_eq
• f.eq_spec -- goes well with .eq_def and .eq_n
• f.def -- probably not, we had this name for what’s now f.eq_def and it broke too much.

Implementation

Various ways to skin the cat, specially with regard to the existing getUnfoldFor? machinery.

One plausible path of least disruption is to have a single (non-extensible) generator for f.eq_unfold that looks at f.eq_def to compute the lemma statement (by moving binders to the RHS), and then proves it by rfl if possible and else using funext and f.eq_def.

Future steps

Once this is in, I imagine it might be a worthwhile refactoring to make it so that only the generation of f.unfold is extensible (wf, structural, non-rec, maybe #3119), and then f.eq_def and f.eq_1 are generated by lean from that definition. More uniform, less code. This should not prevent those theorems to be proven by Eq.rfl if possible.

Community Feedback

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Impact

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[nomeata]

Edit: Suggesting name f.eq_unfold instead of f.unfold to not break as much code out there.