## Stream: new members

### Topic: How to use exists_prop

#### Daniel Fabian (May 17 2020 at 13:00):

I'm working my way through the mathlib tutorial, and right around exercise 0040, there's an ominous comment that you can use exists_prop to rewrite the exercise.

But I don't think that's the case:
exists_prop {p q : Prop} : (∃ (h : p), q) ↔ p ∧ q

But that only works if the proof p is independent of anything, i.e. a pure proposition. In the exercise, however, the statement is exists n >= N,.... This desugars into the existence of an n and a proof, that n >= N. The latter is value-dependent on n. So you necessarily have a dependent type signature.

Here's a repro:

@[simp] theorem exists_prop {p q : Prop} : (∃ h : p, q) ↔ p ∧ q :=
⟨λ ⟨h₁, h₂⟩, ⟨h₁, h₂⟩, λ ⟨h₁, h₂⟩, ⟨h₁, h₂⟩⟩

example (N : ℕ) : ∃ (n : ℕ) (H : n ≥ N), n ≥ N :=
begin
rw exists_prop,
sorry
end


Am I missing something? This comment just confused me, because when I looked around a bit, the quantifiers seem to be mostly dependent statements, not propositions.

#### Patrick Massot (May 17 2020 at 13:06):

The statement n ≥ N is still a Prop. The fact that it depends on n doesn't change this.

#### Patrick Massot (May 17 2020 at 13:08):

That being said, your example is not relevant, because in that case you would use use which does the exists_prop rewriting for you. A more relevant example would have an assumption having this confusing shape.

#### Reid Barton (May 17 2020 at 13:09):

After one use, you get a statement where you can apply exists_prop.

#### Patrick Massot (May 17 2020 at 13:09):

In your example you can write

example (N : ℕ) : ∃ (n : ℕ) (H : n ≥ N), n ≥ N :=
begin
refine ⟨N, _⟩,
rw exists_prop,
sorry
end


to see the rewriting happening, but you can replace both lines by use N

#### Reid Barton (May 17 2020 at 13:09):

oh, does use do this automatically?

#### Patrick Massot (May 17 2020 at 13:09):

No Reid, this is not necessary

Yes

#### Reid Barton (May 17 2020 at 13:10):

After one existsi, you get ... :slight_smile:

#### Patrick Massot (May 17 2020 at 13:10):

You shouldn't live in the past like that.

#### Reid Barton (May 17 2020 at 13:10):

I just use refine

#### Daniel Fabian (May 17 2020 at 13:11):

just to be clear, when you refine \<N, _\>, you just chose N as your witness, yes?

yes

#### Patrick Massot (May 17 2020 at 13:16):

You can try the code

#### Daniel Fabian (May 17 2020 at 13:17):

and @Patrick Massot , I still don't quite see how you'd apply it, sorry. It doesn't matter if I write exists n >= N, n >= N or the way I wrote it. Lean's goal is the same.

My point being, that since it's a double-exists goal, it's not an instance of exists_prop.

As such, you need to eliminate one of the exist statements and if you do it using the use tactic, the second exists disappears.

And of course use is the obvious solution here, but the question was specifically how to use exists_prop in the context of the tutorial. Where you mostly have double-exists.

#### Patrick Massot (May 17 2020 at 13:18):

That being said, your example is not relevant, because in that case you would use use which does the exists_prop rewriting for you. A more relevant example would have an assumption having this confusing shape.

#### Patrick Massot (May 17 2020 at 13:19):

And I need to go on family duty, sorry. I'll just add that you can do the whole tutorials without using this lemma, I only added it as a comment for people who are really confused by these double exists

(deleted)

#### Yury G. Kudryashov (May 17 2020 at 13:34):

I didn't look at the tutorials but I use exists_prop with double exists using simp only [exists_prop].

#### Daniel Fabian (May 17 2020 at 13:39):

ah yes. Now that one actually does something... It takes the double exists and eliminates the inner one and makes the statement a conjunction.

#### Daniel Fabian (May 17 2020 at 13:42):

I think the only really confusing thing is, that apparently in the tutorial the things are not instances of exists_prop so you can't apply it (other than calling simp as you showed, which wasn't taught at that point in the tutorial yet).

And the assumptions are universally quantified.

#### Daniel Fabian (May 17 2020 at 13:43):

Also exists_prop isn't used in the solutions, so you can't look up how it could be used.

#### Jalex Stark (May 17 2020 at 16:26):

Patrick Massot said:

You shouldn't live in the past like that.

Are you suggesting that one should always use use instead of existsi? I often find use being hundreds of milliseconds slower to achieve the same result
(if i'm bothered by that level of slowdown, does it mean I should be more lemmas and shorter proofs?)

#### Johan Commelin (May 17 2020 at 16:29):

If use misbehaves, I fall back on refine \<bla, _\>

#### Johan Commelin (May 17 2020 at 16:29):

I haven't used existsi since use was introduced.

Last updated: May 08 2021 at 20:11 UTC