# Zulip Chat Archive

## Stream: general

### Topic: mathematics in lean error

#### Gihan Marasingha (Jul 30 2020 at 12:28):

Section 3.1 of 'Mathematics in Lean' contains the code

```
variables {α : Type*} (r s t : set α)
example : s ⊆ s :=
by { intros x xs, exact xs }
example : s ⊆ s := λ x xs, xs
```

The last line causes an error in vscode and in the community web editor:

```
type mismatch, term
λ (x : ?m_1) (xs : ?m_2[x]), xs
has type
Π (x : ?m_1), ?m_2[x] → ?m_2[x]
but is expected to have type
s ⊆ s
```

I don't understand why there's an error message.

@Jeremy Avigad

#### Jalex Stark (Jul 30 2020 at 12:33):

notice that the following works

```
import data.set.basic
variables {α : Type*} (s : set α)
example : s ⊆ s := by exact λ x xs, xs
```

(In case it's not clear, the difference here is that `exact`

will use something like `refl`

to unify the goal and the target)

#### Jalex Stark (Jul 30 2020 at 12:34):

this doesn't work

```
import data.set.basic
variables {α : Type*} (s : set α)
local attribute [reducible] set.subset
example : s ⊆ s := λ x xs, xs
```

#### Jalex Stark (Jul 30 2020 at 12:39):

and this does work

```
import data.set.basic
variables {α : Type*} (s : set α)
example : s ⊆ s := set.subset_def.mpr $ λ x xs, xs
```

#### Kenny Lau (Jul 30 2020 at 12:46):

```
variables {α : Type*} (s : set α)
example : s ⊆ s := λ x, id
```

#### Jeremy Avigad (Jul 30 2020 at 13:13):

This is a quirk of Lean's parser: `example`

is parsed like `def`

rather than `theorem`

, which means that the elaborator doesn't use the statement of the theorem to elaborate the proof term. That's why `exact`

works where the proof term doesn't. It will also work if you change `example`

to `theorem foo`

.

I don't remember why Leo implemented `example`

that way, except maybe that he also wanted it to work for definitions, like `example := 3`

. I also don't know why this example made it past the test suite, but we'll fix it. @Gihan Marasingha, thanks for the correction.

#### Gihan Marasingha (Jul 30 2020 at 13:19):

Thanks @Jeremy Avigad . This was really confusing me.

#### Sebastian Ullrich (Jul 30 2020 at 13:32):

Jeremy Avigad said:

I don't remember why Leo implemented

`example`

that way, except maybe that he also wanted it to work for definitions, like`example := 3`

.

I would guess so as well. Fortunately it won't matter in Lean 4 because a `def`

*with* a specified type will behave exactly like `theorem`

does in Lean 3 (except for the parallelism, currently).

#### Mario Carneiro (Jul 30 2020 at 14:41):

This is a surprise to me. Not that `example`

is parsed like `def`

but that `def`

ignores the expected type. Why would it do that? I would have expected that it behaves the same as `def f := show s ⊆ s, from λ x xs, xs`

#### Sebastian Ullrich (Jul 30 2020 at 15:21):

...good question. I hadn't looked very closely at the actual example. I don't think it completely ignores it, but there must be some subtle, probably unintended difference.

#### ZHAO Jinxiang (Aug 16 2020 at 16:21):

Section (3.3. Negation) of 'Mathematics in Lean' contains the code

```
import data.real.basic
-- BEGIN
example :
¬ ∀ {f : ℝ → ℝ}, monotone f → ∀ {a b}, f a ≤ f b → a ≤ b :=
begin
intro h,
let f := λ x : ℝ, (0 : ℝ),
have monof : monotone f,
{ intros x y h1, dsimp [f], reflexivity},
have h' : f 1 ≤ f 0,
dsimp [f],
reflexivity,
have h1: 1 ≤ 0, {
-- exact h monof h',
sorry,
},
linarith,
end
-- END
```

If I remove sorry and use `exact h monof h',`

, it will told me

```
unnamed_1180.lean:15:10
invalid type ascription, term has type
?m_1 ≤ ?m_2
but is expected to have type
1 ≤ 0
state:
h : ∀ {f : ℝ → ℝ}, monotone f → ∀ {a b : ℝ}, f a ≤ f b → a ≤ b,
f : ℝ → ℝ := λ (x : ℝ), 0,
monof : monotone f,
h' : f 1 ≤ f 0
⊢ 1 ≤ 0
```

I don't understand why there's an error message.

@Jeremy Avigad

#### Mario Carneiro (Aug 16 2020 at 16:28):

The statement `h1`

says `(1 : nat) <= 0`

, because there is nothing to suggest what the type should be so lean picks `nat`

by default. Change that line to `have h1: (1 : ℝ) ≤ 0,`

#### Bryan Gin-ge Chen (Aug 16 2020 at 16:31):

That error message is incredibly unhelpful though.

#### ZHAO Jinxiang (Aug 16 2020 at 16:31):

@Mario Carneiro Thank you very much.

#### ZHAO Jinxiang (Aug 16 2020 at 16:37):

Is the 1 here a polymorphic 1, not nat and not real?

#### Kevin Buzzard (Aug 16 2020 at 16:42):

It's polymorphic until lean gives up on the idea that you're going to tell it which one, and then it uses nat

#### ZHAO Jinxiang (Aug 16 2020 at 16:45):

So the `?m_1`

here is (1:nat)

#### Mario Carneiro (Aug 16 2020 at 16:46):

The `1`

at the bottom is `1:nat`

, the `?m_1`

at the top is `?m_1:real`

and that's why there is a type unification error

#### Mario Carneiro (Aug 16 2020 at 16:46):

and yes, the message is incredibly unhelpful, I only know the problem because I've seen it many times

#### ZHAO Jinxiang (Aug 17 2020 at 02:04):

I have another question about this:

```
import data.real.basic
-- BEGIN
example :
¬ ∀ {f : ℝ → ℝ}, monotone f → ∀ {a b}, f a ≤ f b → a ≤ b :=
begin
intro h,
let f := λ x : ℝ, (0 : ℝ),
have monof : monotone f,
{ intros x y h1, dsimp [f], reflexivity},
have h' : f 1 ≤ f 0,
dsimp [f],
reflexivity,
-- have h1: (1:ℝ) ≤ 0,
-- exact h monof h',
have h'':monotone f → ∀ {a b}, f a ≤ f b → a ≤ b,
exact h,
have h''':∀ {a b : ℝ}, f a ≤ f b → a ≤ b,
exact h'' monof,
-- linarith,
end
-- END
```

Error message:

```
invalid type ascription, term has type
f ?m_1 ≤ f ?m_2 → ?m_1 ≤ ?m_2
but is expected to have type
∀ {a b : ℝ}, f a ≤ f b → a ≤ b
```

#### Floris van Doorn (Aug 17 2020 at 02:22):

replace the line where you declare `h''`

by

```
have h'' : monotone f → ∀ a b, f a ≤ f b → a ≤ b,
```

#### Floris van Doorn (Aug 17 2020 at 02:29):

The curly braces mean these are implicit arguments, so `h'' monof`

is interpreted as `@h'' monof _ _`

.

#### Floris van Doorn (Aug 17 2020 at 02:29):

You can also use strict implicit arguments, using `{{a b}}`

or `\{a b\}`

.

#### Kenny Lau (Aug 17 2020 at 05:53):

we really need `{{}}`

to be the default behaviour

#### Johan Commelin (Aug 17 2020 at 06:00):

Can we swap the meaning of `{{ }}`

and `{ }`

in C++?

#### Johan Commelin (Aug 17 2020 at 06:00):

That would save us a huge refactor

#### Kenny Lau (Aug 17 2020 at 06:04):

yeah that's what I mean

#### Mario Carneiro (Aug 17 2020 at 06:18):

I don't know about this. I think there are a significant fraction of `{ }`

that should be `{ }`

, compared to a small fraction where `{{ }}`

is noticeably better or more appropriate

#### Johan Commelin (Aug 17 2020 at 06:42):

@Mario Carneiro I'm slow today. Can you give an example where `{{ }}`

is clearly the wrong choice?

#### Mario Carneiro (Aug 17 2020 at 06:45):

Something like `\forall {{x y}}, f x y = 0`

#### Mario Carneiro (Aug 17 2020 at 06:46):

really anything where there are no explicit arguments after the semi implicit args

#### Mario Carneiro (Aug 17 2020 at 06:46):

because then you can't apply the semi implicit args even if you want to

#### Mario Carneiro (Aug 17 2020 at 06:46):

you would have to write `@foo _ _`

#### Johan Commelin (Aug 17 2020 at 06:47):

Fair enough. Personally, I wouldn't mind having these explicit.

#### Johan Commelin (Aug 17 2020 at 06:48):

For quite some time, I've been thinking that we're marking too many things implicit.

#### Mario Carneiro (Aug 17 2020 at 06:48):

For a local hypothesis that makes sense, but it is not uncommon for type arguments to be implicit like this

#### Mario Carneiro (Aug 17 2020 at 06:50):

my ballpark estimate is that today with `{ }`

default we use `{ }`

99% of the time and prefer `{{ }}`

about 1% of the time, while if `{{ }}`

was the default then we would use it 90% of the time and would find reasons to use `{ }`

10% of the time

#### Johan Commelin (Aug 17 2020 at 06:52):

I'm also talking about things like

```
lemma submodule.neg_mem (hx : x ∈ p) : -x ∈ p := -- x is implicit
```

but I realise that this is somewhat off topic in this thread.

These statements are annoying when you don't have `hx`

yet, and `-x \in p`

is also not exactly the expected type.

#### ZHAO Jinxiang (Aug 17 2020 at 12:46):

```
import data.real.basic
def fn_ub (f : ℝ → ℝ) (a : ℝ) : Prop := ∀ x, f x ≤ a
def fn_has_ub (f : ℝ → ℝ) := ∃ a, fn_ub f a
open_locale classical
variable (f : ℝ → ℝ)
-- BEGIN
example (h : ¬ ∀ a, ∃ x, f x > a) : fn_has_ub f :=
begin
push_neg at h,
exact h
end
example (h : ¬ fn_has_ub f) : ∀ a, ∃ x, f x > a :=
begin
simp only [fn_has_ub, fn_ub] at h,
push_neg at h,
exact h
end
-- END
```

If I change `simp only [fn_has_ub, fn_ub] at h`

to `simp only [fn_has_ub] at h,simp only [fn_ub] at h,`

it will told me:

```
unnamed_1338.lean:19:29
simplify tactic failed to simplify
state:
f : ℝ → ℝ,
h : ¬Exists (fn_ub f)
⊢ ∀ (a : ℝ), ∃ (x : ℝ), f x > a
```

#### Kevin Buzzard (Aug 17 2020 at 12:56):

If this is not an error in the mathematics in Lean book, can I request that we move this to a new thread?

#### Jeremy Avigad (Aug 17 2020 at 15:44):

I am doing my late-morning Zulip catch-up, so I don't know if this moved to a new thread. BA long time ago I suggested exactly this change to Leo, and he tried it and said that lots of things broke with the change. IIRC he came to the conclusion that neither was clearly the more natural default. This is not an argument against trying it again and seeing what happens. It's just a (vaguely remembered) data point.

#### Floris van Doorn (Aug 17 2020 at 15:50):

I think normal implicit is a better default, but there are plenty of cases where strict implicit is desirable. If you like to write `inv_inj.mpr`

or more generally`any_proof_of_an_iff_without_explicit_arguments.mp`

, then you have a hard time with strict implicit arguments.

Last updated: May 09 2021 at 18:17 UTC