# Zulip Chat Archive

## Stream: general

### Topic: error with let substitution

#### Mohammed Eyad Kurd-Misto (Jun 03 2018 at 23:18):

Hello, I'm not sure if this is the correct place to post this. I am Mohammed Eyad Kurd-Misto, a PhD student at Chapman university. I am having an issue with a proof where the let construct does not seem to be working the way I would think. For two steps of the proof, I am simply substituting portions of the current statement from the let, and in one instance there is no issue but in another I am getting an error. The main relevant code is:

```
lemma flat_sharp {B : Type} : Π {n : ℕ}, ∀ (ps: @datum_or_data B ff n), (♭ (♯ ps)) = ps
| 0 datum_or_data.nil := rfl
| (n+1) (datum_or_data.cons p ps) :=
let ⟨as, h⟩ := ♯ ps, a := ♮⁻¹ p in --let (⟨as, h⟩ : vector B n) := (♯ ps : vector B n), a := ♮⁻¹ p in
calc
(♭ (♯ (datum_or_data.cons p ps))) = ♭ (vector.cons (♮⁻¹ p) (♯ ps)) : rfl
... = ♭ (vector.cons (a) (♯ ps)) : rfl --this works
... = ♭ (vector.cons (a) (⟨as, h⟩)) : rfl --@rfl (vector B n) -- this does not work?
... = datum_or_data.cons (♮ (a)) (♭ ⟨as, h⟩) : rfl
...
```

♭ and ♯ are operations for converting data from a vector to a 'datum_or_data' and vice versa so the goal here is simply to show they invert one another. ♮and ♮⁻¹ are operations for converting the individual elements of the data. I'm really not sure why I can't simply substitute ⟨as, h⟩ for ♯ ps since they are defined to be the same in the let and I have been unable to replicate anything similar where I can't simply substitute what is defined in the let construct. Any help would be greatly appreciated!

The error message given is:

Lean]

type mismatch at application

eq.trans (eq.trans (rfl (♭♯datum_or_data.cons p ps)) (rfl (♭(♮⁻¹p) :: ♯ps))) (rfl ?m_2)

term

rfl ?m_2

has type

?m_2 = ?m_2

but is expected to have type

(♭a :: ♯ps) = ♭a :: ⟨as, h⟩

Code to support this lemma is:

`import data.vector`

```
variable {α : Type}
variable {x : α}
variable {xs : list α}
```

```
lemma list_len_cancel {n :ℕ} (h : list.length (x::xs) = n+1)
: list.length xs = n := nat.succ.inj h -- injectivity of successor
```

```
inductive datum_or_data {B : Type} : bool → ℕ → Type
| sg (a : B) : datum_or_data tt 0
| nil : datum_or_data ff 0
| cons {n : ℕ} (a : datum_or_data tt 0) (as : datum_or_data ff n) : datum_or_data ff (n+1)
```

```
@[reducible] def datum_inv {B : Type} : @datum_or_data B tt 0 → B
| (datum_or_data.sg x) := x
```

notation `♮`

x := datum_or_data.sg x

notation `♮⁻¹`

a := datum_inv a

```
@[reducible] definition vector_from_data {B : Type} : Π {n :ℕ }, @datum_or_data B ff n → vector B n
| 0 _ := vector.nil
| (n+1) (datum_or_data.cons p ps) := vector.cons (♮⁻¹ p) (vector_from_data ps)
```

```
@[reducible] definition data_from_vector {B : Type} : Π {n :ℕ }, vector B n → @datum_or_data B ff n
| 0 _ := datum_or_data.nil
| (n+1) ⟨(a::as), h⟩ := datum_or_data.cons (♮ a) (data_from_vector ⟨as,list_len_cancel h⟩ )
```

notation `♯`

p := vector_from_data p

notation `♭`

v := data_from_vector v

```
-- inverses
@[simp] lemma natural {B : Type}: ∀ a : B, (♮⁻¹ (♮ a)) = a := assume a, rfl
@[simp] lemma natural' {B: Type}: ∀ d : @datum_or_data B tt 0, (♮ (♮⁻¹ d)) = d
| (♮ b) := rfl
```

```
--prove that flat and sharp are inverses
lemma flat_sharp {B : Type} : Π {n : ℕ}, ∀ (ps: @datum_or_data B ff n), (♭ (♯ ps)) = ps
| 0 datum_or_data.nil := rfl
| (n+1) (datum_or_data.cons p ps) :=
let ⟨as, h⟩ := ♯ ps, a := ♮⁻¹ p in --let (⟨as, h⟩ : vector B n) := (♯ ps : vector B n), a := ♮⁻¹ p in
calc
(♭ (♯ (datum_or_data.cons p ps))) = ♭ (vector.cons (♮⁻¹ p) (♯ ps)) : rfl
... = ♭ (vector.cons (a) (♯ ps)) : rfl --this works
... = ♭ (vector.cons (a) (⟨as, h⟩)) : rfl --@rfl (vector B n) -- this does not work?
... = datum_or_data.cons (♮ (a)) (♭ ⟨as, h⟩) : rfl
... = datum_or_data.cons p (♭ (♯ ps)) : congr_arg (λ x, datum_or_data.cons x (♭ (♯ ps))) (natural' p)
... = datum_or_data.cons p ps : congr_arg (λ x, datum_or_data.cons p x) (flat_sharp ps)
```

#### Simon Hudon (Jun 03 2018 at 23:22):

The way you use the let is more like a match statement than a local definition. It works but it makes definitional equality a bit weird.

#### Simon Hudon (Jun 03 2018 at 23:24):

You might need to use an actual match statement an include an equality lemma in the match expression. I'll just try something and get back to you.

#### Simon Hudon (Jun 03 2018 at 23:25):

Btw, you might like to know that it's possible to have nice formatting of Lean code by enclosing it in backticks

#### Mohammed Eyad Kurd-Misto (Jun 03 2018 at 23:33):

Thank you, I reformatted the initial post (though the notation portions actually contain backticks in the lean code so I left them as is).

#### Simon Hudon (Jun 03 2018 at 23:35):

If you put three ticks before the first line and three after the last, you don't need to format it line by line. Even better: if you put three ticks followed by `lean`

, you get syntax highlighting

#### Simon Hudon (Jun 03 2018 at 23:37):

What I think you should do is:

lemma flat_sharp {B : Type} : Π {n : ℕ}, ∀ (ps: @datum_or_data B ff n), (♭ (♯ ps)) = ps | 0 datum_or_data.nil := rfl | (n+1) (datum_or_data.cons p ps) := by cases h' : ♯ ps with as h ; calc (♭ (♯ (datum_or_data.cons p ps))) = ♭ (vector.cons (♮⁻¹ p) (♯ ps)) : rfl ... = ♭ (vector.cons (a) (♯ ps)) : rfl --this works ... = ♭ (vector.cons (a) (⟨as, h⟩)) : rw h' ... = datum_or_data.cons (♮ (a)) (♭ ⟨as, h⟩) : rfl ...

#### Mohammed Eyad Kurd-Misto (Jun 03 2018 at 23:55):

Thank you, I'll look at continuing the proof using cases/rw instead.

Last updated: May 13 2021 at 18:26 UTC