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fredrik-bakke committed Oct 11, 2023
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8 changes: 4 additions & 4 deletions .vscode/extensions.json
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@@ -1,6 +1,6 @@
{
"recommendations": [
"nikolaikudasovfizruk.rzk-1-experimental-highlighting",
"esbenp.prettier-vscode"
]
"recommendations": [
"nikolaikudasovfizruk.rzk-1-experimental-highlighting",
"esbenp.prettier-vscode"
]
}
34 changes: 15 additions & 19 deletions src/hott/06-contractible.rzk.md
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Expand Up @@ -533,52 +533,48 @@ A type is contractible if and only if it has singleton induction.

## Identity types of contractible types

We show that any two paths between the same endpoints in a contractible type are
the same.

In a contractible type any path $p : x = y$ is equal to the path constructed in
`eq-is-contr`.
We show that any two paths between the same endpoints in a contractible type are the same.

In a contractible type any path $p : x = y$ is equal to the path constructed in `eq-is-contr`.
```rzk
#define path-eq-path-through-center-is-contr
( A : U)
( is-contr-A : is-contr A)
( x y : A)
( p : x = y)
: ((eq-is-contr A is-contr-A x y) = p)
:=
:=
ind-path
( A)
( x)
( \ y' p' → (eq-is-contr A is-contr-A x y') = p')
( left-inverse-concat A (center-contraction A is-contr-A) x (homotopy-contraction A is-contr-A x))
( y)
( p)
( \ y' p' → (eq-is-contr A is-contr-A x y') = p')
( left-inverse-concat A (center-contraction A is-contr-A) x (homotopy-contraction A is-contr-A x))
( y)
( p)
```

Finally, in a contractible type any two paths between the same end points are
equal. There are many possible proofs of this (e.g. identifying contractible
types with the unit type where it is more transparent), but we proceed by gluing
together the two identifications to the out and back path.
Finally, in a contractible type any two paths between the same end points are equal. There are many possible proofs of this (e.g. identifying contractible types with the unit type where it is more transparent), but we proceed by gluing together the two identifications to the out and back path.

```rzk
#define all-paths-eq-is-contr
#define all-paths-eq-is-contr
( A : U)
( is-contr-A : is-contr A)
( x y : A)
( p q : x = y)
( p q : x = y)
: (p = q)
:=
:=
concat
( x = y)
( p)
( eq-is-contr A is-contr-A x y)
( q)
( rev
(x = y)
(x = y)
( eq-is-contr A is-contr-A x y)
( p)
( p)
( path-eq-path-through-center-is-contr A is-contr-A x y p))
( path-eq-path-through-center-is-contr A is-contr-A x y q)
```


26 changes: 13 additions & 13 deletions src/hott/11-homotopy-pullbacks.rzk.md
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Expand Up @@ -8,9 +8,8 @@ This is a literate `rzk` file:

## Homotopy cartesian squares

We start by fixing the data of a map between two type families `A' → U` and
`A → U`, which we think of as a commutative square

We start by fixing the data of a map between two type families
`A' → U` and `A → U`, which we think of as a commutative square
```
Σ A' → Σ A
↓ ↓
Expand All @@ -36,8 +35,8 @@ We start by fixing the data of a map between two type families `A' → U` and
:= \ (a', c') → (α a', γ a' c')
```

We say that such a square is homotopy cartesian just if it induces an
equivalence componentwise.
We say that such a square is homotopy cartesian
just if it induces an equivalence componentwise.

```rzk
#def is-homotopy-cartesian uses (A)
Expand Down Expand Up @@ -217,8 +216,8 @@ always do this (whether the square is homotopy-cartesian or not).

### Invariance under pullbacks

We can pullback a homotopy cartesian square over `α : A' → A` along any map of
maps `β → α` and obtain another homotopy cartesian square.
We can pullback a homotopy cartesian square over `α : A' → A`
along any map of maps `β → α` and obtain another homotopy cartesian square.

```rzk
#def is-homotopy-cartesian-pullback
Expand Down Expand Up @@ -247,15 +246,16 @@ maps `β → α` and obtain another homotopy cartesian square.

## Pasting calculus for homotopy cartesian squares

Currently our notion of squares is not symmetric, since the vertical maps are
given by type families, i.e. they are _display maps_, while the horizontal maps
are arbitrary. Therefore we distinquish between the vertical and the horizontal
pasting calculus.
Currently our notion of squares is not symmetric,
since the vertical maps are given by type families,
i.e. they are _display maps_,
while the horizontal maps are arbitrary.
Therefore we distinquish between the vertical and the horizontal pasting calculus.

### Vertical calculus

The following vertical composition and cancellation laws follow easily from the
corresponding statements about equivalences established above.
The following vertical composition and cancellation laws follow easily from
the corresponding statements about equivalences established above.

```rzk
#section homotopy-cartesian-vertical-calculus
Expand Down
11 changes: 6 additions & 5 deletions src/simplicial-hott/03-extension-types.rzk.md
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Expand Up @@ -17,12 +17,13 @@ This is a literate `rzk` file:

## Extension up to homotopy

For a shape inclusion `ϕ ⊂ ψ` and any type `A`, we have the inbuilt extension
types `(t : ψ) → A [ϕ t ↦ σ t]` (for every `σ : ϕ → A`).
For a shape inclusion `ϕ ⊂ ψ` and any type `A`,
we have the inbuilt extension types `(t : ψ) → A [ϕ t ↦ σ t]`
(for every `σ : ϕ → A`).

We show that these extension types are equivalent to the fibers of the canonical
restriction map `(ψ → A) → (ϕ → A)`, which we can view as the types of
"extension up to homotopy".
We show that these extension types are equivalent to the fibers
of the canonical restriction map `(ψ → A) → (ϕ → A)`,
which we can view as the types of "extension up to homotopy".

```rzk
#section extensions-up-to-homotopy
Expand Down
75 changes: 38 additions & 37 deletions src/simplicial-hott/04-right-orthogonal.rzk.md
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Expand Up @@ -16,26 +16,26 @@ Some of the definitions in this file rely on naive extension extensionality:

## Right orthogonal maps with respect to shapes

For every shape inclusion `ϕ ⊂ ψ`, we obtain a fibrancy condition for a map
`α : A' → A` in terms of unique extension along `ϕ ⊂ ψ`. This is a relative
version of unique extension along `ϕ ⊂ ψ`.
For every shape inclusion `ϕ ⊂ ψ`,
we obtain a fibrancy condition for a map `α : A' → A`
in terms of unique extension along `ϕ ⊂ ψ`.
This is a relative version of unique extension along `ϕ ⊂ ψ`.

We say that `α : A' → A` is _right orthogonal_ to the shape inclusion `ϕ ⊂ ψ`,
if the square

```
(ψ → A') → (ψ → A)
↓ ↓
(ϕ → A') → (ϕ → A)
```

is homotopy cartesian.

Equivalently, we can interpret this orthogonality as a cofibrancy condition on
the shape inclusion. We say that the shape inclusion `ϕ ⊂ ψ` is _left
orthogonal_ to the map `α`, if `α : A' → A` is right orthogonal to `ϕ ⊂ ψ`.
Equivalently, we can interpret this orthogonality
as a cofibrancy condition on the shape inclusion.
We say that the shape inclusion `ϕ ⊂ ψ` is _left orthogonal_ to the map `α`,
if `α : A' → A` is right orthogonal to `ϕ ⊂ ψ`.

```rzk title="BW23, Section 3"
#def is-right-orthogonal-to-shape
Expand All @@ -62,9 +62,8 @@ We fix a map `α : A' → A`.
#variables A' A : U
#variable α : A' → A
```

Consider nested shapes `ϕ ⊂ χ ⊂ ψ` and the three possible right orthogonality
conditions.
Consider nested shapes `ϕ ⊂ χ ⊂ ψ`
and the three possible right orthogonality conditions.

```rzk
#variable I : CUBE
Expand All @@ -78,18 +77,19 @@ conditions.
-- rzk does not accept these terms after η-reduction
```

Using the vertical pasting calculus for homotopy cartesian squares, it is not
hard to deduce the corresponding composition and cancellation properties for
left orthogonality of shape inclusion with respect to `α : A' → A`.
Using the vertical pasting calculus for homotopy cartesian squares,
it is not hard to deduce the corresponding composition and cancellation properties
for left orthogonality of shape inclusion with respect to `α : A' → A`.

### Σ over an intermediate shape

The only fact that stops some of these laws from being a direct corollary is
that the `Σ`-types appearing in the vertical pasting of the relevant squares
(such as `Σ (\ σ : ϕ → A), ( (t : χ) → A [ϕ t ↦ σ t])`) are not literally equal
to the corresponding extension types (such as `τ → A `). Therefore we have to
occasionally go back or forth along the functorial equivalence
`cofibration-composition-functorial`.
The only fact that stops some of these laws from being a direct corollary
is that the `Σ`-types appearing in the vertical pasting of the relevant squares
(such as `Σ (\ σ : ϕ → A), ( (t : χ) → A [ϕ t ↦ σ t])`)
are not literally equal to the corresponding extension types
(such as `τ → A `).
Therefore we have to occasionally go back or forth along the
functorial equivalence `cofibration-composition-functorial`.

```rzk
#def is-homotopy-cartesian-Σ-is-right-orthogonal-to-shape uses (is-orth-ψ-ϕ)
Expand Down Expand Up @@ -177,8 +177,9 @@ If `ϕ ⊂ χ` and `ϕ ⊂ ψ` are left orthogonal to `α : A' → A`, then so i
τ'
```

If `ϕ ⊂ ψ` is left orthogonal to `α : A' → A` and `χ ⊂ ψ` is a (functorial)
shape retract, then `ϕ ⊂ ψ` is left orthogonal to `α : A' → A`.
If `ϕ ⊂ ψ` is left orthogonal to `α : A' → A`
and `χ ⊂ ψ` is a (functorial) shape retract,
then `ϕ ⊂ ψ` is left orthogonal to `α : A' → A`.

```rzk
#def is-right-orthogonal-to-shape-right-cancel-retract uses (is-orth-ψ-ϕ)
Expand All @@ -203,11 +204,11 @@ shape retract, then `ϕ ⊂ ψ` is left orthogonal to `α : A' → A`.

### Stability under exponentiation

If `ϕ ⊂ ψ` is left orthogonal to `α : A' → A` then so is `χ × ϕ ⊂ χ × ψ` for
every other shape `χ`.
If `ϕ ⊂ ψ` is left orthogonal to `α : A' → A`
then so is `χ × ϕ ⊂ χ × ψ` for every other shape `χ`.

The following proof uses a lot of currying and uncurrying and relies on (the
naive form of) extension extensionality.
The following proof uses a lot of currying and uncurrying
and relies on (the naive form of) extension extensionality.

```rzk
#def is-right-orthogonal-to-shape-× uses (naiveextext)
Expand Down Expand Up @@ -279,8 +280,8 @@ naive form of) extension extensionality.

### Stability under exact pushouts

For any two shapes `ϕ, ψ ⊂ I`, if `ϕ ∩ ψ ⊂ ϕ` is left orthogonal to
`α : A' → A`, then so is `ψ ⊂ ϕ ∪ ψ`.
For any two shapes `ϕ, ψ ⊂ I`, if `ϕ ∩ ψ ⊂ ϕ` is left orthogonal to `α : A' → A`,
then so is `ψ ⊂ ϕ ∪ ψ`.

```rzk
#def is-right-orthogonal-to-shape-pushout
Expand All @@ -304,8 +305,8 @@ For any two shapes `ϕ, ψ ⊂ I`, if `ϕ ∩ ψ ⊂ ϕ` is left orthogonal to

## Types with unique extension

We say that an type `A` has unique extensions for a shape inclusion `ϕ ⊂ ψ`, if
for each `σ : ϕ → A` the type of `ψ`-extensions is contractible.
We say that an type `A` has unique extensions for a shape inclusion `ϕ ⊂ ψ`,
if for each `σ : ϕ → A` the type of `ψ`-extensions is contractible.

```rzk
#def has-unique-extensions
Expand All @@ -318,8 +319,8 @@ for each `σ : ϕ → A` the type of `ψ`-extensions is contractible.
( σ : ϕ → A) → is-contr ( (t : ψ) → A [ϕ t ↦ σ t])
```

The property of having unique extension can be pulled back along any right
orthogonal map.
The property of having unique extension
can be pulled back along any right orthogonal map.

```rzk
#def has-unique-extensions-domain-right-orthogonal-has-unique-extensions-codomain
Expand Down Expand Up @@ -355,9 +356,9 @@ is an equivalence.
is-equiv (ψ → A) (ϕ → A) ( \ τ t → τ t)
```

This follows straightforwardly from the fact that for every `σ : ϕ → A` we have
an equivalence between the extension type `(t : ψ) → A [ϕ t ↦ σ t]` and the
fiber of the restriction map `(ψ → A) → (ϕ → A)`.
This follows straightforwardly from the fact that for every `σ : ϕ → A`
we have an equivalence between the extension type `(t : ψ) → A [ϕ t ↦ σ t]`
and the fiber of the restriction map `(ψ → A) → (ϕ → A)`.

```rzk
#def is-local-type-has-unique-extensions
Expand Down Expand Up @@ -388,8 +389,8 @@ fiber of the restriction map `(ψ → A) → (ϕ → A)`.
#end is-local-type
```

Since the property of having unique extensions passes from the codomain to the
domain of a right orthogonal map, the same is true for locality of types.
Since the property of having unique extensions passes from the codomain to the domain
of a right orthogonal map, the same is true for locality of types.

```rzk
#def is-local-type-domain-right-orthogonal-is-local-type-codomain
Expand Down
8 changes: 4 additions & 4 deletions src/simplicial-hott/05-segal-types.rzk.md
Original file line number Diff line number Diff line change
Expand Up @@ -16,8 +16,8 @@ This is a literate `rzk` file:
- `hott/total-space.md` — We rely on
`#!rzk is-equiv-projection-contractible-fibers` and
`#!rzk projection-total-type` in the proof of Theorem 5.5.
- `02-simplicial-type-theory.rzk.md` — We rely on definitions of simplicies and
their subshapes.
- `02-simplicial-type-theory.rzk.md` — We rely on definitions of simplicies and their
subshapes.
- `03-extension-types.rzk.md` — We use the fubini theorem and extension
extensionality.

Expand Down Expand Up @@ -1779,8 +1779,8 @@ The cofibration Λ²₁ → Δ² is inner anodyne

## Inner fibrations

An inner fibration is a map `α : A' → A` which is right orthogonal to `Λ ⊂ Δ²`.
This is the relative notion of a Segal type.
An inner fibration is a map `α : A' → A` which is right orthogonal
to `Λ ⊂ Δ²`. This is the relative notion of a Segal type.

```rzk
#def is-inner-fibration
Expand Down
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