open import 1Lab.Path
open import 1Lab.Type hiding (id; _∘_)

open import Cat.Base

module Cat.Reasoning {o ℓ} (C : Precategory o ℓ) where

open import Cat.Morphism C public

Reasoning combinators for categories🔗

When doing category theory, we often have to perform many “trivial” algebraic manipulations like reassociation, insertion of identity morphisms, etc. On paper we can omit these steps, but Agda is a bit more picky! We could just do these steps in our proofs one after another, but this clutters things up. Instead, we provide a series of reasoning combinators to make writing (and reading) proofs easier.

Most of these helpers were taken from agda-categories.

private variable
  u v w x y z : Ob
  a a' a'' b b' b'' c c' c'' d d' d'' : Hom x y
  f g g' h h' i : Hom x y

Identity morphisms🔗

id-comm : ∀ {a b} {f : Hom a b} → f ∘ id ≡ id ∘ f
id-comm {f = f} = idr f ∙ sym (idl f)

id-comm-sym : ∀ {a b} {f : Hom a b} → id ∘ f ≡ f ∘ id
id-comm-sym {f = f} = idl f ∙ sym (idr f)

idr2 : ∀ {a b c} (f : Hom b c) (g : Hom a b) → f ∘ g ∘ id ≡ f ∘ g
idr2 f g = ap (f ∘_) (idr g)

module _ (a≡id : a ≡ id) where abstract
  eliml : a ∘ f ≡ f
  eliml {f = f} =
    a ∘ f ≡⟨ ap (_∘ f) a≡id ⟩≡
    id ∘ f ≡⟨ idl f ⟩≡
    f ∎

  elimr : f ∘ a ≡ f
  elimr {f = f} =
    f ∘ a ≡⟨ ap (f ∘_) a≡id ⟩≡
    f ∘ id ≡⟨ idr f ⟩≡
    f ∎

  elim-inner : f ∘ a ∘ h ≡ f ∘ h
  elim-inner {f = f} = ap (f ∘_) eliml

  introl : f ≡ a ∘ f
  introl = sym eliml

  intror : f ≡ f ∘ a
  intror = sym elimr

  intro-inner : f ∘ h ≡ f ∘ a ∘ h
  intro-inner {f = f} = ap (f ∘_) introl

Reassocations🔗

We often find ourselves in situations where we have an equality involving the composition of 2 morphisms, but the association is a bit off. These combinators aim to address that situation.

module _ (ab≡c : a ∘ b ≡ c) where abstract
  pulll : a ∘ (b ∘ f) ≡ c ∘ f
  pulll {f = f} =
    a ∘ b ∘ f   ≡⟨ assoc a b f ⟩≡
    (a ∘ b) ∘ f ≡⟨ ap (_∘ f) ab≡c ⟩≡
    c ∘ f ∎

  pullr : (f ∘ a) ∘ b ≡ f ∘ c
  pullr {f = f} =
    (f ∘ a) ∘ b ≡˘⟨ assoc f a b ⟩≡˘
    f ∘ (a ∘ b) ≡⟨ ap (f ∘_) ab≡c ⟩≡
    f ∘ c ∎

  pull-inner : (f ∘ a) ∘ (b ∘ g) ≡ f ∘ c ∘ g
  pull-inner {f = f} = sym (assoc _ _ _) ∙ ap (f ∘_) pulll

module _ (abc≡d : a ∘ b ∘ c ≡ d) where abstract
  pulll3 : a ∘ (b ∘ (c ∘ f)) ≡ d ∘ f
  pulll3 {f = f} =
    a ∘ b ∘ c ∘ f   ≡⟨ ap (a ∘_) (assoc _ _ _) ⟩≡
    a ∘ (b ∘ c) ∘ f ≡⟨ assoc _ _ _ ⟩≡
    (a ∘ b ∘ c) ∘ f ≡⟨ ap (_∘ f) abc≡d ⟩≡
    d ∘ f           ∎

  pullr3 : ((f ∘ a) ∘ b) ∘ c ≡ f ∘ d
  pullr3 {f = f} =
    ((f ∘ a) ∘ b) ∘ c ≡˘⟨ assoc _ _ _ ⟩≡˘
    (f ∘ a) ∘ b ∘ c   ≡˘⟨ assoc _ _ _ ⟩≡˘
    f ∘ a ∘ b ∘ c     ≡⟨ ap (f ∘_) abc≡d ⟩≡
    f ∘ d ∎

module _ (c≡ab : c ≡ a ∘ b) where abstract
  pushl : c ∘ f ≡ a ∘ (b ∘ f)
  pushl = sym (pulll (sym c≡ab))

  pushr : f ∘ c ≡ (f ∘ a) ∘ b
  pushr = sym (pullr (sym c≡ab))

  push-inner : f ∘ c ∘ g ≡ (f ∘ a) ∘ (b ∘ g)
  push-inner {f = f} = ap (f ∘_) pushl ∙ assoc _ _ _

module _ (d≡abc : d ≡ a ∘ b ∘ c) where abstract
  pushl3 : d ∘ f ≡ a ∘ (b ∘ (c ∘ f))
  pushl3 = sym (pulll3 (sym d≡abc))

  pushr3 : f ∘ d ≡ ((f ∘ a) ∘ b) ∘ c
  pushr3 = sym (pullr3 (sym d≡abc))

module _ (p : f ∘ h ≡ g ∘ i) where abstract
  extendl : f ∘ (h ∘ b) ≡ g ∘ (i ∘ b)
  extendl {b = b} =
    f ∘ (h ∘ b) ≡⟨ assoc f h b ⟩≡
    (f ∘ h) ∘ b ≡⟨ ap (_∘ b) p ⟩≡
    (g ∘ i) ∘ b ≡˘⟨ assoc g i b ⟩≡˘
    g ∘ (i ∘ b) ∎

  extendr : (a ∘ f) ∘ h ≡ (a ∘ g) ∘ i
  extendr {a = a} =
    (a ∘ f) ∘ h ≡˘⟨ assoc a f h ⟩≡˘
    a ∘ (f ∘ h) ≡⟨ ap (a ∘_) p ⟩≡
    a ∘ (g ∘ i) ≡⟨ assoc a g i ⟩≡
    (a ∘ g) ∘ i ∎

  extend-inner : a ∘ f ∘ h ∘ b ≡ a ∘ g ∘ i ∘ b
  extend-inner {a = a} = ap (a ∘_) extendl

module _ (p : a ∘ b ∘ c ≡ d ∘ f ∘ g) where abstract
  extendl3 : a ∘ (b ∘ (c ∘ h)) ≡ d ∘ (f ∘ (g ∘ h))
  extendl3 = pulll3 p ∙ sym (pulll3 refl)

  extendr3 : ((h ∘ a) ∘ b) ∘ c ≡ ((h ∘ d) ∘ f) ∘ g
  extendr3 = pullr3 p ∙ sym (pullr3 refl)

We also define some useful combinators for performing repeated pulls/pushes.

abstract
  centralize
    : f ∘ g ≡ a ∘ b
    → h ∘ i ≡ c ∘ d
    → f ∘ g ∘ h ∘ i ≡ a ∘ (b ∘ c) ∘ d
  centralize {f = f} {g = g} {a = a} {b = b} {h = h} {i = i} {c = c} {d = d} p q =
    f ∘ g ∘ h ∘ i   ≡⟨ pulll p ⟩≡
    (a ∘ b) ∘ h ∘ i ≡⟨ pullr (pushr q) ⟩≡
    a ∘ (b ∘ c) ∘ d ∎

  centralizel
    : f ∘ g ≡ a ∘ b
    → f ∘ g ∘ h ∘ i ≡ a ∘ (b ∘ h) ∘ i
  centralizel p = centralize p refl

  centralizer
    : h ∘ i ≡ c ∘ d
    → f ∘ g ∘ h ∘ i ≡ f ∘ (g ∘ c) ∘ d
  centralizer p = centralize refl p

  disperse
    : f ∘ g ≡ a ∘ b
    → h ∘ i ≡ c ∘ d
    → f ∘ (g ∘ h) ∘ i ≡ a ∘ b ∘ c ∘ d
  disperse {f = f} {g = g} {a = a} {b = b} {h = h} {i = i} {c = c} {d = d} p q =
    f ∘ (g ∘ h) ∘ i ≡⟨ pushr (pullr q) ⟩≡
    (f ∘ g) ∘ c ∘ d ≡⟨ pushl p ⟩≡
    a ∘ b ∘ c ∘ d ∎

  dispersel
    : f ∘ g ≡ a ∘ b
    → f ∘ (g ∘ h) ∘ i ≡ a ∘ b ∘ h ∘ i
  dispersel p = disperse p refl

  disperser
    : h ∘ i ≡ c ∘ d
    → f ∘ (g ∘ h) ∘ i ≡ f ∘ g ∘ c ∘ d
  disperser p = disperse refl p

Cancellation🔗

These lemmas do 2 things at once: rearrange parenthesis, and also remove things that are equal to id.

module _ (inv : h ∘ i ≡ id) where abstract
  cancell : h ∘ (i ∘ f) ≡ f
  cancell {f = f} =
    h ∘ (i ∘ f) ≡⟨ pulll inv ⟩≡
    id ∘ f      ≡⟨ idl f ⟩≡
    f           ∎

  cancelr : (f ∘ h) ∘ i ≡ f
  cancelr {f = f} =
    (f ∘ h) ∘ i ≡⟨ pullr inv ⟩≡
    f ∘ id      ≡⟨ idr f ⟩≡
    f           ∎

  insertl : f ≡ h ∘ (i ∘ f)
  insertl = sym cancell

  insertr : f ≡ (f ∘ h) ∘ i
  insertr = sym cancelr

  cancel-inner : (f ∘ h) ∘ (i ∘ g) ≡ f ∘ g
  cancel-inner = pulll cancelr

  insert-inner : f ∘ g ≡ (f ∘ h) ∘ (i ∘ g)
  insert-inner = pushl insertr

  deleter : (f ∘ g ∘ h) ∘ i ≡ f ∘ g
  deleter = pullr cancelr

  deletel : h ∘ (i ∘ f) ∘ g ≡ f ∘ g
  deletel = pulll cancell

module _ (inv : g ∘ h ∘ i ≡ id) where abstract
  cancell3 : g ∘ (h ∘ (i ∘ f)) ≡ f
  cancell3 {f = f} = pulll3 inv ∙ idl f

  cancelr3 : ((f ∘ g) ∘ h) ∘ i ≡ f
  cancelr3 {f = f} = pullr3 inv ∙ idr f

  insertl3 : f ≡ g ∘ (h ∘ (i ∘ f))
  insertl3 = sym cancell3

  insertr3 : f ≡ ((f ∘ g) ∘ h) ∘ i
  insertr3 = sym cancelr3

We also have combinators which combine expanding on one side with a cancellation on the other side:

lswizzle : g ≡ h ∘ i → f ∘ h ≡ id → f ∘ g ≡ i
lswizzle {g = g} {h = h} {i = i} {f = f} p q =
  f ∘ g     ≡⟨ ap₂ _∘_ refl p ⟩≡
  f ∘ h ∘ i ≡⟨ cancell q ⟩≡
  i         ∎

rswizzle : g ≡ i ∘ h → h ∘ f ≡ id → g ∘ f ≡ i
rswizzle {g = g} {i = i} {h = h} {f = f} p q =
  g ∘ f       ≡⟨ ap₂ _∘_ p refl ⟩≡
  (i ∘ h) ∘ f ≡⟨ cancelr q ⟩≡
  i           ∎

The following “swizzle” operation can be pictured as flipping a commutative square along an axis, provided the morphisms on that axis are invertible.

swizzle : f ∘ g ≡ h ∘ i → g ∘ g' ≡ id → h' ∘ h ≡ id → h' ∘ f ≡ i ∘ g'
swizzle {f = f} {g = g} {h = h} {i = i} {g' = g'} {h' = h'} p q r =
  lswizzle (sym (assoc _ _ _ ∙ rswizzle (sym p) q)) r

Isomorphisms🔗

These lemmas are useful for proving that partial inverses to an isomorphism are unique. There’s a helper for proving uniqueness of left inverses, of right inverses, and for proving that any left inverse must match any right inverse.

module _ {y z} (f : y ≅ z) where abstract
  open _≅_

  left-inv-unique
    : ∀ {g h}
    → g ∘ f .to ≡ id → h ∘ f .to ≡ id
    → g ≡ h
  left-inv-unique {g = g} {h = h} p q =
    g                   ≡⟨ intror (f .invl) ⟩≡
    g ∘ f .to ∘ f .from ≡⟨ extendl (p ∙ sym q) ⟩≡
    h ∘ f .to ∘ f .from ≡⟨ elimr (f .invl) ⟩≡
    h                   ∎

  left-right-inv-unique
    : ∀ {g h}
    → g ∘ f .to ≡ id → f .to ∘ h ≡ id
    → g ≡ h
  left-right-inv-unique {g = g} {h = h} p q =
    g                    ≡⟨ intror (f .invl) ⟩≡
    g ∘ f .to ∘ f .from  ≡⟨ cancell p ⟩≡
    f .from              ≡⟨ intror q ⟩≡
    f .from ∘ f .to ∘ h  ≡⟨ cancell (f .invr) ⟩≡
    h                    ∎

  right-inv-unique
    : ∀ {g h}
    → f .to ∘ g ≡ id → f .to ∘ h ≡ id
    → g ≡ h
  right-inv-unique {g = g} {h} p q =
    g                     ≡⟨ introl (f .invr) ⟩≡
    (f .from ∘ f .to) ∘ g ≡⟨ pullr (p ∙ sym q) ⟩≡
    f .from ∘ f .to ∘ h   ≡⟨ cancell (f .invr) ⟩≡
    h                     ∎

If we have a commuting triangle of isomorphisms, then we can flip one of the sides to obtain a new commuting triangle of isomorphisms.

Iso-swapr :
  ∀ {a : x ≅ y} {b : y ≅ z} {c : x ≅ z}
  → a ∘Iso b ≡ c
  → a ≡ c ∘Iso (b Iso⁻¹)
Iso-swapr {a = a} {b = b} {c = c} p = ≅-path $
  a .to                     ≡⟨ introl (b .invr) ⟩≡
  (b .from ∘ b .to) ∘ a .to ≡⟨ pullr (ap to p) ⟩≡
  b .from ∘ c .to           ∎

Iso-swapl :
  ∀ {a : x ≅ y} {b : y ≅ z} {c : x ≅ z}
  → a ∘Iso b ≡ c
  → b ≡ (a Iso⁻¹) ∘Iso c
Iso-swapl {a = a} {b = b} {c = c} p = ≅-path $
  b .to                   ≡⟨ intror (a .invl) ⟩≡
  b .to ∘ a .to ∘ a .from ≡⟨ pulll (ap to p) ⟩≡
  c .to ∘ a .from         ∎

Assume we have a prism of isomorphisms, as in the following diagram:

If the top, front, left, and right faces all commute, then so does the bottom face.

Iso-prism : ∀ {a : u ≅ v} {b : v ≅ w} {c : u ≅ w}
      → {d : u ≅ x} {e : v ≅ y} {f : w ≅ z}
      → {g : x ≅ y} {h : y ≅ z} {i : x ≅ z}
      → a ∘Iso b ≡ c
      → a ∘Iso e ≡ d ∘Iso g
      → b ∘Iso f ≡ e ∘Iso h
      → c ∘Iso f ≡ d ∘Iso i
      → g ∘Iso h ≡ i
Iso-prism {a = a} {b} {c} {d} {e} {f} {g} {h} {i} top left right front =
  ≅-path $
    h .to ∘ g .to                                           ≡⟨ ap₂ _∘_ (ap to (Iso-swapl (sym right))) (ap to (Iso-swapl (sym left)) ∙ sym (assoc _ _ _)) ⟩≡
    ((f .to ∘ b .to) ∘ e .from) ∘ (e .to ∘ a .to ∘ d .from) ≡⟨ cancel-inner (e .invr) ⟩≡
    (f .to ∘ b .to) ∘ (a .to ∘ d .from)                     ≡⟨ pull-inner (ap to top) ⟩≡
    f .to ∘ c .to ∘ d .from                                 ≡⟨ assoc _ _ _ ∙ sym (ap to (Iso-swapl (sym front))) ⟩≡
    i .to ∎

Notation🔗

When doing equational reasoning, it’s often somewhat clumsy to have to write ap (f ∘_) p when proving that f ∘ g ≡ f ∘ h. To fix this, we steal some cute mixfix notation from agda-categories which allows us to write ≡⟨ refl⟩∘⟨ p ⟩ instead, which is much more aesthetically pleasing!

_⟩∘⟨_ : f ≡ h → g ≡ i → f ∘ g ≡ h ∘ i
_⟩∘⟨_ = ap₂ _∘_

infixr  _⟩∘⟨_

refl⟩∘⟨_ : g ≡ h → f ∘ g ≡ f ∘ h
refl⟩∘⟨_ {f = f} p = ap (f ∘_) p

_⟩∘⟨refl : f ≡ h → f ∘ g ≡ h ∘ g
_⟩∘⟨refl {g = g} p = ap (_∘ g) p

infix  refl⟩∘⟨_
infix  _⟩∘⟨refl