open import 1Lab.Path

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.

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)

module _ (a≡id : a ≡ id) where

  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 ∎

  introl : f ≡ a ∘ f
  introl = sym eliml

  intror : f ≡ f ∘ a
  intror = sym elimr

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

  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 ∎

module _ (c≡ab : c ≡ a ∘ b) where

  pushl : c ∘ f ≡ a ∘ (b ∘ f)
  pushl = sym (pulll (sym c≡ab))

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

module _ (p : f ∘ h ≡ g ∘ i) where
  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

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

  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

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
  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                     ∎

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 define 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 _∘_

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