{-
  Definitions for functions
-}
{-# OPTIONS --cubical --safe #-}
module Cubical.Foundations.Function where

open import Cubical.Core.Everything

open import Cubical.Foundations.Prelude

-- The identity function
idfun : ∀ {ℓ} → (A : Type ℓ) → A → A
idfun _ x = x

infixr  _∘_

_∘_ : ∀ {ℓ ℓ′ ℓ″} {A : Type ℓ} {B : A → Type ℓ′} {C : (a : A) → B a → Type ℓ″}
        (g : {a : A} → (b : B a) → C a b) → (f : (a : A) → B a) → (a : A) → C a (f a)
g ∘ f = λ x → g (f x)

∘-assoc : ∀ {ℓ ℓ′ ℓ″ ℓ‴} {A : Type ℓ} {B : A → Type ℓ′} {C : (a : A) → B a → Type ℓ″} {D : (a : A) (b : B a) → C a b → Type ℓ‴}
            (h : {a : A} {b : B a} → (c : C a b) → D a b c) (g : {a : A} → (b : B a) → C a b) (f : (a : A) → B a)
          → (h ∘ g) ∘ f ≡ h ∘ (g ∘ f)
∘-assoc h g f i x = h (g (f x))

∘-idˡ : ∀ {ℓ ℓ′} {A : Type ℓ} {B : A → Type ℓ′} (f : (a : A) → B a) → f ∘ idfun A ≡ f
∘-idˡ f i x = f x

∘-idʳ : ∀ {ℓ ℓ′} {A : Type ℓ} {B : A → Type ℓ′} (f : (a : A) → B a) → (λ {a} → idfun (B a)) ∘ f ≡ f
∘-idʳ f i x = f x


const : ∀ {ℓ ℓ′} {A : Type ℓ} {B : Type ℓ′} → A → B → A
const x = λ _ → x


case_of_ : ∀ {ℓ ℓ'} {A : Type ℓ} {B : A → Type ℓ'} → (x : A) → (∀ x → B x) → B x
case x of f = f x

case_return_of_ : ∀ {ℓ ℓ'} {A : Type ℓ} (x : A) (B : A → Type ℓ') → (∀ x → B x) → B x
case x return P of f = f x

uncurry
  : ∀{ℓ ℓ′ ℓ″} {A : Type ℓ} {B : A → Type ℓ′} {C : (a : A) → B a → Type ℓ″}
  → ((x : A) → (y : B x) → C x y)
  → (p : Σ A B) → C (fst p) (snd p)
uncurry f (x , y) = f x y

module _ {ℓ ℓ'} {A : Type ℓ} {B : Type ℓ'} where
  -- Notions of 'coherently constant' functions for low dimensions.
  -- These are the properties of functions necessary to e.g. eliminate
  -- from the propositional truncation.

  -- 2-Constant functions are coherently constant if B is a set.
  2-Constant : (A → B) → Type _
  2-Constant f = ∀ x y → f x ≡ f y

  2-Constant-isProp : isSet B → (f : A → B) → isProp (2-Constant f)
  2-Constant-isProp Bset f link1 link2 i x y j
    = Bset (f x) (f y) (link1 x y) (link2 x y) i j

  -- 3-Constant functions are coherently constant if B is a groupoid.
  record 3-Constant (f : A → B) : Type (ℓ-max ℓ ℓ') where
    field
      link : 2-Constant f
      coh₁ : ∀ x y z → Square refl (link x y) (link x z) (link y z)

    coh₂ : ∀ x y z → Square (link x y) (link x z) (link y z) refl
    coh₂ x y z i j
      = hcomp (λ k → λ
              { (j = i0) → link x y i
              ; (i = i0) → link x z (j ∧ k)
              ; (j = i1) → link x z (i ∨ k)
              ; (i = i1) → link y z j
              })
          (coh₁ x y z j i)

    link≡refl : ∀ x → refl ≡ link x x
    link≡refl x i j
      = hcomp (λ k → λ
              { (i = i0) → link x x (j ∧ ~ k)
              ; (i = i1) → link x x j
              ; (j = i0) → f x
              ; (j = i1) → link x x (~ i ∧ ~ k)
              })
          (coh₁ x x x (~ i) j)

    downleft : ∀ x y → Square refl (link x y) refl (link y x)
    downleft x y i j
      = hcomp (λ k → λ
              { (i = i0) → link x y j
              ; (i = i1) → link≡refl x (~ k) j
              ; (j = i0) → f x
              ; (j = i1) → link y x i
              })
          (coh₁ x y x i j)

    link≡symlink : ∀ x y → link x y ≡ sym (link y x)
    link≡symlink x y i j
      = hcomp (λ k → λ
              { (i = i0) → link x y j
              ; (i = i1) → link y x (~ j ∨ ~ k)
              ; (j = i0) → f x
              ; (j = i1) → link y x (i ∧ ~ k)
              })
          (downleft x y i j)