156 lines
5.2 KiB
Text
156 lines
5.2 KiB
Text
/-
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Copyright (c) 2014 Microsoft Corporation. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Author: Leonardo de Moura, Jeremy Avigad, Haitao Zhang
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General operations on functions.
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-/
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import logic.cast
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namespace function
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variables {A : Type} {B : Type} {C : Type} {D : Type} {E : Type}
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definition compose [reducible] [unfold-f] (f : B → C) (g : A → B) : A → C :=
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λx, f (g x)
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definition compose_right [reducible] [unfold-f] (f : B → B → B) (g : A → B) : B → A → B :=
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λ b a, f b (g a)
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definition compose_left [reducible] [unfold-f] (f : B → B → B) (g : A → B) : A → B → B :=
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λ a b, f (g a) b
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definition id [reducible] [unfold-f] (a : A) : A :=
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a
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definition on_fun [reducible] [unfold-f] (f : B → B → C) (g : A → B) : A → A → C :=
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λx y, f (g x) (g y)
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definition combine [reducible] [unfold-f] (f : A → B → C) (op : C → D → E) (g : A → B → D)
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: A → B → E :=
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λx y, op (f x y) (g x y)
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definition const [reducible] [unfold-f] (B : Type) (a : A) : B → A :=
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λx, a
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definition dcompose [reducible] [unfold-f] {B : A → Type} {C : Π {x : A}, B x → Type}
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(f : Π {x : A} (y : B x), C y) (g : Πx, B x) : Πx, C (g x) :=
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λx, f (g x)
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definition swap [reducible] [unfold-f] {C : A → B → Type} (f : Πx y, C x y) : Πy x, C x y :=
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λy x, f x y
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definition app [reducible] {B : A → Type} (f : Πx, B x) (x : A) : B x :=
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f x
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definition curry [reducible] [unfold-f] : (A × B → C) → A → B → C :=
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λ f a b, f (a, b)
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definition uncurry [reducible] [unfold-c 5] : (A → B → C) → (A × B → C) :=
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λ f p, match p with (a, b) := f a b end
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theorem curry_uncurry (f : A → B → C) : curry (uncurry f) = f :=
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rfl
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theorem uncurry_curry (f : A × B → C) : uncurry (curry f) = f :=
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funext (λ p, match p with (a, b) := rfl end)
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precedence `∘'`:60
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precedence `on`:1
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precedence `$`:1
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infixr ∘ := compose
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infixr ∘' := dcompose
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infixl on := on_fun
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infixr $ := app
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notation f `-[` op `]-` g := combine f op g
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lemma left_id (f : A → B) : id ∘ f = f := rfl
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lemma right_id (f : A → B) : f ∘ id = f := rfl
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theorem compose.assoc (f : C → D) (g : B → C) (h : A → B) : (f ∘ g) ∘ h = f ∘ (g ∘ h) := rfl
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theorem compose.left_id (f : A → B) : id ∘ f = f := rfl
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theorem compose.right_id (f : A → B) : f ∘ id = f := rfl
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theorem compose_const_right (f : B → C) (b : B) : f ∘ (const A b) = const A (f b) := rfl
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definition injective (f : A → B) : Prop := ∀ ⦃a₁ a₂⦄, f a₁ = f a₂ → a₁ = a₂
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theorem injective_compose {g : B → C} {f : A → B} (Hg : injective g) (Hf : injective f) :
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injective (g ∘ f) :=
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take a₁ a₂, assume Heq, Hf (Hg Heq)
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definition surjective (f : A → B) : Prop := ∀ b, ∃ a, f a = b
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theorem surjective_compose {g : B → C} {f : A → B} (Hg : surjective g) (Hf : surjective f) :
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surjective (g ∘ f) :=
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take c,
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obtain b (Hb : g b = c), from Hg c,
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obtain a (Ha : f a = b), from Hf b,
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exists.intro a (eq.trans (congr_arg g Ha) Hb)
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definition bijective (f : A → B) := injective f ∧ surjective f
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theorem bijective_compose {g : B → C} {f : A → B} (Hg : bijective g) (Hf : bijective f) :
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bijective (g ∘ f) :=
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obtain Hginj Hgsurj, from Hg,
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obtain Hfinj Hfsurj, from Hf,
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and.intro (injective_compose Hginj Hfinj) (surjective_compose Hgsurj Hfsurj)
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-- g is a left inverse to f
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definition left_inverse (g : B → A) (f : A → B) : Prop := ∀x, g (f x) = x
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definition has_left_inverse (f : A → B) : Prop := ∃ finv : B → A, left_inverse finv f
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-- g is a right inverse to f
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definition right_inverse (g : B → A) (f : A → B) : Prop := left_inverse f g
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definition has_right_inverse (f : A → B) : Prop := ∃ finv : B → A, right_inverse finv f
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theorem injective_of_has_left_inverse {f : A → B} : has_left_inverse f → injective f :=
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assume h, take a b, assume faeqfb,
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obtain (finv : B → A) (inv : left_inverse finv f), from h,
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calc a = finv (f a) : by rewrite inv
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... = finv (f b) : faeqfb
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... = b : by rewrite inv
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theorem right_inverse_of_injective_of_left_inverse {f : A → B} {g : B → A}
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(injf : injective f) (lfg : left_inverse f g) :
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right_inverse f g :=
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take x,
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have H : f (g (f x)) = f x, from lfg (f x),
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injf H
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theorem surjective_of_has_right_inverse {f : A → B} : has_right_inverse f → surjective f :=
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assume h, take b,
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obtain (finv : B → A) (inv : right_inverse finv f), from h,
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let a : A := finv b in
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have h : f a = b, from calc
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f a = (f ∘ finv) b : rfl
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... = id b : by rewrite inv
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... = b : rfl,
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exists.intro a h
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theorem left_inverse_of_surjective_of_right_inverse {f : A → B} {g : B → A}
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(surjf : surjective f) (rfg : right_inverse f g) :
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left_inverse f g :=
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take y,
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obtain x (Hx : f x = y), from surjf y,
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calc
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f (g y) = f (g (f x)) : Hx
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... = f x : rfg
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... = y : Hx
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theorem injective_id : injective (@id A) := take a₁ a₂ H, H
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theorem surjective_id : surjective (@id A) := take a, exists.intro a rfl
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theorem bijective_id : bijective (@id A) := and.intro injective_id surjective_id
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end function
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-- copy reducible annotations to top-level
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export [reduce-hints] [unfold-hints] function
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