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Lists, added All and Any

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wadler 2018-02-14 19:34:10 -02:00
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1
README.md
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@ -52,7 +52,6 @@ https://github.com/agda/agda/blob/master/src/data/emacs-mode/agda-input.el#L194
C-c C-c x split on variable x
C-c C-space fill in hole
Also see [here](
http://agda.readthedocs.io/en/latest/tools/emacs-mode.html
).

160
src/Lists.lagda
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@ -13,8 +13,12 @@ introducing polymorphic types and functions.
import Relation.Binary.PropositionalEquality as Eq
open Eq using (_≡_; refl; sym; trans; cong)
open Eq.≡-Reasoning
open import Data.Nat using (ℕ; zero; suc; _+_; _*_)
open import Data.Nat using (ℕ; zero; suc; _+_; _*_; _≤_; s≤s; z≤n)
open import Data.Nat.Properties using (+-assoc; +-identityˡ; +-identityʳ; *-assoc; *-identityˡ; *-identityʳ)
open import Relation.Nullary using (¬_)
open import Data.Product using (_×_) renaming (_,_ to ⟨_,_⟩)
open import Isomorphism using (_≃_)
open import Function using (_∘′_)
\end{code}
## Lists
@ -601,7 +605,12 @@ Use fold to define a function to find the product of a list of numbers.
Show that fold and append are related as follows.
\begin{code}
postulate
foldr-++ : ∀ {A B : Set} (_⊗_ : A → B → B) (e : B) (xs ys : List A) →
foldr _⊗_ e (xs ++ ys) ≡ foldr _⊗_ (foldr _⊗_ e ys) xs
\end{code}
*Exercise* (`map-is-foldr`) <!-- `mapIsFold` in Data.List.Properties -->
@ -660,9 +669,9 @@ list, are all examples of monoids.
If `_⊕_` and `e` form a monoid, then we can re-express fold on the
same operator and an arbitrary value.
\begin{code}
fold-monoid : ∀ {A : Set} (_⊗_ : A → A → A) (e : A) → Monoid _⊗_ e →
∀ (y : A) (xs : List A) → foldr _⊗_ e xs ⊗ y ≡ foldr _⊗_ y xs
fold-monoid _⊗_ e ⊗-monoid y [] =
foldr-monoid : ∀ {A : Set} (_⊗_ : A → A → A) (e : A) → Monoid _⊗_ e →
∀ (xs : List A) (y : A) → foldr _⊗_ e xs ⊗ y ≡ foldr _⊗_ y xs
foldr-monoid _⊗_ e ⊗-monoid [] y =
begin
foldr _⊗_ e [] ⊗ y
≡⟨⟩
@ -672,35 +681,158 @@ fold-monoid _⊗_ e ⊗-monoid y [] =
≡⟨⟩
foldr _⊗_ y []
∎
fold-monoid _⊗_ e ⊗-monoid y (x ∷ xs) =
foldr-monoid _⊗_ e ⊗-monoid (x ∷ xs) y =
begin
foldr _⊗_ e (x ∷ xs) ⊗ y
≡⟨⟩
(x ⊗ foldr _⊗_ e xs) ⊗ y
≡⟨ assoc ⊗-monoid x (foldr _⊗_ e xs) y ⟩
x ⊗ (foldr _⊗_ e xs ⊗ y)
≡⟨ cong (x ⊗_) (fold-monoid _⊗_ e ⊗-monoid y xs) ⟩
≡⟨ cong (x ⊗_) (foldr-monoid _⊗_ e ⊗-monoid xs y) ⟩
x ⊗ (foldr _⊗_ y xs)
≡⟨⟩
foldr _⊗_ y (x ∷ xs)
∎
\end{code}
As a consequence of `foldr-++` and `fold-monoid`, it is easy to show
As a consequence, using a previous exercise, we have the following.
\begin{code}
foldr-monoid-++ : ∀ {A : Set} (_⊗_ : A → A → A) (e : A) → Monoid _⊗_ e →
(xs ys : List A) → foldr _⊗_ e (xs ++ ys) ≡ foldr _⊗_ e xs ⊗ foldr _⊗_ e ys
foldr-monoid-++ _⊗_ e monoid-⊗ xs ys =
begin
foldr _⊗_ e (xs ++ ys)
≡⟨ foldr-++ _⊗_ e xs ys ⟩
foldr _⊗_ (foldr _⊗_ e ys) xs
≡⟨ sym (foldr-monoid _⊗_ e monoid-⊗ xs (foldr _⊗_ e ys)) ⟩
foldr _⊗_ e xs ⊗ foldr _⊗_ e ys
∎
\end{code}
foldr _⊗_ e (xs ++ ys) ≡ foldr _⊗_ e xs ⊗ foldr _⊗_ e ys
## All and Any
We can also define predicates over lists. Two of the most important are `All` and `Any`.
## Element
Predicate `All P` holds if predicate `P` is satisfied by every element of a list.
\begin{code}
data All {A : Set} (P : A → Set) : List A → Set where
[] : All P []
_∷_ : {x : A} {xs : List A} → P x → All P xs → All P (x ∷ xs)
\end{code}
The type has two constructors, reusing the names of the same constructors for lists.
The first asserts that `All P` always holds of the empty list.
The second asserts that if `P` holds of the head of a list, and if `All P`
holds of the tail of the list, then `All P` holds for the entire list.
Agda uses type information to distinguish whether the constructor is building
a list, or evidence that `All P` holds.
For example, `All (_≤ 2)` holds of a list where every element is less
than or equal to two. Recall that `z≤n` proves `zero ≤ n` for any
`n`, and that if `m≤n` proves `m ≤ n` then `s≤s m≤n` proves `suc m ≤
suc n`, for any `m` and `n`.
\begin{code}
ex₁₀ : All (_≤ 2) ([ 0 , 1 , 2 ])
ex₁₀ = z≤n ∷ (s≤s z≤n) ∷ (s≤s (s≤s z≤n)) ∷ []
\end{code}
Here `_∷_` and `[]` are the constructors of `All P` rather than of `List A`.
The three items are proofs of `0 ≤ 2`, `1 ≤ 2`, and `2 ≤ 2`, respectively.
Predicate `Any P` holds if predicate `P` is satisfied by some element of a list.
\begin{code}
data Any {A : Set} (P : A → Set) : List A → Set where
here : {x : A} {xs : List A} → P x → Any P (x ∷ xs)
there : {x : A} {xs : List A} → Any P xs → Any P (x ∷ xs)
\end{code}
The first constructor provides evidence that the head of the list satisfies `P`,
while the second provides evidence that the tail of the list satisfies `Any P`.
For example, we can define list membership as follows.
\begin{code}
infix 4 _∈_
_∈_ : ∀ {A : Set} (x : A) (xs : List A) → Set
x ∈ xs = Any (x ≡_) xs
_∉_ : ∀ {A : Set} (x : A) (xs : List A) → Set
x ∉ xs = ¬ (x ∈ xs)
\end{code}
For example, we have `0 ∈ ([ 0 , 1 , 0 ])`. Indeed, we can demonstrate
this fact in two different ways, corresponding to the two different
occurrences of zero in the list, as the first element and as the third element.
\begin{code}
ex₁₁ ex₁₂ : 0 ∈ ([ 0 , 1 , 0 ])
ex₁₁ = here refl
ex₁₂ = there (there (here refl))
\end{code}
Further, we can demonstrate that two is not in the list, because
any possible proof that it is in the list leads to contradiction.
\begin{code}
ex₁₃ : 2 ∉ ([ 0 , 1 , 0 ])
ex₁₃ (here ())
ex₁₃ (there (here ()))
ex₁₃ (there (there (here ())))
ex₁₃ (there (there (there ())))
\end{code}
The four occurrences of `()` attest to the fact that there is no
possible evidence for the facts `2 ≡ 0`, `2 ≡ 1`, `2 ≡ 0`, and
`2 ∈ []`, respectively.
A predicate holds for every element of one list appended to another if and
only if it holds for every element of each list. Indeed, an even stronger
result is true, as we can show that the two types are isomorphic.
\begin{code}
All-++ : ∀ {A : Set} {P : A → Set} (xs ys : List A) → All P (xs ++ ys) ≃ (All P xs × All P ys)
All-++ xs ys =
record
{ to = to xs ys
; fro = fro xs ys
; invˡ = invˡ xs ys
; invʳ = invʳ xs ys
}
where
to : ∀ {A : Set} {P : A → Set} (xs ys : List A) → All P (xs ++ ys) → (All P xs × All P ys)
to [] ys AllPys = ⟨ [] , AllPys ⟩
to (x ∷ xs) ys (Px ∷ AllPxsys) with to xs ys AllPxsys
... | ⟨ AllPxs , AllPys ⟩ = ⟨ Px ∷ AllPxs , AllPys ⟩
fro : ∀ { A : Set} {P : A → Set} (xs ys : List A) → All P xs × All P ys → All P (xs ++ ys)
fro [] ys ⟨ [] , AllPys ⟩ = AllPys
fro (x ∷ xs) ys ⟨ Px ∷ AllPxs , AllPys ⟩ = Px ∷ fro xs ys ⟨ AllPxs , AllPys ⟩
invˡ : ∀ { A : Set} {P : A → Set} (xs ys : List A) → (AllPxsys : All P (xs ++ ys)) →
fro xs ys (to xs ys AllPxsys) ≡ AllPxsys
invˡ [] ys AllPys = refl
invˡ (x ∷ xs) ys (Px ∷ AllPxsys) = cong (Px ∷_) (invˡ xs ys AllPxsys)
invʳ : ∀ { A : Set} {P : A → Set} (xs ys : List A) → (AllPxsAllPys : All P xs × All P ys) →
to xs ys (fro xs ys AllPxsAllPys) ≡ AllPxsAllPys
invʳ [] ys ⟨ [] , AllPys ⟩ = refl
invʳ (x ∷ xs) ys ⟨ Px ∷ AllPxs , AllPys ⟩ rewrite invʳ xs ys ⟨ AllPxs , AllPys ⟩ = refl
\end{code}
*Exercise* `Any-++`
Prove a result similar to `All-++`, but with `Any` in place of `All`, and a suitable
replacement for `_×_`. As a consequence, demonstrate an isomorphism relating
`_∈_` and `_++_`.
*Exercise* `¬Any≃All¬`
Show that `Any` and `All` satisfy a version of De Morgan's Law.
\begin{code}
data _∈_ {A : Set} (x : A) : List A → Set where
here : {y : A} {ys : List A} → x ≡ y → x ∈ y ∷ ys
there : {y : A} {ys : List A} → x ∈ ys → x ∈ y ∷ ys
infix 4 _∈_
postulate
¬Any≃All¬ : ∀ {A : Set} (P : A → Set) (xs : List A) → ¬ Any P xs ≃ All (λ x → ¬ P x) xs
\end{code}
Do we also have the following?
¬ All P xs ≃ Any (λ x → ¬ P x) xs
If so, prove; if not, explain why.
## Unicode
∷ U+2237 PROPORTION (\::)

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src/Logic.lagda
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@ -833,6 +833,22 @@ excluded-middle-irrefutable : ∀ {A : Set} → ¬ ¬ (A ⊎ ¬ A)
excluded-middle-irrefutable k = k (inj₂ (λ a → k (inj₁ a)))
\end{code}
*Exercise* (`¬⊎≃׬`)
Show that conjunction, disjunction, and negation are related by a
version of De Morgan's Law.
\begin{code}
postulate
¬⊎≃׬ : ∀ {A B : Set} → ¬ (A ⊎ B) ≃ (¬ A) × (¬ B)
\end{code}
This result is an easy consequence of something we've proved previously.
Do we also have the following?
¬ (A × B) ≃ (¬ A) ⊎ (¬ B)
If so, prove; if not, explain why.
## Intuitive and Classical logic
@ -878,7 +894,7 @@ a disjoint sum.
(The passage above is taken from "Propositions as Types", Philip Wadler,
*Communications of the ACM*, December 2015.)
**Exercise** Prove the following six formulas are equivalent.
**Exercise** Prove the following five formulas are equivalent.
\begin{code}
lone ltwo : Level
@ -890,9 +906,7 @@ ltwo = lsuc lone
excluded-middle = ∀ {A : Set} → A ⊎ ¬ A
peirce = ∀ {A B : Set} → (((A → B) → A) → A)
implication = ∀ {A B : Set} → (A → B) → ¬ A ⊎ B
de-morgan = ∀ {A B : Set} → ¬ (¬ A × ¬ B) → A ⊎ B
de-morgan′ = ∀ {A B : Set} → ¬ (¬ A ⊎ ¬ B) → A × B
de-morgan = ∀ {A B : Set} → ¬ (A × B) → ¬ A ⊎ ¬ B
\end{code}
It turns out that an assertion such as `Set : Set` is paradoxical.

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src/extra/Dummy.agda
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@ -1,4 +1,6 @@
import Data.List.Properties
import Data.List.All
import Data.Nat.Properties
import Data.Nat.Properties.Simple
import Relation.Nullary