Programming Language Foundations in Agda

module Assignment2 where

YOUR NAME AND EMAIL GOES HERE

Introduction

You must do all the exercises labelled “(recommended)”.

Exercises labelled “(stretch)” are there to provide an extra challenge. You don’t need to do all of these, but should attempt at least a few.

Exercises labelled “(practice)” are included for those who want extra practice.

Submit your homework using Gradescope. Go to the course page under Learn. Select Assessment, then select Assignment Submission. Please ensure your files execute correctly under Agda!

Good Scholarly Practice.

Please remember the University requirement as regards all assessed work. Details about this can be found at:

https://web.inf.ed.ac.uk/infweb/admin/policies/academic-misconduct

Furthermore, you are required to take reasonable measures to protect your assessed work from unauthorised access. For example, if you put any such work on a public repository then you must set access permissions appropriately (generally permitting access only to yourself, or your group in the case of group practicals).

Connectives

module Connectives where

Imports

  import Relation.Binary.PropositionalEquality as Eq
  open Eq using (_≡_; refl)
  open Eq.≡-Reasoning
  open import Data.Nat using (ℕ)
  open import Function using (_∘_)
  open import plfa.part1.Isomorphism using (_≃_; _≲_; extensionality; _⇔_)
  open plfa.part1.Isomorphism.≃-Reasoning

  open import plfa.part1.Connectives
    hiding (⊎-weak-×; ⊎×-implies-×⊎)

Exercise `⇔≃×` (practice)

Show that A ⇔ B as defined earlier is isomorphic to (A → B) × (B → A).

  -- Your code goes here

Exercise `⊎-comm` (recommended)

Show sum is commutative up to isomorphism.

  -- Your code goes here

Exercise `⊎-assoc` (practice)

Show sum is associative up to isomorphism.

  -- Your code goes here

Exercise `⊥-identityˡ` (recommended)

Show empty is the left identity of sums up to isomorphism.

  -- Your code goes here

Exercise `⊥-identityʳ` (practice)

Show empty is the right identity of sums up to isomorphism.

  -- Your code goes here

Exercise `⊎-weak-×` (recommended)

Show that the following property holds:

  postulate
    ⊎-weak-× : ∀ {A B C : Set} → (A ⊎ B) × C → A ⊎ (B × C)

This is called a weak distributive law. Give the corresponding distributive law, and explain how it relates to the weak version.

  -- Your code goes here

Exercise `⊎×-implies-×⊎` (practice)

Show that a disjunct of conjuncts implies a conjunct of disjuncts:

  postulate
    ⊎×-implies-×⊎ : ∀ {A B C D : Set} → (A × B) ⊎ (C × D) → (A ⊎ C) × (B ⊎ D)

Does the converse hold? If so, prove; if not, give a counterexample.

  -- Your code goes here

Negation

module Negation where

Imports

  open import Relation.Binary.PropositionalEquality using (_≡_; refl)
  open import Data.Nat using (ℕ; zero; suc)
  open import plfa.part1.Isomorphism using (_≃_; extensionality)
  open import plfa.part1.Connectives
  open import plfa.part1.Negation hiding (Stable)

Exercise `<-irreflexive` (recommended)

Using negation, show that strict inequality is irreflexive, that is, n < n holds for no n.

  -- Your code goes here

Exercise `trichotomy` (practice)

Show that strict inequality satisfies trichotomy, that is, for any naturals m and n exactly one of the following holds:

m < n
m ≡ n
m > n

Here “exactly one” means that not only one of the three must hold, but that when one holds the negation of the other two must also hold.

  -- Your code goes here

Exercise `⊎-dual-×` (recommended)

Show that conjunction, disjunction, and negation are related by a version of De Morgan’s Law.

¬ (A ⊎ B) ≃ (¬ A) × (¬ B)

This result is an easy consequence of something we’ve proved previously.

  -- Your code goes here

Do we also have the following?

¬ (A × B) ≃ (¬ A) ⊎ (¬ B)

If so, prove; if not, can you give a relation weaker than isomorphism that relates the two sides?

Exercise `Classical` (stretch)

Consider the following principles:

Excluded Middle: A ⊎ ¬ A, for all A
Double Negation Elimination: ¬ ¬ A → A, for all A
Peirce’s Law: ((A → B) → A) → A, for all A and B.
Implication as disjunction: (A → B) → ¬ A ⊎ B, for all A and B.
De Morgan: ¬ (¬ A × ¬ B) → A ⊎ B, for all A and B.

Show that each of these implies all the others.

  -- Your code goes here

Exercise `Stable` (stretch)

Say that a formula is stable if double negation elimination holds for it:

  Stable : Set → Set
  Stable A = ¬ ¬ A → A

Show that any negated formula is stable, and that the conjunction of two stable formulas is stable.

  -- Your code goes here

Quantifiers

module Quantifiers where

Imports

  import Relation.Binary.PropositionalEquality as Eq
  open Eq using (_≡_; refl)
  open import Data.Nat using (ℕ; zero; suc; _+_; _*_)
  open import Relation.Nullary using (¬_)
  open import Function using (_∘_)
  open import plfa.part1.Isomorphism using (_≃_; extensionality; ∀-extensionality)
  open import plfa.part1.Connectives
    hiding (Tri)
  open import plfa.part1.Quantifiers
    hiding (∀-distrib-×; ⊎∀-implies-∀⊎; ∃-distrib-⊎; ∃×-implies-×∃; ∃¬-implies-¬∀; Tri)

Exercise `∀-distrib-×` (recommended)

Show that universals distribute over conjunction:

  postulate
    ∀-distrib-× : ∀ {A : Set} {B C : A → Set} →
      (∀ (x : A) → B x × C x) ≃ (∀ (x : A) → B x) × (∀ (x : A) → C x)

Compare this with the result (→-distrib-×) in Chapter Connectives.

Hint: you will need to use ∀-extensionality.

Exercise `⊎∀-implies-∀⊎` (practice)

Show that a disjunction of universals implies a universal of disjunctions:

  postulate
    ⊎∀-implies-∀⊎ : ∀ {A : Set} {B C : A → Set} →
      (∀ (x : A) → B x) ⊎ (∀ (x : A) → C x) → ∀ (x : A) → B x ⊎ C x

Does the converse hold? If so, prove; if not, explain why.

Exercise `∀-×` (practice)

Consider the following type.

  data Tri : Set where
    aa : Tri
    bb : Tri
    cc : Tri

Let B be a type indexed by Tri, that is B : Tri → Set. Show that ∀ (x : Tri) → B x is isomorphic to B aa × B bb × B cc.

Hint: you will need to use ∀-extensionality.

Exercise `∃-distrib-⊎` (recommended)

Show that existentials distribute over disjunction:

  postulate
    ∃-distrib-⊎ : ∀ {A : Set} {B C : A → Set} →
      ∃[ x ] (B x ⊎ C x) ≃ (∃[ x ] B x) ⊎ (∃[ x ] C x)

Exercise `∃×-implies-×∃` (practice)

Show that an existential of conjunctions implies a conjunction of existentials:

  postulate
    ∃×-implies-×∃ : ∀ {A : Set} {B C : A → Set} →
      ∃[ x ] (B x × C x) → (∃[ x ] B x) × (∃[ x ] C x)

Does the converse hold? If so, prove; if not, explain why.

Exercise `∃-⊎` (practice)

Let Tri and B be as in Exercise ∀-×. Show that ∃[ x ] B x is isomorphic to B aa ⊎ B bb ⊎ B cc.

Exercise `∃-even-odd` (practice)

How do the proofs become more difficult if we replace m * 2 and 1 + m * 2 by 2 * m and 2 * m + 1? Rewrite the proofs of ∃-even and ∃-odd when restated in this way.

  -- Your code goes here

Exercise `∃-+-≤` (practice)

Show that y ≤ z holds if and only if there exists a x such that x + y ≡ z.

  -- Your code goes here

Exercise `∃¬-implies-¬∀` (recommended)

Show that existential of a negation implies negation of a universal:

  postulate
    ∃¬-implies-¬∀ : ∀ {A : Set} {B : A → Set}
      → ∃[ x ] (¬ B x)
        --------------
      → ¬ (∀ x → B x)

Does the converse hold? If so, prove; if not, explain why.

Exercise `Bin-isomorphism` (stretch)

Recall that Exercises Bin, Bin-laws, and Bin-predicates define a datatype Bin of bitstrings representing natural numbers, and asks you to define the following functions and predicates:

to   : ℕ → Bin
from : Bin → ℕ
Can  : Bin → Set

And to establish the following properties:

from (to n) ≡ n

----------
Can (to n)

Can b
---------------
to (from b) ≡ b

Using the above, establish that there is an isomorphism between ℕ and ∃[ b ] Can b.

We recommend proving the following lemmas which show that, for a given binary number b, there is only one proof of One b and similarly for Can b.

≡One : ∀ {b : Bin} (o o′ : One b) → o ≡ o′

≡Can : ∀ {b : Bin} (cb cb′ : Can b) → cb ≡ cb′

Many of the alternatives for proving to∘from turn out to be tricky. However, the proof can be straightforward if you use the following lemma, which is a corollary of ≡Can.

proj₁≡→Can≡ : {cb cb′ : ∃[ b ] Can b} → proj₁ cb ≡ proj₁ cb′ → cb ≡ cb′

  -- Your code goes here

Decidable

module Decidable where

Imports

  import Relation.Binary.PropositionalEquality as Eq
  open Eq using (_≡_; refl)
  open Eq.≡-Reasoning
  open import Data.Nat using (ℕ; zero; suc)
  open import Data.Product using (_×_) renaming (_,_ to ⟨_,_⟩)
  open import Data.Sum using (_⊎_; inj₁; inj₂)
  open import Relation.Nullary using (¬_)
  open import Relation.Nullary.Negation using ()
    renaming (contradiction to ¬¬-intro)
  open import Data.Unit using (⊤; tt)
  open import Data.Empty using (⊥; ⊥-elim)
  open import plfa.part1.Relations using (_<_; z<s; s<s)
  open import plfa.part1.Isomorphism using (_⇔_)

  open import plfa.part1.Decidable
    hiding (_<?_; _≡ℕ?_; ∧-×; ∨-⊎; not-¬; _iff_; _⇔-dec_; iff-⇔)

Exercise `_<?_` (recommended)

Analogous to the function above, define a function to decide strict inequality:

  postulate
    _<?_ : ∀ (m n : ℕ) → Dec (m < n)

  -- Your code goes here

Exercise `_≡ℕ?_` (practice)

Define a function to decide whether two naturals are equal:

  postulate
    _≡ℕ?_ : ∀ (m n : ℕ) → Dec (m ≡ n)

  -- Your code goes here

Exercise `erasure` (practice)

Show that erasure relates corresponding boolean and decidable operations:

  postulate
    ∧-× : ∀ {A B : Set} (x : Dec A) (y : Dec B) → ⌊ x ⌋ ∧ ⌊ y ⌋ ≡ ⌊ x ×-dec y ⌋
    ∨-⊎ : ∀ {A B : Set} (x : Dec A) (y : Dec B) → ⌊ x ⌋ ∨ ⌊ y ⌋ ≡ ⌊ x ⊎-dec y ⌋
    not-¬ : ∀ {A : Set} (x : Dec A) → not ⌊ x ⌋ ≡ ⌊ ¬? x ⌋

Exercise `iff-erasure` (recommended)

Give analogues of the _⇔_ operation from Chapter Isomorphism, operation on booleans and decidables, and also show the corresponding erasure:

  postulate
    _iff_ : Bool → Bool → Bool
    _⇔-dec_ : ∀ {A B : Set} → Dec A → Dec B → Dec (A ⇔ B)
    iff-⇔ : ∀ {A B : Set} (x : Dec A) (y : Dec B) → ⌊ x ⌋ iff ⌊ y ⌋ ≡ ⌊ x ⇔-dec y ⌋

  -- Your code goes here

Exercise `False` (practice)

Give analogues of True, toWitness, and fromWitness which work with negated properties. Call these False, toWitnessFalse, and fromWitnessFalse.

Exercise `Bin-decidable` (stretch)

Recall that Exercises Bin, Bin-laws, and Bin-predicates define a datatype Bin of bitstrings representing natural numbers, and asks you to define the following predicates:

One  : Bin → Set
Can  : Bin → Set

Show that both of the above are decidable.

One? : ∀ (b : Bin) → Dec (One b)
Can? : ∀ (b : Bin) → Dec (Can b)

YOUR NAME AND EMAIL GOES HERE

Introduction

Good Scholarly Practice.

Connectives

Imports

Exercise ⇔≃× (practice)

Exercise ⊎-comm (recommended)

Exercise ⊎-assoc (practice)

Exercise ⊥-identityˡ (recommended)

Exercise ⊥-identityʳ (practice)

Exercise ⊎-weak-× (recommended)

Exercise ⊎×-implies-×⊎ (practice)

Negation

Imports

Exercise <-irreflexive (recommended)

Exercise trichotomy (practice)

Exercise ⊎-dual-× (recommended)

Exercise Classical (stretch)

Exercise Stable (stretch)

Quantifiers

Imports

Exercise ∀-distrib-× (recommended)

Exercise ⊎∀-implies-∀⊎ (practice)

Exercise ∀-× (practice)

Exercise ∃-distrib-⊎ (recommended)

Exercise ∃×-implies-×∃ (practice)

Exercise ∃-⊎ (practice)

Exercise ∃-even-odd (practice)

Exercise ∃-+-≤ (practice)

Exercise ∃¬-implies-¬∀ (recommended)

Exercise Bin-isomorphism (stretch)

Decidable

Imports

Exercise _<?_ (recommended)

Exercise _≡ℕ?_ (practice)

Exercise erasure (practice)

Exercise iff-erasure (recommended)

Exercise False (practice)

Exercise Bin-decidable (stretch)

Exercise `⇔≃×` (practice)

Exercise `⊎-comm` (recommended)

Exercise `⊎-assoc` (practice)

Exercise `⊥-identityˡ` (recommended)

Exercise `⊥-identityʳ` (practice)

Exercise `⊎-weak-×` (recommended)

Exercise `⊎×-implies-×⊎` (practice)

Exercise `<-irreflexive` (recommended)

Exercise `trichotomy` (practice)

Exercise `⊎-dual-×` (recommended)

Exercise `Classical` (stretch)

Exercise `Stable` (stretch)

Exercise `∀-distrib-×` (recommended)

Exercise `⊎∀-implies-∀⊎` (practice)

Exercise `∀-×` (practice)

Exercise `∃-distrib-⊎` (recommended)

Exercise `∃×-implies-×∃` (practice)

Exercise `∃-⊎` (practice)

Exercise `∃-even-odd` (practice)

Exercise `∃-+-≤` (practice)

Exercise `∃¬-implies-¬∀` (recommended)

Exercise `Bin-isomorphism` (stretch)

Exercise `_<?_` (recommended)

Exercise `_≡ℕ?_` (practice)

Exercise `erasure` (practice)

Exercise `iff-erasure` (recommended)

Exercise `False` (practice)

Exercise `Bin-decidable` (stretch)