# Constructing Boolean algebra using type theory

Here we look at constructing logic using type theory.

module Logic.logicBasics where

open import Agda.Primitive using (Level; _⊔_; lsuc; lzero)

## Objects of Logic

### Empty / False

Type with no object - cannot ever create an object of this type. This makes it possible to define absurd functions, which map ⟂ to anything, as given nothing, one can create anything. Looking at it from a different angle, assuming any False statement can serve as a proof for anything, which is absurd.

data ⟂ : Set where

Absurd can imply anything, be it true or not. Thus, any statement can be proven using absurdity.

However, it is impossible to create an object of absurd type (as ⟂ has no constructor) and hence these functions make no sense in the “real world”, as in they can never be invoked.

### Singleton / True

Set with only one object: True.

data ⊤ : Set where
singleton : ⊤

We also have another representation of boolean objects, Bool.

open import Lang.dataStructures using (Bool; true; false)

## Operations on these objects

### Negation or the logical NOT

We use the fact that a negation of a proposition P (to exist and be true) implies ¬ P has to be false, ⟂.

¬ : ∀ {a} → Set a → Set a
¬ P = P → ⟂

for our other representation:

not : Bool → Bool
not true = false
not false = true

### Product, conjunction ∧ or the logical AND

The logic AND and OR are pretty straightforward for both representations:

data _∧_ (A B : Set) : Set where
AND : A → B → A ∧ B

infixr 4 _∧_

similarly:

_&&_ : Bool → Bool → Bool
true && x = x
false && x = false

infixr 4 _&&_

### Co-product, disjunction or the logical OR

data _∨_ (A B : Set) : Set where
inj1 : A → A ∨ B
inj2 : B → A ∨ B

infixr 4 _∨_
_||_ : Bool → Bool → Bool
true || x = true
false || x  = false

infixr 4 _||_

## Higher-order operations

These higher-order operations are built by composing the lower ones in various ways:

### Implication

Implication, or if P then Q, is derived from the well-known construction

$a ⟹ b = ¬ a ∨ b$

data _⟹_ (A B : Set) : Set where
it⟹ : (¬ A) ∨ B → A ⟹ B
impl : Bool → Bool → Bool
impl x y = (not x) || y

Notice how the two implementations are different: ⟹ is constructive, whereas impl is computatational.

### The exclusive or or XOR

data _⨁_ (A B : Set) : Set where
⨁₁ : A → B → (A ∨ B) ∧ (¬ (A ∧ B)) → A ⨁ B
xor : Bool → Bool → Bool
xor x y = (x || y) && (not (x && y))

### Absurd

The absurd pattern for proving Whatever, we discussed above:

⟂-elim : ∀ {w} {Whatever : Set w}
→ ⟂ → Whatever
⟂-elim ()

A proposition P can always be true ⊤ if ¬ P is always false ⟂. If one were to assume a contradictory proposition P, one could prove anything as a contradiction makes P absurd. This is called as “ex falso quodlibet” or “from falsity, whatever you like”. We can ⟂-elim it in such a way to prove this:

contradiction : ∀ {p w} {P : Set p} {Whatever : Set w}
→ P
→ ¬ P
→ Whatever
contradiction p (¬p) = ⟂-elim (¬p p)

### Contraposition

Consider two propositions, A and B. Now if A → B is true, then ¬ A → ¬ B will be true. In other words, if a statement A always implies another B, then B not being true would imply A not being true. This is called a contraposition and is proven in the following manner:

contrapos : ∀ {a b} {A : Set a}{B : Set b}
→ (A → B)
→ ¬ B → ¬ A
contrapos f ¬b a = contradiction (f a) ¬b

### Boolean construction

We can obviously go ahead now and implement the if-then-else semantic:

if_then_else_ : {A : Set} -> Bool -> A -> A -> A
if true  then x else y = x
if false then x else y = y

-- a more pythonic style P ? if_true : if_false (with universe polymorphism)
_then_else_ : ∀ {a} {A : Set a} -> Bool -> A -> A -> A
true then x else y = x
false then x else y = y

Note again that since this is a computational semantic, we provide implementation for only the concrete type Bool.

### Existential Quantification

Existential Quantification, or better known as “there exists” or ∃, essential indicates if a proposition can exist:

data ∃ {A : Set} (P : A → Set) : Set where
exists : {x : A}
→ P x
→ ∃ P

### Dependent proposition type

This is a dependent type, which creates propositions from a proposition family P:

∏ : {A : Set}(P : A -> Set) -> Set
∏ {A} P = (x : A) → P x

Equality