Martin Escardo 2011.


{-# OPTIONS --without-K --exact-split --safe #-}

module DecidableAndDetachable where

open import SpartanMLTT
open import Two


We look at decidable propositions and subsets (using the terminogy
"detachable" for the latter").


decidable : U  U
decidable A = A + ¬ A 

¬¬-elim : {A : U}  

 decidable A  ¬¬ A  A

¬¬-elim (inl a) f = a
¬¬-elim (inr g) f = ∅-elim(f g)

negation-preserves-decidability : {A : U}  

 decidable A  decidable(¬ A)

negation-preserves-decidability (inl a) = inr  f  f a)
negation-preserves-decidability (inr g) = inl g

which-of : {A B : U}  
 A + B  Σ \(b : 𝟚)  (b    A) × (b    B)

which-of (inl a) =  , ((λ r  a) ,  ()))
which-of (inr b) =  , ((λ ()) ,  r  b))


Notice that in Agda the term λ () is a proof of an implication that
holds vacuously, by virtue of the premise being false.  In the above
example, the first occurrence is a proof of ₀ ≡ ₁ → B, and the second
one is a proof of ₁ ≡ ₀ → A. 

The following is a special case we are interested in:


truth-value : {A : U}  
 decidable A  Σ \(b : 𝟚)  (b    A) × (b    ¬ A)

truth-value = which-of


Notice that this b is unique (Agda exercise) and that the converse
also holds. In classical mathematics it is posited that all
propositions have binary truth values, irrespective of whether they
have BHK-style witnesses. And this is precisely the role of the
principle of excluded middle in classical mathematics.  The following
requires choice, which holds in BHK-style constructive mathematics:


indicator : {X : U}  {A B : X  U}  
 ((x : X)  A x + B x)
   Σ \(p : X  𝟚)  (x : X)  (p x    A x) × (p x    B x)

indicator {X} {A} {B} h = 
  x  pr₁(lemma₁ x)) ,  x  pr₂(lemma₁ x))
  lemma₀ : (x : X)  (A x + B x)  Σ \b  (b    A x) × (b    B x)
  lemma₀ x = which-of {A x} {B x}

  lemma₁ : (x : X)  Σ \b  (b    A x) × (b    B x)
  lemma₁ = λ x  lemma₀ x (h x)


We again have a particular case of interest.  Detachable subsets,
defined below, are often known as decidable subsets. Agda doesn't
allow overloading of terminology, and hence we gladly accept the
slighly non-universal terminology.


detachable : {X : U}  (A : X  U)  U
detachable A =  x  decidable(A x)

characteristic-function : {X : U}  {A : X  U}  
 detachable A 
    Σ \(p : X  𝟚)  (x : X)  (p x    A x) × (p x    ¬(A x))

characteristic-function = indicator

co-characteristic-function : {X : U} {A : X  U}  
 detachable A 
    Σ \(p : X  𝟚)  (x : X)  (p x    ¬(A x)) × (p x    A x)

co-characteristic-function d = indicator x  +-commutative(d x))

Notice that p is unique (Agda exercise - you will need extensionality).