# Lecture 4: Simply typed λ-calculus as CCC

Teacher: Paul-André Melliès


## Reminders

An adjoint pair is a pair of functors

$\begin{xy} \xymatrix{ A \ar@/^/[r]^{ L } \ar@{<-}@/_/[r]_{ R } & B \\ } \end{xy}$

equipped with a family of bijections:

$ϕ_{a, b}: B(La, b) ≅ A(a, Rb)$

natural in $a$ and $b$.

This operation defines functors:

$B(L\bullet, \bullet): A^{op} × B ⟶ Set\\ A(\bullet, R\bullet): A × B^{op} ⟶ Set$

NB: in Homotopy Type Theory, we don’t currently know how to deal with these opposite categories.

Naturality here amounts to every commutative square

$\begin{xy} \xymatrix{ LA \ar[rr]^{ g } \ar@{<-}[d]_{ Lh_A } && B \ar[d]^{ h_B } \\ LA' \ar[rr]_{ f } && B' } \end{xy}$

being turned into a commutative square:

$\begin{xy} \xymatrix{ A \ar[rr]^{ ϕ_{A,B}(g) } \ar@{<-}[d]_{ h_A } && RB \ar[d]^{ Rh_B } \\ A' \ar[rr]_{ ϕ_{A',B'}(f) } && RB' } \end{xy}$

### Mac Lane’s Parameter theorem

In a cartesian category, we already know that the cartesian product defines a bifunctor.

Similarly, in a CCC: (Mac Lane’s Parameter theorem) the family of cartesian exponentiations

$(A ⇒ \bullet)_{A ∈ 𝒞}: 𝒞 ⟶ 𝒞$

defines a unique bifunctor:

$A, B ⟼ A ⇒ B: 𝒞^{op} × 𝒞 ⟶ 𝒞$

such that the bijections $ϕ_{A, B, C}$ are natural in $A, B, C$.

#### Every CCC is enriched over itself

In a cartesian closed category, the functor ${\rm Hom}$ functors as:

$𝒞^{op} × 𝒞 \xto {⇒} 𝒞 \xto{𝒞(1,\bullet)} Set$

NB: the terminal object $1$ exists, as $𝒞$ is cartesian (in the category of types, $1$ is the empty context: closed terms: $1 \xto t A$)

NB: the converse is not true: a category may be enriched over itself but not cartesian closed.

Example: Topological spaces: not cartesian closed. But a sub-category thereof is:

the category of compactly generated topological spaces (in which every set is closed iff its intersection with every compact is closed) and continuous functios is a CCC.

# Simply typed $λ$-calculus as CCC

Inference rules/typing judgement:

### Logical rules

$\cfrac{}{x:A ⊢ x:A}\text{(variable)}\\ \, \\ \cfrac{Γ, x:A ⊢ P:B}{Γ ⊢ λx.P:A ⇒ B}\text{(abstraction)}\\ \, \\ \cfrac{Γ ⊢ P: A ⇒ B \qquad Δ ⊢ Q:A}{Γ, Δ ⊢ PQ: B}\text{(abstraction)}$

### Structural rules

$\cfrac{Γ ⊢ P:B}{Γ, x:A ⊢ P:B}\text{(weakening)}\\ \, \\ \cfrac{Γ, x:A, y:A ⊢ P:B}{Γ, z:A ⊢ P[x←z, y←z]: B}\text{(contraction)}\\ \, \\ \cfrac{Γ, x:A, y:B, Δ ⊢ P: C}{Γ, y:B,x:A, Δ ⊢ P: C}\text{(exchange)}$
$x_1: A_1, …, x_k:A_k ⊢ P:B$

will be interpreted as

$A_1 × ⋯ × A_k \xto P B$

For every type variable $α$, we’re given an object $ξ(α)$

Interpretation of a type $A$: $⟦A⟧$

Then, by structural induction, we define $⟦\bullet⟧$ for every type.

Every sequent $x_1: A_1, …, x_k:A_k ⊢ P:B$ is translated into $⟦A_1⟧ × ⟦A_k⟧ ⟶ ⟦B⟧$

by induction on the derivation tree:

• Variable: $⟦A⟧ \xto {id_{⟦A⟧}} ⟦A⟧$

• Lambda/Curryfication: $A × Γ \xto f B$ becomes, by adjunction: $Γ \xto {ϕ_{A,Γ,B}(f)} A ⇒ B$

• Application: $Γ \xto f A \qquad \text{ and } \qquad Δ \xto g A⇒B$

$Γ × Δ \xto {f × g} A × (A ⇒ B) \xto {eval_{A,B}} B$
• Contraction $Γ × A × A \xto f B$ becomes $Γ × A \xto {Γ × δ_A} Γ × A × A \xto f B$

• Weakening $Γ × A \xto {π_1} Γ \xto f B$

which is isomorphic to

$Γ × A \xto {Γ × wk} Γ × 1 ≃ Γ \xto f B$
• Exchange: analogous

## Soundness theorem

In every CCC $𝒞$, the interpretation $⟦\bullet⟧$ is an invariant modulo $β, η$.

If $Γ ⊢ (λx. M): A⇒B$ and $Δ ⊢ N:A$:

$⟦Γ, Δ ⊢ (λx. M)N: B⟧ = ⟦Γ, Δ ⊢ M[x:=N]: B⟧$

If $Γ ⊢ M: A ⇒ B$:

$⟦Γ, Δ ⊢ (λx. Mx): A ⇒ B⟧ = ⟦Γ ⊢ M: A ⇒ B⟧$

Proof: Similar to the one of subject reduction (preservation of type by $β$-reduction and $η$-expansion): by induction on the typing judgment.

Linear Logic: an intermediate notion

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