Lecture 18: PCF Böhm trees, Innocent strategies, HylandOng Games
Lecture 18
$λ$calculus: Normal forms of the form
\[M \; ≝ \; λx_0, …, x_n. W \\ W \; ≝ n \; \mid \; \texttt{case } x^A M_1 ⋯ M_p \texttt{ is } [n_1 → W_1 … n_q → W_q]\\ \text{ where } A = σ_1 → ⋯ → σ_p → nat\]NB: this term is not allowed $λf. f$, where $f: nat → nat$ in PCF Böhm trees, because of the form of $A$ above. Only way to fix it: $η$expand it: $λf,x. f x$
Exercise: Show that any real PCF term reduces to a some (infinite) Böhm tree.
⟹ Infinite Böhm trees are $η$long normal forms.
\[σ \; ≝ \; nat \; \mid \; σ → σ\]NB: so $σ$ is of the form $σ_1 → ⋯ → σ_n → nat$.
Typing rules:
\[\cfrac{⊢ W: nat}{⊢ λ x_1^{σ_1}, …, x_n^{σ_n}. W: σ_1 → ⋯ → σ_n → nat}\] \[\cfrac{}{⊢ n: nat}\] \[\cfrac{⊢ M_1: σ_1 \quad ⋯ \quad M_p: σ_p \qquad ⊢ W_1: nat \quad ⋯ \quad ⊢ W_q: nat}{x: σ_1 → ⋯ → σ_p → nat ⊢ \texttt{case } x \vec{M} \texttt{ is } [… n_i → W_i …] : nat}\]First goal: Compile PCF Btrees into “strategies” written using the following vocabulary:
$(nat → nat) → nat → nat$ is turned into
digraph {
rankdir=TB;
"nat_11" > "nat_1" > "nat_ε";
"nat_2" > "nat_ε";
}
The vocabulary will consist of moves of the form
\[?_u \qquad n_u\]with the convention that
 $?_ε$: Opponent (O)
 $n_ε$: Player (P)
and
 If $?_v$ is O for $σ_i$, then $?_{iv}$ is a P for $A$
 If $?_v$ is P for $σ_i$, then $?_{iv}$ is a O for $A$
NB: this is reminiscient of values of $M ⇒ M’$ being either values of $M’$ or cells of $M$
+ an equipement of pointers from P moves back to O moves
cf. picture
Innocent Strategies
Innocent strategies: between memoryless ones and those retaining complete history.
Warmup Example:
\[lor: nat_1 → nat_2 → nat_ε\] \[h \; ≝ \; λf. \texttt{ case } f (3) \texttt{ is } [4 → 7, 6 → 9] : (nat_{11} → nat_1) → nat_ε\]Kierstead (student of Curry (who was working on higherorder recursive functions)) terms:
From now on,
\[\texttt{case } x \vec{M} \quad \text{ is an abbrev. for } \quad \texttt{case } x \vec{M} \texttt{ of } \underbrace{[0 → 0, 1 → 1, …, 2 → 2, …]}_{\text{infinite list}}\] \[Kierstead: ((nat → nat) → nat) → nat\\ Kierstead_1: \quad λf. \texttt{case } f \, (λx. \texttt{case } f \, (λy. \texttt{case } x))\\ Kierstead_2: \quad λf. \texttt{case } f \, (λx. \texttt{case } f \, (λy. \texttt{case } y))\\\]cf. picture
We will define a cartesian closed category of HylandOng Games.

Morphisms = innocent strategies
Two equivalent forms:

meager (set of views):

where Opponent moves point to their direct Player parent move

composition via abstract machines


fat (set of plays)
 composition is defined in one line
NB: it’s similar to what we had with sequential algorithms: meager (AM) vs. fat (abstract algorithms)

Example: cf. picture
Kierstead applied to $h$
Arenas (HO)
An arena is given by
\[(M, λ: M → \lbrace O, P \rbrace)\]where
 $M$ stands for the moves

$⊢ ⊆ M ∪ M × M$
 if $⊢ m$, then $m$ is an O
 if $m ⊢ n$, then $m$ and $n$ are of opposite polarities
Cartesian Product
\[A = (M, λ, ⊢) \qquad A' = (M', λ', ⊢')\] \[A × A' = (\underbrace{(M.1) ∪ (M'.2)}_{M''}, λ'', ⊢'') \\\]where
\[λ''(m.1) = λ(m)\\ \cfrac{⊢ m}{⊢'' m.1}\\ \; \\ \cfrac{⊢' n}{⊢'' n.2}\\ \; \\ \cfrac{m_1 ⊢ m_2}{m_1.1 ⊢'' m_2.1}\\ \; \\\]Function Space
\[A ⇒ A' = (M'', λ'', ⊢'')\] \[M'' = M.1 ∪ M'.2\\ λ''(m.1) = \overline{λ(m)} \qquad \text{ (P becomes O and vice versa)}\\ λ''(n.2) = \overline{λ'(n)}\\\]And the rules are the same as the ones of the product, except the intial rules:
\[\cfrac{⊢' n }{⊢'' n.2}\] \[\cfrac{⊢ m \qquad ⊢' n}{n.2 ⊢'' m.1}\] \[\cfrac{m_1 ⊢ m_2}{m_1.1 ⊢'' m_2.1}\] \[\cfrac{n_1 ⊢ n_2}{n_1.2 ⊢'' n_2.2}\]Strategies (morphisms in our category)
A play is an alternating sequence of moves in $M$ starting with O, equipped with pointers from all occurrences (of noninitial moves) in the sequence in such a way that
 if $m_i$ points to $m_j$ for $j < i$, then $m_j ⊢ m_i$
(cf. picture)
 A strategy:

is a set $σ$ of evenlength plays closed under even prefixes
NB: definition by Russ Harmer. It is very practical for nondeterminism.
 A strategy $σ$ is deterministic:

if moreover: whenever $s m_1, \, s m_2 ∈ σ$, where the $m_i$’s point in $s$, it comes that $m_2 = m_1$
Innocent strategies
 A strategy $σ$ of $A$ is called innocent (Hyland Ong (HO)):

if for all $s$ (ranging over plays of $A$):
\[s ∈ σ \quad ⟺ \quad \underbrace{\lceil s \rceil}_{\text{P view of } s} ∈ σ\]where the view of a play is defined as:
\[\lceil ε \rceil = ε\\ \lceil s n \rceil = \lceil s \rceil n \qquad \text{ if } λ(n) = P\\ \lceil s m \rceil = m \qquad \text{ if } λ(m) = O \text{ and } ⊢ m\\ \lceil s n s' m \rceil = \lceil s n \rceil m \qquad \text{ if } m \text{ points inside } n\\\]
Exercise: When we had the interaction $h(f)$, we spotted a play $s$ that belonged to $h$. Compute $\lceil s \rceil$.
 A view:

is a sequence $s$ such that $\lceil s \rceil = s$
Meager strategies
Given an innocent strategy $σ$, we can define
\[meager(σ) \; ≝ \; \lbrace \text{set of views } s \; \mid \; s ∈ σ \rbrace\]Prop: $meager$ is injective.
Innocent strategies can be composed in two ways:
 either as strategies (parallel composition + hiding)
 or as meager strategies (abstract machines)
Parallel composition
Let
 $σ$ be a strategy of $A ⇒ A’$
 $σ’$ be a strategy of $A’ ⇒ A’’$
Then we define
\[σ' \circ σ \; ≝ \; \lbrace v \; \mid \; v ∈ plays (A ⇒ A'') \text{ and } ∃ \underbrace{u}_{\rlap{\text{sequence of moves taken in } A, A', A'' \text{ together with pointers}}} \text{ st } v = u_{ A, A''}, \quad w ≝ u_{ A, A'} ∈ σ, \quad w' ≝ u_{ A', A''} ∈ σ' \rbrace\]NB:
 Hiding: because the everything related to $A’$ has been kept private (hidden) in the final result

Parallel composition: because $w$ and $w’$ are obtained from the same $u$
 This definition works even without determinism nor innocence, but the equivalence with meager strategies holds only when we have innocence
\[nat = \lbrace \lbrace ? \rbrace ∪ ℕ \; \mid \; \begin{cases} λ (?) = O, \quad λ (n) = P \\ ⊢ ?, \quad ? ⊢ n \end{cases}\rbrace\]
Next time:

give a precise definition of the compilation from PCF Böhm trees to innocent strategies

Suggest: the characterisation of the image of this compilation (which is injective: two different Böhm tress give rise to different strategies)

Wellparenthesisation: exclude
catch
andor_tester

How to interpret some imperative features in the setting of HO games (imperative features ⟶ noninnocent strategies)
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