# Notes on Focusing

I’ve been spending a lot of time whacking my head on focusing literature. I’d like to jot down some intuition around what a focused system is and how it relates to the rest of the world. I’m going to steer clear of actually proving things but I will point out where a proof would be needed.

### What Is Focusing

In a nutshell, focusing is a strategy to create proofs that minimizes the amount of choices available at each step. Focusing is thus amenable to mechanization since a computer is very good at applying a lot of deterministic procedures but incredibly bad at nondeterministic choice.

Now when we set out to define a focused system we usually do something like

- Formalize our logical framework with natural deduction
- Translate our framework into a sequent calculus
- Transform our sequent calculus into a focused variant

At each of these steps there’s a proof that says something like “System 2 is sound and complete with respect to System 1”. We can then chain these proofs together to get that we can transform any nonfocused proof into a focused one (focalization) and the reverse (de-focalization).

In order to actually carry out these proofs there’s a fair amount of work and pain. Usually we’ll need something like cut elimination and/or identity expansion.

### Groundwork

Now before we go on to define an example logic, let’s notice a few things. First off, in sequent calculus there are left and right rules. Left rules decompose known facts into other known facts while right rules transform our goal. There’s also an identity sequent which more or less just states

```
A is an atom
—————————————
Γ, A → A
```

This is a bit boring though.

Now certain rules are invertible: their conclusion implies their premise in addition to the reverse. For example if I said you must prove `A ∧ B`

clearly we’ll have to prove both `A`

and `B`

in order to prove `A ∧ B`

; there’s no alternative set of rule applications that let us circumvent proving `A`

and `B`

.

This means that if we were mechanically trying to prove something of the form `A ∧ B`

we can immediately apply the right rule that decomposes `∧`

into 2 goals.

We can these sort of rules invertible or asynchronous. Dually, there are rules that when applied transform our goal into something impossible to prove. Consider `⊥ ∨ ⊤`

, clearly apply the rule that transforms this into `⊥`

would be a bad idea!

Now if we begin classifying all the left and write rules we’ll notice that the tend to all into 2 categories

- Things with invertible left rules and noninvertible right rules
- Things with noninvertible left rules and invertible right rules

We dub the first group “positive” things and the second “negative” things. This is called polarization and isn’t strictly necessary but greatly simplifies a lot of our system.

Now there are a few things that could be considered both positive and negative. For example we can consider `∧`

as positive with

```
Γ → A⁺ Γ → B⁺
———————————————
Γ → A⁺ ∧ B⁺
Γ, A⁺, B⁺ → C
—————————————————
Γ, A⁺ ∧ B⁺ → C
```

In this case, the key determiner for the polarity of ∧ comes from its subcomponents. We can just treat ∧ as positive along with its subcomponents and with an appropriate dual ∧⁻, our proof system will still be complete.

As a quick example, implication `⊃`

is negative. the right rule

```
Γ, A → B
——————————
Γ → A ⊃ B
```

While its left rule isn’t

```
Γ, A ⊃ B → A Γ, B, A ⊃ B → C
——————————————————————————————
Γ, A ⊃ B → C
```

Since we could easily have something like `⊥ ⊃ ⊤`

but using this rule would entail (heh) proving `⊥`

! Urk. If our system applied this rules remorselessly, we’d quickly end up in a divergent proof search.

### An Actual Focused System

*Do note that these typing rules are straight out of Rob Simmons’ paper, linked below*

Now that we’ve actually seen some examples of invertible rules and polarized connectives, let’s see how this all fits into a coherent system. There is one critical change we must make to the structure of our judgments: an addition to the form `_ → _`

. Instead of just an unordered multiset on the left, in order to properly do inversion we change this to `Γ; Ω ⊢ A`

where Ω is an ordered list of propositions we intend to focus on.

Furthermore, since we’re dealing with a polarized calculus, we occasionally want to view positive things as negative and vice versa. For this we have shifts, ↓ and ↑. When we’re focusing on some proposition and we reach a shift, we pop out of the focused portion of our judgment.

Our system is broken up into 3 essentially separate judgments. In this judgment we basically apply as many invertible rules as many places as we can.

```
Γ, A⁻; Q ⊢ U
——————————————
Γ; ↓A⁻, Q ⊢ U
Γ; A⁺, Ω ⊢ U Γ; B+; Ω ⊢ U
———————————————————————————
Γ; A⁺ ∨ B⁺, Ω ⊢ U
Γ; A⁺, B⁺, Ω ⊢ U
————————————————————
Γ; A⁺ ∧ B⁺, Ω ⊢ U
——————————————
Γ; ⊥, Ω ⊢ U
```

We first look at how to break down Ω into simpler forms. The idea is that we’re going to keep going till there’s nothing left in Ω. Ω can only contain positive propositions so eventually we’ll decompose everything to shifts (which we move into Γ) ⊤+ (which we just drop on the floor) or ⊥ (which means we’re done). These are all invertible rules to we can safely apply them eagerly and we won’t change the provability of our goal.

Once we’ve moved everything out of Ω we can make a choice. If `U`

is “stable” meaning that we can’t break it down further easily, we can pick a something negative out of our context and focus on it

```
Γ; [A⁻] ⊢ U
————————————–
Γ, A⁻; • ⊢ U
```

This pops us into the next judgment in our calculus. However, if U is not stable, then we have to decompose it further as well.

```
Γ; • ⊢ A⁺
——————————————
Γ; • ⊢ ↑ A⁺
———————————
Γ; • ⊢ ⊤⁻
Γ; A⁺ ⊢ B⁻
—————————————
Γ; • ⊢ A⁺ ⊃ B⁻
Γ; • ⊢ A⁻ Γ; • ⊢ B⁻
—————————————————————
Γ; • ⊢ A⁻ ∧ B⁻
```

If we have a negative connective at the top level we can decompose that further, leaving us with a strictly smaller goal. Finally, we may reach a positive proposition with nothing in Ω. In this case we focus on the right.

```
Γ ⊢ [A⁺]
———————————
Γ; • ⊢ A⁺
```

Now we’re in a position to discuss these two focused judgments. If we focus on the right we decompose positive connectives

```
——————————
Γ ⊢ [⊤⁺]
Γ; • ⊢ A⁻
—————————
Γ ⊢ ↓ A⁻
Γ ⊢ [A⁺]
—————————————
Γ ⊢ [A⁺ ∨ B⁺]
Γ ⊢ [B⁺]
—————————————
Γ ⊢ [A⁺ ∨ B⁺]
Γ ⊢ [A⁺] Γ ⊢ [B⁺]
———————————————————
Γ ⊢ [A⁺ ∧ B⁺]
```

These judgments follow the ones we’ve already seen. If we encounter a shift, we stop focusing. Otherwise we decompose the topmost positive connective. Now looking at these, you should see that sometimes these rules we’ll lead us to a “mistake”. Imagine if we applied the 4th rule to `⊤ ∨ ⊥`

! This is why these rules are segregated into a separate judgment.

In this judgment’s dual we essentially apply the exact same rules to the left of the turnstile and on negative connectives.

```
Γ; A⁺ ⊢ U
————————————
Γ; [↑A⁺] ⊢ U
Γ ⊢ [A⁺] Γ; [B⁻] ⊢ U
——————————————————————
Γ; [A⁺ ⊃ B⁻] ⊢ U
Γ; [A⁻] ⊢ U
—————————————————
Γ; [A⁻ ∧ B⁻] ⊢ U
Γ; [B⁻] ⊢ U
—————————————————
Γ; [A⁻ ∧ B⁻] ⊢ U
```

That wraps up our focused system. The idea is now we have this much more limited system which can express the same things our original, unfocused system could. A computer can be easily programmed to do a focused search since there’s much less backtracking everywhere leading to fewer rules being applicable at each step. I think Pfenning has referred to this as removing most of the “don’t-care” nondeterminism from our rules.

### Wrap Up

I’m going to wrap up the post here. Proving focalization or even something like cut elimination is quite fiddly and I have no desire at all to try to transcribe it (painfully) into markdown and get it wrong in the process.

Instead, now that you have some idea of what focusing is about, go read Rob Simmons’ paper. It provides a clear account of proving everything necessary prove a focused system is complete and sound with respect to its unfocused counterpart.

Cheers

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