# Notes on Quotients Types

Lately I’ve been reading a lot of type theory literature. In effort to help my future self, I’m going to jot down a few thoughts on quotient types, the subject of some recent google-fu.

## But Why!

The problem quotient types are aimed at solving is actually a very common one. I’m sure at some point or another you’ve used a piece of data you’ve wanted to compare for equality. Additionally, that data properly needed some work to determine whether it was equal to another piece.

A simple example might would be representing rational numbers. A rational number is a fraction of two integers, so let’s just say

` type Rational = (Integer, Integer)`

Now all is well, we can define a `Num`

instance and what not. But what about equality? Clearly we want equivalent fractions to be equal. That should mean that `(2, 4) = (1, 2)`

since they both represent the same number.

Now our implementation has a sticky point, clearly this isn’t the case on its own! What we really want to say is “`(2, 4) = (1, 2)`

up to trivial rejiggering”.

Haskell’s own `Rational`

type solves this by not exposing a raw tuple. It still exists under the hood, but we only expose smart constructors that will reduce our fractions as far as possible.

This is displeasing from a dependently typed setting however, we want to be able to formally prove the equality of some things. This “equality modulo normalization” leaves us with a choice. Either we can really provide a function which is essentially

```
foo : (a b : Rational)
-> Either (reduce a = reduce b) (reduce a /= reduce b)
```

This doesn’t really help us though, there’s no way to express that `a`

should be observationally equivalent to `b`

. This is a problem seemingly as old as dependent types: How can we have a simple representation of equality that captures all the structure we want and none that we don’t.

Hiding away the representation of rationals certainly buys us something, we can use a smart constructor to ensure things are normalized. From there we could potentially prove a (difficult) theorem which essentially states that

```
=-with-norm : (a b c d : Integer)
-> a * d = b * c -> mkRat a b = mkRat c d
```

This still leaves us with some woes however, now a lot of computations become difficult to talk about since we’ve lost the helpful notion that `denominator o mkRat a = id`

and similar. The lack of transparency shifts a lot of the burden of proof onto the code privy to the internal representation of the type, the only place where we know enough to prove such things.

Really what we want to say is “Hey, just forget about a bit of the structure of this type and just consider things to be identical up to `R`

”. Where `R`

is some equivalence relation, eg

`a R a`

`a R b`

implies`b R a`

`a R b`

and`b R c`

implies`a R c`

If you’re a mathematician, this should sound similar. It’s a lot like how we can take a set and partition it into equivalence classes. This operation is sometimes called “quotienting a set”.

For our example above, we really mean that our rational is a type quotiented by the relation `(a, b) R (c, d)`

iff `a * c = b * d`

.

Some other things that could potentially use quotienting

- Sets
- Maps
- Integers
- Lots of Abstract Types

Basically anything where we want to hide some of the implementation details that are irrelevant for their behavior.

## More than Handwaving

Now that I’ve spent some time essentially waving my hand about quotient types what are they? Clearly we need a rule that goes something like

```
Γ ⊢ A type, E is an equivalence relation on A
———————————————–———————————————————————————————
Γ ⊢ A // E type
```

Along with the typing rule

```
Γ ⊢ a : A
——————————————————
Γ ⊢ a : A // E
```

So all members of the original type belong to the quotiented type, and finally

```
Γ ⊢ a : A, Γ ⊢ b : A, Γ ⊢ a E b
–——————————————–——————————————————
Γ ⊢ a ≡ b : A // E
```

Notice something important here, that `≡`

is the fancy shmancy judgmental equality baked right into the language. This calls into question decidability. It seems that `a E b`

could involve some non-trivial proof terms.

More than that, in a constructive, proof relevant setting things can be a bit trickier than they seem. We can’t just define a quotient to be the same type with a different equivalence relation, since that would imply some icky things.

To illustrate this problem, imagine we have a predicate `P`

on a type `A`

where `a E b`

implies `P a ⇔ P b`

. If we just redefine the equivalence relation on quotes, `P`

would not be a wellformed predicate on `A // E`

, since `a ≡ b : A // E`

doesn’t mean that `P a ≡ P b`

. This would be unfortunate.

Clearly some subtler treatment of this is needed. To that end I found this paper discussing some of the handling of NuRPL’s quotients enlightening.

## How NuPRL Does It

The paper I linked to is a discussion on how to think about quotients in terms of other type theory constructs. In order to do this we need a few things first.

The first thing to realize is that NuPRL’s type theory is different than what you are probably used to. We don’t have this single magical global equality. Instead, we define equality inductively across the type. This notion means that our equality judgment doesn’t have to be natural in the type it works across. It can do specific things at each case. Perhaps the most frequent is that we can have functional extensionality.

`f = g ⇔ ∀ a. f a = g a`

Okay, so now that we’ve tossed aside the notion of a single global equality, what else is new? Well something new is the lens through which many people look at NuRPL’s type theory: PER semantics. Remember that PER is a relationship satisfying

`a R b → then b R a`

`a R b ∧ b R c → a R c`

In other words, a PER is an equivalence relationship that isn’t necessarily reflexive at all points.

The idea is to view types not as some opaque “thingy” but instead to be partial equivalence relations across the set of untyped lambda calculus terms. Inductively defined equality falls right out of this idea since we can just define `a ≡ b : A`

to be equivalent to `(a, b) ∈ A`

.

Now another problem rears it head, what does `a : A`

mean? Well even though we’re dealing with PERs, but it’s quite reasonable to say something is a member of a type if it’s reflexive. That is to say each relation is a full equivalence relation for the things we call members of that type. So we can therefore define `a : A`

to be `(a, a) ∈ A`

.

Another important constraint, in order for a type family to be well formed, it needs to respect the equality of the type it maps across. In other words, for all `B : A → Type`

, we have `(a, a') ∈ A' ⇒ (B a = B a') ∈ U`

. This should seem on par with how we defined function equality and we call this “type functionality”.

Let’s all touch on another concept: squashed types. The idea is to take a type and throw away all information other than whether or not it’s occupied. There are two basic types of squashing, extensional or intensional. In the intensional we consider two squashed things equal if and only if the types they’re squashing are equal

```
A = B
————————————
[A] = [B]
```

Now we can also consider only the behavior of the squashed type, the extensional view. Since the only behavior of a squashed type is simply existing, our extensional squash type has the equivalence

```
∥A∥ ⇔ ∥B∥
————————–
∥A∥ = ∥B∥
```

Now aside from this, the introduction of these types are basically the same: if we can prove that a type is occupied, we can grab a squashed type. Similarly, when we eliminate a type all we get is the trivial occupant of the squashed type, called •.

```
Γ ⊢ A
———————
Γ ⊢ [A]
Γ, x : |A|, Δ[̱•] ⊢ C[̱•]
——————————————————————————
Γ, x : |A|, Δ[x] ⊢ C[x]
```

What’s interesting is that when proving an equality judgment, we can unsquash obth of these types. This is only because NuRPL’s equality proofs computationally trivial.

Now with all of that out of the way, I’d like to present two typing rules. First

```
Γ ⊢ A ≡ A'; Γ, x : A, y : A ⊢ E[x; y] = E'[x; y]; E and E' are PERS
————————————————————————————————————————————————————————————————————
Γ ⊢ A // E ≡ A' // E'
```

In English, two quotients are equal when the types and their quotienting relations are equal.

```
Γ, u : x ≡ y ∈ (A // E), v :
∥x E y∥, Δ[u] ⊢ C [u]
———————————————————————————————————————————————————–
Γ, u : x ≡ y ∈ (A // E), Δ[u] ⊢ C [u]
```

There are a few new things here. The first is that we have a new `Δ [u]`

thing. This is a result of dependent types, can have things in our context that depend on `u`

and so to indicate that we “split” the context, with `Γ, u, Δ`

and apply the depend part of the context `Δ`

to the variable it depends on `u`

.

Now the long and short of this is that when we’re of this is that when we’re trying to use an equivalence between two terms in a quotient, we only get the squashed term. This done mean that we only need to provide a squash to get equality in the first place though

```
Γ ⊢ ∥ x E y
∥; Γ ⊢ x : A; Γ ⊢ y : A
——————————————————————————————————–
Γ ⊢ x ≡ y : A // E
```

Remember that we can trivially form an `∥ A ∥`

from `A`

’.

Now there’s just one thing left to talk about, using our quotiented types. To do this the paper outlines one primitive elimination rule and defines several others.

```
Γ, x : A, y : A, e : x E y, a : ND, Δ[ndₐ{x;y}] ⊢ |C[ndₐ{x;y}]|
——————————————————————————————————————————————————————————————–
Γ, x : A // E, Δ[x] ⊢ |C[x]|
```

`ND`

is a admittedly odd type that’s supposed to represent nondeterministic choice. It has two terms, `tt`

and `ff`

and they’re considered “equal” under `ND`

. However, `nd`

returns its first argument if it’s fed `tt`

and the second if it is fed `ff`

. Hence, nondeterminism.

Now in our rule we use this to indicate that if we’re eliminating some quotiented type we can get *any* value that’s considered equal under `E`

. We can only be assured that when we eliminate a quotiented type, it will be related by the equivalence relation to `x`

. This rule captures this notion by allowing us to randomly choose some `y : A`

so that `x E y`

.

Overall, this rule simply states that if `C`

is occupied for any term related to `x`

, then it is occupied for `C[x]`

.

## Wrap up

As with my last post, here’s some questions for the curious reader to pursue

- What elimination rules can we derive from the above?
- If we’re of proving equality can we get more expressive rules?
- What would an extensional quotient type look like?
- Why would we want intensional or extensional?
- How can we express quotient types with higher inductive types from HoTT

The last one is particularly interesting.

*Thanks to Jon Sterling for proof reading*