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module Types.universe where

open import Lang.dataStructures using (
  Bool; true; false;
;;; List;
  zero; one)

open import Agda.Primitive renaming (
  Level to AgdaLevel;
  lzero to alzero;
  lsuc to alsuc;
  __ to _⊔⊔_)

Universes and families

A universe can be thought of as a container for all of mathematics. There is no mathematics that is possible outside of its universe. Thus, one can think of universes of types to contain all types, a universe of sets contains all sets and so on. One could also think of universes as a collection of entities that one needs to work with − for e.g. for proving a theorem on sets, one could work in a universe of sets.

Formally, the structure of the universe used in type theory are Russel-style and Tarski-style universes though we use the former as it is easier and sufficient for our purposes. There are other kinds of universes in mathematics, for example the Grothendieck universe, Von Neumann universe.

The type of all types is called Set in agda. Now, in constructing this type of all types naively we encounter a bunch of paradoxes, namely Russel’s Paradox, Cantor’s Paradox, Girard’s Paradox etc. These can be avoided by constructing the type of all types as “universes” in a hierarchically cumulative way. When we consider our universe to be the set of all types, we say that our universe is constructed hierarchically, with an index i such that universe Uᵢ ∈ Uᵢ₊₁ and so on.

\[ U_{0} \in U_{1} \in U_{2} \in ... \in U_{i} \in U_{i+1} \in ... \in U_{\infty} \]

This avoids the problem of Russel’s paradox, which implies that the set of all sets itself is not a set. Namely, if there were such a set U, then one could form the subset A ⊆ U of all sets that do not contain themselves. Then we would have A ∈ A if and only if A ∉ A, a contradiction.

Let us define the above index i of universe Uᵢ, called Level in Agda’s standard library:

infixl 6 __

  Level : Set

We define it as a postulate so we don’t have to provide an implementation yet. We continue to define some operations on it, i.e.:

  lzero : Level
  lsuc  : (: Level)  Level
  __   : (ℓ₁ ℓ₂ : Level)  Level

And finally, we define universe as:

record Universe u e : Set (lsuc (u ⊔ e)) where
    -- Codes.
    U : Set u

    -- Decoding function.
    El : U  Set e
Figure 1: Universes

A “family” of types varying over a given type are called, well “families of types”. An example of this would be the finite set, Fin where every finite set has n elements where n ∈ ℕ and hence Fin, the creator of finite sets, is dependent on ℕ.

Universes in Agda

In some implementations, universes are represented using a different keyword Type instead of Set in order to avoid confusion:

Type : (i : AgdaLevel)  Set (alsuc i)
Type i = Set i

Type₀ = Type alzero
Type0 = Type alzero

Type₁ = Type (alsuc alzero)
Type1 = Type (alsuc alzero)

Universe Polymorphism

Now, given that we have infinite hierarchial universes, we would have to define the same functions, data types, proofs and other machinery for each universe level, which would be pretty tedious to say the least. However, we observe how our universes are defined and note that the level-based indexing system, that connects each successive universe, provides us with the mechanics to define math for all universe levels . Programmers would find this is nothing but the widely used pattern called Polymorphism:

id : {: AgdaLevel} {A : Set} (x : A)  A
id x = x

Here id represents a family of identity functions given a type A and its level and works for all universe levels.

infixr 5 _::_
data List₁ {: AgdaLevel} (A : Set) : Set (alsuc ℓ) where
  [] : List₁ A
  _::_ : A  List₁ A  List₁ A

someList : List₁ ℕ
someList = (one :: zero :: [])

sameList : List₁ ℕ
sameList = id someList

Machinery on Types

We now define some utility machinery for operating on types.

Type of

We obviously need a means to check types:

typeof :  {i} (A : Type i) (u : A)  A
typeof A u = u

infix 40 typeof
syntax typeof A u =  u :> A

Equality of types

infix 30 _==_
data _==_ {i} {A : Type i} (a : A) : A  Type i where
  idp : a == a

Identity type

The equality of types is itself a type - the identity type:

Path = _==_

The reason for calling this a Path has a huge backstory, which we will explore in Homotopy Type Theory.