Ordinals in Gödel's system T and in Martin-Löf Type Theory ------------------------------------------------------------ Martin Escardo, 2010, coded in Agda November 2011. This is based on work of Coquand, Setzer and Hancock, in particular: (i) Coquand, Hancock and Setzer (1997) http://www.cse.chalmers.se/~coquand/ordinal.ps (ii) Hancock (Russell'08 Proof Theory meets Type Theory, Swansea) https://web.archive.org/web/20220723181746/http://www.cs.swan.ac.uk/~csetzer/russell08/slides/hancock.pdf See also his MGS'08 lecture notes https://www.cs.bham.ac.uk/~mhe/events/MGS08/notes/proofTheory.pdf An interesting and powerful idea is their use of "lenses", which allows to define rather large ordinals, in particular in the presence of dependent types and universes. Another idea is to use Church encodings of ordinals. Here I do something more modest, without lenses, but still with Church encodings. I explicitly define addition, multiplication and exponentiation of ordinals, and there may be a small contribution here. In the Goedel system T fragment of Agda, these arithmetic operations cannot be uniformly typed, but they still have neat definitions. In particular, because of the non-uniform typing, we can only dominate ordinals strictly below ε₀ - this is not a limitation of our approach, but rather of system T. Using dependent types (products in fact will be enough) and a universe, we can get a uniform typing of the arithmetic operations, and hence the ordinal ε₀ and much higher indeed. But I will content myself with defining ε₀, which is not definable in system T, as an illustration of what dependent types and universes add in terms of expressivity. But it is easy to get higher ordinals using what is defined here. We proceed in three steps to define addition, multiplication and exponentiation, and hence ε₀ and much higher. (1): We essentially use Goedel's system T and work with a type O X = X → (X → X) → ((ℕ → X) → X) → X of Church encodings of ordinal trees, where X is a parameter, and define the basic arithmetic operations on ordinals with the non-uniform types add : O X → O X → O X mul : O X → O (O X) → O X exp : O (O X) → O (O X) → O X These types are the best one can do in system T. With this we can define ordinals abitrarily close to, and strictly below, the ordinal ε₀. (2): We use the first universe and dependent products to define O' X = Π (n : ℕ) → Oⁿ⁺¹ X and hence the arithmetic operations with uniform types add', mul', exp' : O' X → O' X → O' X from add, mul, exp defined in step (1). With this we can now define ε₀, not only in O' X, but also in O X. So you can see the type O' X as an auxiliary construction to get more in O X. (3): We inductively define a (standard) W-type of ordinal trees (e.g. studied by Brouwer, by Howard in an extension of system T, and mentioned by Martin-Loef in some of his papers), and show how to define complex Brouwer ordinal trees *without* using (structural) recursion on ordinals trees, using step (2). All (primitive) recursions in the development of (1)-(3) are on the set ℕ. This is followed by exercises, now using recursion and induction on Brouwer ordinal trees. \begin{code} {-# OPTIONS --safe --without-K #-} module Ordinals.Codes where open import MLTT.Spartan \end{code} Step (1). Church ordinal trees: \begin{code} O : 𝓤₀ ̇ → 𝓤₀ ̇ O X = X → (X → X) → ((ℕ → X) → X) → X zer : {X : 𝓤₀ ̇ } → O X zer = λ z → λ s → λ l → z suc : {X : 𝓤₀ ̇ } → O X → O X suc a = λ z → λ s → λ l → s (a z s l) lim : {X : 𝓤₀ ̇ } → (ℕ → O X) → O X lim as = λ z → λ s → λ l → l (λ i → as i z s l) O-rec : {X : 𝓤₀ ̇ } → X → (X → X) → ((ℕ → X) → X) → (O X → X) O-rec z s l a = a z s l \end{code} Notice that by uncurrying, flipping and currying the type of O-rec is isomorphic to {X : 𝓤₀ ̇ } → O X → O X, but the above form is more convenient for recursive definitions. In this first step, we have natural definitions but the types are not uniform: \begin{code} add : {X : 𝓤₀ ̇ } → O X → O X → O X add a b = λ z → λ s → λ l → a (b z s l) s l mul : {X : 𝓤₀ ̇ } → O X → O (O X) → O X mul a = O-rec zer (λ r → add r a) lim exp : {X : 𝓤₀ ̇ } → O (O X) → O (O X) → O X exp a = O-rec (suc zer) (λ r → mul r a) lim \end{code} Remark: if we had used O-rec to define add, it would instead have the type {X : 𝓤₀ ̇ } → O X → O (O X) → O X, and then mul would have the type {X : 𝓤₀ ̇ } → O (O X) → O (O X) → O X, with the same definition, but the same definition of exp then cannot be typed using iterations of O. In step (2) we will consider all finite iterations of O to define a type O', and give a uniform type {X : 𝓤₀ ̇ } → O' X → O' X → O' X to add, mul, and exp. We will not use the following: \begin{code} down : {X : 𝓤₀ ̇ } → O (O X) → O X down = O-rec zer suc lim \end{code} There is a term up : {X : 𝓤₀ ̇ } → O X → O (O X), but no such term has the desired behaviour of being a (left or right) inverse of down. Before using the first universe, we can dominate any ordinal below ε₀. The sequence of finite ordinals first: \begin{code} finite : {X : 𝓤₀ ̇ } → ℕ → O X finite = rec zer suc \end{code} Its limit: \begin{code} ω : {X : 𝓤₀ ̇ } → O X ω = lim finite \end{code} Now the iterated powers of ω, which can't be defined uniformly without universes or W-types or impredicativity etc. \begin{code} ω^ω : {X : 𝓤₀ ̇ } → O X ω^ω = exp ω ω ω^ω^ω : {X : 𝓤₀ ̇ } → O X ω^ω^ω = exp ω ω^ω ω^ω^ω^ω : {X : 𝓤₀ ̇ } → O X ω^ω^ω^ω = exp ω ω^ω^ω \end{code} And so on. Although the definitions look uniform, they are not. In fact, the candidate for the recursion step doesn't have type O X → O X, but rather: \begin{code} step : {X : 𝓤₀ ̇ } → O (O X) → O X step = exp ω \end{code} If you try to define ω-tower : {X : 𝓤₀ ̇ } → ℕ → O X ω-tower = rec ω (exp ω) then Agda rightfully complains that this would need X = O X, which is impossible. Moreover, e.g. in the definition of ω₃ the use of ω has its type X instantiated to O (O (O (O X))), if I counted properly. Thus, although we always write ω in the definitions of ω₁, ω₂, ω₃, ..., if we are strictly working in system T we need a different definition of ω in each case (with the same raw term but with a different type). Step (2). We now use the first universe to reach ε₀ and beyond. We build a type O' X of ordinals based on O X. It is the definition of rec₁, used to construct O', that uses the first universe. So we move from higher-type primitive recursion (system T) to even higher primitive recursion using a universe. \begin{code} rec₁ : 𝓤₀ ̇ → (𝓤₀ ̇ → 𝓤₀ ̇ ) → ℕ → 𝓤₀ ̇ rec₁ X F zero = X rec₁ X F (succ n) = F (rec₁ X F n) \end{code} We define O' X = Π (n : ℕ) → Oⁿ⁺¹ X as follows in Agda notation: \begin{code} O' : 𝓤₀ ̇ → 𝓤₀ ̇ O' X = (n : ℕ) → O (rec₁ X O n) zer' : {X : 𝓤₀ ̇ } → O' X zer' = λ n → zer suc' : {X : 𝓤₀ ̇ } → O' X → O' X suc' a = λ n → suc (a n) lim' : {X : 𝓤₀ ̇ } → (ℕ → O' X) → O' X lim' as = λ n → lim (λ i → as i n) add' : {X : 𝓤₀ ̇ } → O' X → O' X → O' X add' a b = λ n → add (a n) (b n) mul' : {X : 𝓤₀ ̇ } → O' X → O' X → O' X mul' a b = λ n → mul (a n) (b (succ n)) exp' : {X : 𝓤₀ ̇ } → O' X → O' X → O' X exp' a b = λ n → exp (a (succ n)) (b (succ n)) ω' : {X : 𝓤₀ ̇ } → O' X ω' = λ n → ω ω-tower' : {X : 𝓤₀ ̇ } → ℕ → O' X ω-tower' = rec ω' (exp' ω') \end{code} The ordinal ε₀ can now be defined in O' X (and hence in O X - see below). \begin{code} ε₀' : {X : 𝓤₀ ̇ } → O' X ε₀' = lim' ω-tower' \end{code} Because we now have addition, multiplication, exponentiation and limits in a uniform way, we can of course get much higher than ε₀. For example, ε₁, ε₂, ... can be defined uniformly and hence we can define εω. Then proceeding in the same way we can get εα for α = ε₀, and much higher indeed. Now, using this last step (2), we can project to step (1) and define ε₀ as an element of O X using the following coersion: \begin{code} O'↦O : {X : 𝓤₀ ̇ } → O' X → O X O'↦O a = a zero ε₀ : {X : 𝓤₀ ̇ } → O X ε₀ = O'↦O ε₀' \end{code} Notice that the following doesn't type check: O↦O' : {X : 𝓤₀ ̇ } → O X → O' X O↦O' a = λ n → a But it does type check for some particular a, such as ω in the above definition of ω'. Step (3). Brouwer's ordinal trees. I will use the letters u,v to range over B, and us,vs to range over forests, that is, sequences ℕ → B. \begin{code} data B : 𝓤₀ ̇ where Z : B S : B → B L : (ℕ → B) → B \end{code} Firstly we can go from O X to B when X=B: \begin{code} O↦B : O B → B O↦B u = u Z S L \end{code} We can now define a very tall ordinal tree without recursion on B: \begin{code} B-ε₀ : B B-ε₀ = O↦B ε₀ \end{code} Step (4): But the above is not the end of the story. This step, not mentioned above, is started but not completed. We leave the completion as an exercise at the time of writing. We can define the tree B-ε₀ by recursion on B and we should show, in Agda, that this produces the same result as the above recursion-free definition. \begin{code} B-rec : {X : 𝓤₀ ̇ } → X → (X → X) → ((ℕ → X) → X) → B → X B-rec {X} z s l = h where h : B → X h Z = z h (S u) = s (h u) h (L us) = l (λ i → h (us i)) \end{code} By suitable uncurrying, flipping and currying, the type of B-rec is isomorphic to {X : 𝓤₀ ̇ } → B → O X, but the above form is more convenient for recursive definitions: \begin{code} B↦O : {X : 𝓤₀ ̇ } → B → O X B↦O u s z l = B-rec s z l u \end{code} Ordinal tree arithmetic: \begin{code} B-add : B → B → B B-add u = B-rec u S L B-mul : B → B → B B-mul u = B-rec Z (λ r → B-add r u) L B-exp : B → B → B B-exp u = B-rec (S Z) (λ r → B-mul r u) L B-finite : ℕ → B B-finite = rec Z S B-ω : B B-ω = L B-finite B-ω-tower : ℕ → B B-ω-tower = rec B-ω (B-exp B-ω) B-ε₀-alternative : B B-ε₀-alternative = L B-ω-tower \end{code} We are almost ready to formulate the exercise. We need to define extensional equality on B. \begin{code} data _≣_ : B → B → 𝓤₀ ̇ where ≣-Z : Z ≣ Z ≣-S : (u v : B) → u ≣ v → S u ≣ S v ≣-L : (us vs : ℕ → B) → ((i : ℕ) → us i ≣ vs i) → L us ≣ L vs \end{code} Exercises: Construct elements of the following Exercise* types: \begin{code} Exercise₀ = B-ε₀ ≣ B-ε₀-alternative \end{code} Here is a sketch of how this can be approached: \begin{code} Exercise₁ = (u v : B) → B-add u v ≣ O↦B (add (B↦O u) (B↦O v)) Exercise₂ = (u v : B) → B-mul u v ≣ O↦B (mul (B↦O u) (B↦O v)) Exercise₃ = (u v : B) → B-exp u v ≣ O↦B (exp (B↦O u) (B↦O v)) \end{code} We need more coersions: \begin{code} B↦O' : {X : 𝓤₀ ̇ } → B → O' X B↦O' u = λ n → B↦O u O'↦B : O' B → B O'↦B a = O↦B (O'↦O a) Exercise₁' = (u v : B) → B-add u v ≣ O'↦B (add' (B↦O' u) (B↦O' v)) Exercise₂' = (u v : B) → B-mul u v ≣ O'↦B (mul' (B↦O' u) (B↦O' v)) Exercise₃' = (u v : B) → B-exp u v ≣ O'↦B (exp' (B↦O' u) (B↦O' v)) \end{code} And, to solve the above exercises, you will need induction on B (which amounts to "primitive recursion" on B, rather than simple recursion or iteration B-rec on B): \begin{code} B-induction : {A : B → 𝓤₀ ̇ } → A Z → ((u : B) → A u → A (S u)) → ((us : ℕ → B) → ((i : ℕ) → A (us i)) → A (L us)) → ----------------------------------------------------------- ((u : B) → A u) B-induction {A} z s l = h where h : (u : B) → A u h Z = z h (S u) = s u (h u) h (L us) = l us (λ i → h (us i)) \end{code}