是否存在可表示同形状归纳类型等价的类型论及相关证明机制?
Great question! This is exactly the sort of problem that homotopy type theory (HoTT) and its cubical variants excel at solving, and observational type theory (OTT) also has built-in mechanisms for this. Let's break down how each approach works:
Yes, This Is Possible in HoTT/Cubical Type Theory (via Univalence)
The key here is the univalence axiom, which states that two types are equal if and only if they are equivalent (i.e., there exists a pair of mutually inverse functions between them). For your list1 and list2 types, this is straightforward to apply:
Construct an equivalence between
list1 Aandlist2 A:
We can define bidirectional functions that map each constructor oflist1to the corresponding constructor oflist2(and vice versa), then prove these functions are inverses. For example (in Cubical Agda, a cubical TT implementation with built-in univalence):open import Cubical.Core.Everything open import Cubical.Foundations.Everything data list1 (A : Type) : Type where nil1 : list1 A cons1 : A → list1 A → list1 A data list2 (A : Type) : Type where nil2 : list2 A cons2 : A → list2 A → list2 A -- Forward map: list1 → list2 list1→list2 : ∀ {A} → list1 A → list2 A list1→list2 nil1 = nil2 list1→list2 (cons1 x l) = cons2 x (list1→list2 l) -- Reverse map: list2 → list1 list2→list1 : ∀ {A} → list2 A → list1 A list2→list1 nil2 = nil1 list2→list1 (cons2 x l) = cons1 x (list2→list1 l) -- Prove these form a valid equivalence list1≃list2 : ∀ {A} → list1 A ≃ list2 A list1≃list2 = isoToEquiv (iso list1→list2 list2→list1 (λ l → inductive-refl) -- Inductively prove list2→list1∘list1→list2 = id (λ l → inductive-refl)) -- Inductively prove list1→list2∘list2→list1 = idUse univalence to turn the equivalence into a type equality:
Theua(univalence axiom) function converts our equivalencelist1≃list2into a pathlist1 A ≡ list2 A—a formal proof that the two types are equal.Transport predicates and functions across the equality:
With a type equalityp : list1 A ≡ list2 A, thetransportfunction lets us "carry" any predicateP : list1 A → Typeover tolist2 A, producingP' : list2 A → Type. Similarly, any functionf : list1 A → Xcan be transported tof' : list2 A → X. This exactly matches your requirement: the structure of proofs/functions forlist1is preserved, with only constructor names/types replaced.
Observational Type Theory (OTT) Also Handles This Natively
OTT takes a different approach: equality is defined by observational equivalence. For inductive types, two types are observationally equal if they have identical constructor structures (same number of constructors, each constructor has parameters of observationally equal types).
In OTT, list1 and list2 would be judged observationally equal by definition, no extra axioms needed. This means you can directly use a predicate or function defined for list1 A with list2 A, and vice versa—no explicit transport step is required. This aligns perfectly with your intuition that same-shaped inductive types should be interchangeable.
Defining Generic "List-Like" Functions
To define a function like reverse : list-like A → list-like A that works for both list1 and list2, you have a few options:
- In HoTT/cubical TT: Define
reverseon a standardlist Atype, then transport it tolist1 Aandlist2 Ausing their equivalences tolist A. Alternatively, you can define a higher inductive type that abstracts the "list signature" (nil + cons), then showlist1andlist2are instances of this signature. - In OTT: Since
list1andlist2are observationally equal, you can definereversefor one and use it directly with the other, or define it generically over any type with the list-shaped constructor structure.
内容的提问来源于stack exchange,提问作者LogicChains

