3

suppose

Structure : (M : Type l -> Type l) -> Type (lsuc l)
Structure M = Σ (Type l) (λ A -> M A)

I want

homomorphism :
 {M : Type l -> Type l} -> (sA sB : Structure M)
 (f : sA.fst -> sB.fst) -> Type l

such that homomorphism (A , MA) (B , MB) f would represent the proposition that f is a function from A to B that preserves the extra structure (M) (I dont know how to formally define the notion of extra structure and preservation)

for example, if M A = A -> A -> A, then Structure M would the type of all Magmas and homomorphism (A , _*A_) (B , _*B_) f would represent the proposition that f is a magma homomorphism from A to B

The Wikipedia page on homomorphisms formally defines homomorphisms only when M A consists of functions of A, I feel like this is too restrictive.

That being said, I feel like that is not possible without restricting Structure A somewhat

So really, I am asking two questions:

  1. What are the suitable and satisfactory restrictions I can place on Structure A?
  2. What should the definition of homomorphism be?

by satisfactory I want to disallow solutions I think cheat (One such dissatisfactory solution is requiring the existence of a category with Structure M as the type of objects, as it means that we have to manually give the type of morphisms (which will be the type of homomorphisms))

Assuming the univalence axiom, the type Id (Structure M) (A , MA) (B , MB) is almost what I need, as from it I can extract a function f : A -> B that is a bijective homomorphism.

asked Dec 18, 2025 at 20:19
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