Coq: How to refer to the types generated by a specific constructor?

Question

For example, if I define a function from nat to nat, it would be

Definition plusfive (a : nat) : nat := a + 5.

However, I would like to define a function whose arguments are nats constructed using the S constructor (i.e. nonzero) is that possible to directly specify as a type? something like

Definition plusfive (a : nat.S) : nat := a + 5.

(I know that for this case I could also add an argument with a proof that a is nonzero, but I am wondering if it is possible to directly name the type based on the S constructor).

Answer

Functions have to be complete, so you will have to use some subtype instead of nat, or add an argument that reduces input space, for instance (H: a <> 0)

Definition plusfive (a : nat) (H : a <> 0) :=
  match a as e return a = e -> _ with
  | S _ => fun _  => a + 5
  | _   => fun H0 => match (H H0) with end
  end eq_refl.

However, these kinds of tricks have been discovered to be very cumbersome to work with in large developments, and often one instead uses complete functions on the base type that return dummy values for bad input values, and prove that the function is called with correct arguments separately from the function definition. See for example how division is defined in the standard library.

Require Import Nat.
Print div.
div = 
fun x y : nat =>
match y with
| 0 => y
| S y' => fst (divmod x y' 0 y')
end
     : nat -> nat -> nat

Arguments div (_ _)%nat_scope

So Compute (div 1 0). gives you 0.

The nice thing is that you can use div in expressions directly, without having to interleave proofs that the denominator is non-zero. Proving that an expression is correct is then done after it has been defined, not at the same time.