GADTs - Different behaviour in different files

Learning
4

Follow-up question.
Is there a way to “decomplete” a type without rebuilding every element?

Basically,

let decomplete : complete t -> incomplete t = fun x -> x

is technically valid, but obviously type-incorrect, so I have to write:

let decomplete ty = 
  match ty with
  | Ptr t -> Ptr (decomplete t)
  | Number -> Number

And that will allocate memory for no reason. Is there a way to write that easily?