How to convert non-built-in types to string in Ocaml?

I'm trying to write a module that convert something like x:int = if true then 3 else 5 to a string

Here's the code I have so far

module Ast =structtype typ = Bool | Inttype var = A | B | C | D | E | Ftype exp = Const of int * typ           | App of string * exp list           | If of exp * exp * exp           | And of exp * exp           | Or of exp * exp       | Id of var * typ * explet rec toString (t) =    let formatDec1(va,ty,e) = ???    match t with    Const(n, _) -> print_int n      | App(id, [e1; e2]) -> formatter("" ^ id ^ "", e1, e2)      | App(id, [e1]) -> formatter("" ^ id ^ "", e1, Const(0, Int))      | App(id, _) -> formatter("" ^ id ^ "", Const(0, Int), Const(0, Int))      | If(e1, e2, e3) -> formatIf(e1, e2, e3)      | And(e1, e2) -> formatter("&& ", e1, e2)      | Or(e1, e2) -> formatter(" || ", e1, e2)      | Id(va,ty,e) -> formatDecl(va,ty,e)end

I'm still a beginner in OCaml and couldn't find anything about converting to a string online. Thanks!