I have 3 files:

1) cpf0.ml

type string = char list
type url = string
type var = string
type name = string
type symbol =
| Symbol_name of name

2) problem.ml:

type symbol =
  | Ident of string

3) test.ml

open Problem;;
open Cpf0;;

let symbol b = function
  | Symbol_name n -> Ident n

When I combine test.ml: ocamlc -c test.ml. I received an error:

This expression has type Cpf0.name = char list but an expression was expected of type string

Could you please help me to correct it? Thank you very much

EDIT: Thank you for your answer. I want to explain more about these 3 files: Because I am working with extraction in Coq to Ocaml type: cpf0.ml is generated from cpf.v :

 Require Import String.
 Definition string := string.
 Definition name := string.
 Inductive symbol := 
  | Symbol_name : name -> symbol.

The code extraction.v:

Set Extraction Optimize.
Extraction Language Ocaml.
Require ExtrOcamlBasic ExtrOcamlString.
Extraction Blacklist cpf list.

where ExtrOcamlString

I opened: open Cpf0;; in problem.ml, and I got a new problem because in problem.ml they have another definition for type string

This expression has type Cpf0.string = char list but an expression was expected of type Util.StrSet.elt = string

Here is a definition in util.ml defined type string:

module Str = struct type t = string end;;
module StrOrd = Ord.Make (Str);;
module StrSet = Set.Make (StrOrd);;
module StrMap = Map.Make (StrOrd);;

let set_add_chk x s =
  if StrSet.mem x s then failwith (x ^ " already declared")
  else StrSet.add x s;;

I was trying to change t = string to t = char list, but if I do that I have to change a lot of function it depend on (for example: set_add_chk above). Could you please give me a good idea? how I would do in this case.