OCaml：此表达式的类型为 int，但表达式应为 MyFoo.ty 类型（MyFoo.ty 为 int 除外）

Question

这是我的代码：

foo.ml:

module type ITy = sig
   type ty
end

module type IFoo = sig
   type ty
   val doo: ty -> ty
end

module Make (Ty: ITy): IFoo = struct
   type ty = Ty.ty
   let doo (x:ty) = x
end

main.ml:

module MyFoo = Foo.Make(struct
   type ty = int
end)

let () = assert(MyFoo.doo 3 = 3)

要编译，我使用：

$ corebuild main.native

这给了我：

+ ocamlfind ocamlc -c -w A-4-33-40-41-42-43-34-44 -strict-sequence -g -bin-annot -short-paths -thread -package core -ppx 'ppx-jane -as-ppx' -o main.cmo main.ml
File "main.ml", line 5, characters 26-27:
5 | let () = assert(MyFoo.doo 3 = 3)
                              ^
Error: This expression has type int but an expression was expected of type
         MyFoo.ty
Command exited with code 2.
Hint: Recursive traversal of subdirectories was not enabled for this build,
  as the working directory does not look like an ocamlbuild project (no
  '_tags' or 'myocamlbuild.ml' file). If you have modules in subdirectories,
  you should add the option "-r" or create an empty '_tags' file.

  To enable recursive traversal for some subdirectories only, you can use the
  following '_tags' file:

      true: -traverse
      <dir1> or <dir2>: traverse

Compilation unsuccessful after building 4 targets (3 cached) in 00:00:01.

我希望我的代码能够成功编译 b.c。我是一个 OCaml 菜鸟，当我在 main.ml 中调用 Foo.Make 函子来创建 MyFoo 时，我给它一个输入结构 type ty = int。我可能做错了什么，但未能找到专门讨论此问题的文档。也许我在某处使用了错误的运算符。任何帮助将不胜感激。

Answer 1

这是你想要完成的吗？

module type ITy = sig
   type ty
end

module type IFoo = sig
   type ty
   val doo: ty -> ty
end

module Make (Ty: ITy): (IFoo with type ty := Ty.ty) = 
struct
   let doo x = x
end

module MyFoo = Make(struct
   type ty = int
end)

let () = assert(MyFoo.doo 3 = 3)