OCaml 中的访问者设计模式

Question

我正在尝试使用 OCaml 的 OO 构造和类型系统来实现访问者设计模式，但在实例化元素时遇到了问题。

class virtual ['hrRep] employee = object 
 method virtual receiveEvaluation : 'hrRep -> unit
 method virtual getName : string
end;;

class ['hrRep] accountant myName = object (self : 'a)
 inherit ['hrRep]employee
 val name = myName
 method receiveEvaluation rep = rep#visitAccountant self
 method getName = name
end;;

class ['hrRep] salesman myName = object (self : 'a)
 inherit ['hrRep]employee
 val name = myName
 method receiveEvaluation rep = rep#visitSalesman self
 method getName = name
end;;

class virtual ['accountant, 'salesman] hrRep = object (self)
 method virtual visitSalesman : 'salesman -> unit
 method virtual visitAccountant : 'accountant -> unit
end;;

class ['employee, 'salesman] lowerLevelHRRep = 
      object (self) inherit ['employee, 'salesman]hrRep
 method visitSalesman s = print_endline ("Visiting salesman "^s#getName)
 method visitAccountant a = 
       print_endline ("Visiting accountant "^a#getName)
end;;

let s1 : (<visitSalesman : 'a -> unit>) salesman = new salesman "Bob";;
let a1 : (<visitAccountant : 'a -> unit>) accountant = new accountant "Mary";;
let s2 : (<visitSalesman : 'a -> unit>) salesman = new salesman "Sue";;
let h1 : (<getName : string>, <getName : string>) lowerLevelHRRep = new lowerLevelHRRep;;

s1#receiveEvaluation h1;;

我在编译时得到的错误是：

The type of this expression, <visitSalesman : 'a -> unit; _.. > salesman as 'a, 
contains type variables that cannot be generalized.

然而，代码编译时减去了实例化销售员的行。

如何在维护类功能的同时实例化 salesman ？

编辑 调用 receiveEvaluation 时收到错误：

This expression has type (<getName:string>, < getName:string>) lowerLevelHRRep 
but is here used with type <visitSalesman : 'a salesman -> unit > as 'a. 

第二个对象类型没有方法 visitAccountant。

Answer 1

编辑 - 将答案分为 3 个要点：初始编译错误的解决方案、递归解决方案和参数化解决方案

编译错误的解决方案

您的代码在顶层工作正常：

# let s = new salesman ();;
val s : < visitSalesman : 'a -> unit; _.. > salesman as 'a = <obj>

请注意， 类型编译错误通常通过添加类型注释来帮助编译器确定类型来解决。 正如顶层善意地告诉我们它是什么一样，我们可以修改实例化：

let s : (< visitSalesman : 'a -> unit>) salesman = new salesman ();;

这可以编译！

递归解决方案

通过使用递归类可以降低复杂性。 这完全消除了对参数化类的需要，但意味着所有对象都需要在同一个源文件中定义。

class virtual employee =
object
  method virtual receiveEvaluation:(hrrep -> unit)
end

and accountant = 
object(self)
  inherit employee
  method receiveEvaluation:(hrrep -> unit) = fun rep -> rep#visitAccountant (self :> accountant)
end

and salesman = 
object (self)
  inherit employee
  method receiveEvaluation:(hrrep -> unit) = fun rep -> rep#visitSalesman (self :> salesman)
end

and hrrep = 
object
  method visitSalesman:(salesman -> unit) = fun s -> print_endline ("Visiting salesman")
  method visitAccountant:(accountant -> unit) = fun s -> print_endline ("Visiting accountant")
end

let s = new salesman;;
let e = (s :> employee);;
let v = new hrrep;;

e#receiveEvaluation v;;

这将打印“拜访推销员”。 对员工的强制只是为了让这更接近现实世界的场景。

参数化的解决方案

再看一下问题，我认为没有必要有一个参数化的hrRep，因为此时，所有其他类型都是已知的。 通过仅对员工类进行参数化，我得到：

class virtual ['a] employee = 
object
  method virtual receiveEvaluation : 'a -> unit
  method virtual getName : string
end

class ['a] accountant name =
object(self)
  inherit ['a] employee
  val name = name
  method receiveEvaluation rep = rep#visitAccountant self
  method getName = "A " ^ name
end

class ['a] salesman name =
object(self)
  inherit ['a] employee
  val name = name
  method receiveEvaluation rep = rep#visitSalesman self
  method getName = "S " ^ name
end

class virtual hrRep = 
object
  method virtual visitAccountant : hrRep accountant -> unit
  method virtual visitSalesman : hrRep salesman -> unit
end

class lowerLevelHRRep =
object
  inherit hrRep
  method visitAccountant a = print_endline ("Visiting accountant " ^ a#getName)
  method visitSalesman s = print_endline ("Visiting salesman " ^ s#getName)
end;;

let bob = new salesman "Bob";;
let mary = new accountant "Mary";;
let sue = new salesman "Sue";;
let h = new lowerLevelHRRep;;
bob#receiveEvaluation h;;
mary#receiveEvaluation h;;
sue#receiveEvaluation h;;

此返回：

拜访销售员 S Bob

拜访会计师 A Mary

拜访推销员 S Sue

该解决方案的优点是员工不需要了解访客，因此可以在他们自己的编译单元，从而在添加新类型的员工时产生更清晰的代码和更少的重新编译。