Question

我正在尝试使用OCaml的OO结构和类型系统来实现访问者设计模式，并在实例化元素时遇到问题。

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;;

我在编译时遇到的错误是：

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

但是，代码编译减去实例化salesman的行。

如何在保持类功能的同时实例化salesman？

修改通过调用receiveEvaluation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

解决编译错误

请注意，您的代码在顶级版中运行良好：

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

这种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。由于顶层告诉我们它是什么，我们可以修改实例化：

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

这个编译！

递归解决方案

通过使用递归类可以降低复杂性。这完全不需要参数化类，但意味着需要在同一个源文件中定义所有对象。

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;;

打印“访问推销员”。对员工的强制只是为了使其更接近现实世界的情景。

参数化解决方案

再次查看问题，我认为没有必要使用参数化的hrRep，因为此时所有其他类型都是已知的。通过简化员工类的参数化，我得到了这个：

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;;

返回：

拜访推销员S Bob

访问会计师A Mary

拜访推销员S Sue

此解决方案的优点是员工无需了解访问者，因此可以在他们自己的编译单元中进行定义，从而在添加新类型的员工时可以实现更清晰的代码和更少的重新编译。

OCaml中的访客设计模式

1 个答案: