我如何在F＃中编写此成员约束？ [英] How do I write this member constraint in F#?

问题描述

对于类型

 类型Cow（）= 
 class 
 member this.Walk（） = Console.WriteLine（The cow walks。）
 end 
  

我可以写一个方法强制成员约束的方法Walk像

  let inline walk_the_creature creature = 
（^ a：（member Walk ：unit  - > unit）creature）
 //然后做
 walk_the_creature（Cow（））
  

在这种情况下，推断类型。我无法明确地写出这样的生物参数约束。

  //不编译
 //查找基于这个
 //程序点之前的信息的不确定类型的对象。可能需要类型注释... 
 let inline walk_the_creature_2（生物：^ a当^ a :(成员步行：单位 - >单位））= 
 creature.Walk（）
  

我做错了什么？

解决方案

它没有明确写出约束是问题，而是语法不够好，您可以将一个成员约束放在一个参数上，然后以通常的方式调用该成员。 walk_the_creature 和 walk_the_creature2 的主体在这里是一样的：

  let inline walk_the_creature_2（生物：^ a当^ a :(成员Walk：单位 - >单位））= 
（^ a :(成员Walk：单位 - >单位）生物）
  

For a type

type Cow() =
    class
        member this.Walk () = Console.WriteLine("The cow walks.")
    end

I can write a method which enforces a member constrain for method Walk like

let inline walk_the_creature creature =  
    (^a : (member Walk : unit -> unit) creature)
// and then do
walk_the_creature (Cow())

In this case the type is inferred. I am unable to explicitly write a constraint on the creature parameter like this

// Does not compile
// Lookup on object of indeterminate type based on information prior to this 
// program point. A type annotation may be needed...
let inline walk_the_creature_2 (creature:^a when ^a:(member Walk : unit -> unit)) =
    creature.Walk()

What am I doing wrong?

解决方案

It's not explicitly writing the constraints that is the issue, it's that the syntax is not so nice that you can place a member constraint on a parameter and then invoke the member in the usual way. The body of walk_the_creature and walk_the_creature2 would be the same here:

let inline walk_the_creature_2 (creature:^a when ^a:(member Walk : unit -> unit)) =
    (^a : (member Walk : unit -> unit) creature)

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