如何在 F# 中编写此成员约束?

本文介绍了如何在 F# 中编写此成员约束?的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

对于一个类型

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

我可以编写一个方法来强制方法 Walk 的成员约束，例如

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())

在这种情况下，类型是推断出来的.我无法像这样明确地对生物参数编写约束

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()

我做错了什么?

推荐答案

问题不是明确地编写约束，而是语法不太好，您可以在参数上放置成员约束，然后调用成员以通常的方式.walk_the_creature 和 walk_the_creature2 的主体在这里是一样的:

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)