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)

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