类型的功能(单位->单位)可以在F#中具有静态解析的类型参数吗? [英] Can a function of type (unit -> unit) have statically resolved type parameters in F#?

问题描述

为什么不允许这样做?

type Foo() =
    static member Bar() = ()

let inline bar<^a>() = //ERROR: unexpected infix operator in pattern
    (^a : (static member Bar : unit -> unit)())

//Hypothetical usage: let _ = bar<Foo>()

...但这很好吗?

type Foo() =
    static member Bar() = new Foo()

let inline bar() : ^a =
    (^a : (static member Bar : unit -> ^a)())

let x : Foo = bar()

是否需要具有静态解析类型参数的函数来返回解析类型的实例?

Are functions with statically resolved type parameters required to return an instance of the resolved type?

推荐答案

正如您所注意到的，F#将字符<^的序列视为中缀运算符，因此您需要用空格分隔它们.关于何时需要显式指定约束的问题，我认为规则是，当您显式提供函数类型参数时，还需要指定任何必要的约束.否则，如果F#可以推断类型参数和约束，则无需指定它们.

As you noticed, F# treats the sequence of characters <^ as an infix operator, so you need to separate them with a space. As to the question of when you need to explicitly specify constraints, I believe that the rule is that when you explicitly give a function type parameters then you also need to specify any necessary constraints. Otherwise, if F# can infer the type parameters and constraints, you don't need to specify them.

因此，您提出的问题中的这个示例很好用:

So this example from your question works fine:

let inline bar() :^a =
  (^a : (static member Bar : unit -> ^a)())

是这样的:

let inline bar(x : ^a) =
  (^a : (static member Bar : unit -> unit)())

因为有一个泛型类型参数，但是您尚未将其显式放置在函数上，因此F#可以推断出所需的约束.

because there's a generic type parameter, but you haven't explicitly placed it on the function, and F# can infer the needed constraint.

另一方面，如果您尝试修改其他示例以省略显式的泛型参数:

On the other hand, if you try to modify your other example to omit the explicit generic parameter:

let inline bar() =
  (^a : (static member Bar : unit -> unit)())

您将看到F#不允许这样做，因为它无法解决如何为bar()的任何给定调用实例化^a的方法.因此，您需要显式提供type参数，并且一旦完成，还必须显式提供约束.

You'll see that F# won't allow this because there's no way for it to figure out how to instantiate ^a for any given call of bar(). Thus, you need to provide the type parameter explicitly, and once you do, you also have to explicitly provide the constraint.

这篇关于类型的功能(单位->单位)可以在F#中具有静态解析的类型参数吗?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！