.net - 类中的 let 绑定中的显式类型参数

Question

当我尝试创建一些类时

type MyType () =
    let func<'T> () = ()

编译器说有一个错误：

显式类型参数只能用于模块或成员绑定

但是MSDN说：

模块级别、类型或计算表达式中的 let 绑定可以具有显式类型参数。表达式中的 let 绑定（例如函数定义中）不能有类型参数。

为什么文档和编译器会说不同的东西？

Answer 1

This appears to be a syntactic restriction on let bindings inside a class. However, you can still define a generic local function, you just have to specify the type parameters in type annotations:

type MyType () =
   let func (x : 'T) : 'T = x

I do not think this is explicitly syntactically forbidden by the specification, because the specification says that a class definition has the following structure:

type type-name patopt as-defnopt =
      class-inherits-decl_opt
      class-function-or-value-defns_opt
      type-defn-elements

and class-or-value-defn is defined as:

class-function-or-value-defn := attributes_opt static_opt let rec_opt function-or-value-defns

where function-or-value-defns may be a function definition with explicit type parameters:

function-defn :=
inline_opt access_opt ident-or-op typar-defns_opt argument-pats return-typeopt = expr

Answer 2

要添加到 Tomas 的答案，如果您需要类型参数但没有该类型的值，则可以使用带有幻像类型参数的类型。例如：

open System

type Foo() =
    member x.PrintType<'T> () = printfn "%s" typeof<'T>.Name

type TypeParameter<'a> = TP

let foo = Foo ()

let callFoo (typeParameter : TypeParameter<'a>) =
    foo.PrintType<'a> ()

callFoo (TP : TypeParameter<string>)
callFoo (TP : TypeParameter<DateTime>)