类中let绑定中的显式类型参数 [英] Explicit type parameters in let bindings in classes

问题描述

当我尝试创建类似的类

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

编译器说有错误:

显式类型参数只能在模块或成员绑定上使用

Explicit type parameters may only be used on module or member bindings

但是 MSDN 说:

在模块级别，类型或计算表达式中的let绑定可以具有显式类型参数.在表达式中(例如在函数定义中)的let绑定不能具有类型参数.

A let binding at the module level, in a type, or in a computation expression can have explicit type parameters. A let binding in an expression, such as within a function definition, cannot have type parameters.

为什么文档和编译器说的不同?

Why documentation and compiler say different things?

推荐答案

这似乎是对类内 let 绑定的语法限制.但是，您仍然可以定义通用的局部函数，只需要在类型注释中指定类型参数即可:

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

我不认为这在语法上明确地在语法上是禁止的，因为该规范说类定义具有以下结构:

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
类型定义元素

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

和 class-or-value-defn 定义为:

class-function-or-value-defn:=属性 _opt 静态 _opt let rec _opt 函数或值定义

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

其中 function-or-value-defns 可能是带有显式类型参数的函数定义:

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 参数-pats return-typeopt = expr

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

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