“基本"值只能用于直接调用被覆盖成员的基本实现 [英] 'base' values may only be used to make direct calls to the base implementations of overridden members

问题描述

为什么我不能在这里调用f的base实现:

type Base = 
    abstract f : int -> int -> int
    default this.f (x : int) (y : int) : int = x + y

type Derived = 
    inherit Base
    override this.f (x : int) (y : int) : int = base.f -x -y

对base.f的调用引发此编译器错误:

error FS0419: 'base' values may only be used to make direct calls to the base implementations of overridden members

如果我将f更改为采用单个参数，则它将编译.大概这与咖喱参数和元组参数有关，但是上面的代码对我来说很好.

我相信问题在于base不能被闭包捕获-调用必须直接进行.但是，由于仅立即应用第一个参数，因此覆盖咖喱函数会自动创建一个闭包.因此，即使您看起来确实确实在使用base值直接调用重写成员的基本实现，您实际上还是在闭包中使用了base值，这是非法的.

不幸的是，我认为没有什么好方法可以解决此问题.通常，您应尽可能避免使用咖喱成员，但这是一种选择:

type Base = 
    abstract f : int -> (int -> int)
    default this.f (x : int) = fun y -> x + y

type Derived = 
    inherit Base
    override this.f x = 
       let fn = base.f -x
       fun y -> fn -y

Why can't I call the base implementation of f here:

type Base = 
    abstract f : int -> int -> int
    default this.f (x : int) (y : int) : int = x + y

type Derived = 
    inherit Base
    override this.f (x : int) (y : int) : int = base.f -x -y

The call to base.f elicits this compiler error:

error FS0419: 'base' values may only be used to make direct calls to the base implementations of overridden members

If I change f to take a single argument then it compiles. Presumably this is something to do with curried parameters vs tupled parameters, but the above code looks fine to me.

解决方案

I believe that the issue is that base can't be captured by a closure - the call has to be made directly. However, overriding a curried function automatically creates a closure since only the first argument is applied immediately. Therefore, even though it looks like you are indeed using the base value to make a direct call to the base implementation of the overridden member, you're actually using the base value within a closure, which is illegal.

Unfortunately, I don't think there's any great way to work around this issue. Generally, you should avoid curried members when possible, but here's one alternative:

type Base = 
    abstract f : int -> (int -> int)
    default this.f (x : int) = fun y -> x + y

type Derived = 
    inherit Base
    override this.f x = 
       let fn = base.f -x
       fun y -> fn -y

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