Nullable可以用作C#中的函子吗? [英] Can Nullable be used as a functor in C#?

问题描述

在C#中考虑以下代码.

Consider the following code in C#.

public int Foo(int a)
{
    // ...
}

// in some other method

int? x = 0;

x = Foo(x);

最后一行将返回编译错误cannot convert from 'int?' to 'int'，这很合理.但是，例如在Haskell中有Maybe，它与C#中的Nullable对应.由于Maybe是Functor，因此我可以使用fmap将Foo应用于x. C#是否具有类似的机制?

The last line will return a compilation error cannot convert from 'int?' to 'int' which is fair enough. However, for example in Haskell there is Maybe which is a counterpart to Nullable in C#. Since Maybe is a Functor I would be able to apply Foo to x using fmap. Does C# have a similar mechanism?

推荐答案

我们可以自己实现此类功能:

We can implement such functionality ourselves:

public static class FuncUtils {

    public static Nullable<R> Fmap<T, R>(this Nullable<T> x, Func<T, R> f)
        where T : struct
        where R : struct {
        if(x != null) {
            return f(x.Value);
        } else {
            return null;
        }
    }

}

然后我们可以将其用于:

Then we can use it with:

int? x = 0;
x = x.Fmap(Foo);

如果x不是null，它将因此调用函数Foo.它将结果包装回Nullable<R>中.如果x是null，它将返回带有null的Nullable<R>.

It will thus call the function Foo if x is not null. It will wrap the result back in a Nullable<R>. In case x is null, it will return a Nullable<R> with null.

或者我们可以编写一个更等效的函数(例如Haskell中的fmap)，其中我们有一个函数Fmap，该函数将Func<T, R>作为输入并返回Func<Nullable<T>, Nullable<R>>，这样我们就可以将其用于某些x:

Or we can write a more equivalent function (like fmap in Haskell) where we have a function Fmap that takes as input a Func<T, R> and returns a Func<Nullable<T>, Nullable<R>> so that we can then use it for a certain x:

public static class FuncUtils {

    public static Func<Nullable<T>, Nullable<R>> Fmap<T, R>(Func<T, R> f)
        where T : struct
        where R : struct {
        return delegate (Nullable<T> x) {
            if(x != null) {
                return f(x.Value);
            } else {
                return null;
            }
        };
    }

}

然后我们可以像这样使用它:

We can then use it like:

var fmapf = FuncUtils.Fmap<int, int>(Foo);
fmapf(null);  // -> null
fmapf(12);    // -> Foo(12) as int?

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