Question

我尝试使用C ++概念实现Functor和其他各种类别理论概念，但是我遇到了编译错误：

http://coliru.stacked-crooked.com/a/e8b6eb387229bddf

这是我的完整代码（我知道要求fmap<int, int>不会对任何两种类型验证fmap，我计划将其更改为fmap<int, std::string>或其他内容以实现稍微强一点的测试 - 或者可能改变Functor概念，以便它除了F，两种类型T和U之外还需要验证fmap<T, U>的存在{1}}，但在我弄清楚如何解决我得到的错误之后，这就是全部：

#include <functional>
#include <iostream>
#include <vector>

// empty Functor_Impl struct - specialize for each functor
template<template<class> class F> struct Functor_Impl {};

// std::vector Functor implementation
template<>
struct Functor_Impl<std::vector> {
    template<class T, class U>
    static std::vector<U> fmap(std::vector<T> x, std::function<U(T)> f) {
        std::vector<U> out;
        out.reserve(x.size());
        for (int i = 0; i < x.size(); i++) {
            out.push_back(f(x[i]));
        }
        return out;
    }
};

// Functor concept requires Functor_Impl<F> to have fmap
template<template<class> class F>
concept bool Functor = requires(F<int> x) {
    {Functor_Impl<F>::template fmap<int, int>(x)} -> F<int>;
};

// Test function using constraint.
template<template<class> class F, class T>
requires Functor<F>
F<T> mult_by_2(F<T> a) {
    return Functor_Impl<F>::template fmap<T, T>(a, [](T x) {
        return x * 2;
    });
}

int main() {
    std::vector<int> x = {1, 2, 3};
    std::vector<int> x2 = mult_by_2(x);
    for (int i = 0; i < x2.size(); i++) {
        std::cout << x2[i] << std::endl;
    }
}

编译错误：

lol@foldingmachinebox:~/p/website-editor$ g++ foo.cpp -std=c++17 -fconcepts -o foo
foo.cpp: In function ‘int main()’:
foo.cpp:39:38: error: cannot call function ‘F<T> mult_by_2(F<T>) [with F = std::vector; T = int]’
     std::vector<int> x2 = mult_by_2(x);
                                      ^
foo.cpp:31:6: note:   constraints not satisfied
 F<T> mult_by_2(F<T> a) {
      ^~~~~~~~~
foo.cpp:24:14: note: within ‘template<template<class> class F> concept const bool Functor<F> [with F = std::vector]’
 concept bool Functor = requires(F<int> x) {
              ^~~~~~~
foo.cpp:24:14: note:     with ‘std::vector<int> x’
foo.cpp:24:14: note: the required expression ‘Functor_Impl<F>::fmap<int, int>(x)’ would be ill-formed

我猜测我的概念本身的语法是错误的 - 它将变量视为一个函数，反之亦然，因为我不熟悉{{1语法，此外concept上的一些示例代码不能在GCC的实现下编译（例如cppreference.com不编译，必须更改为concept EqualityComparable）

如果我从concept bool EqualityComparable函数声明中删除requires Functor<F>，则代码将编译并运行。

Answer 1

问题正是错误消息所说的：Functor_Impl<F>::template fmap<int, int>(x)不是有效的表达式。 Functor_Impl<std::vector>::fmap有两个参数，而不是一个。

需要静态模板方法的模板模板概念不满足约束

1 个答案: