Reader functor in C++

Question

I'm trying to implement a reader functor in C++. Corresponding Haskell definition is fmap :: (a -> b) -> (r -> a) -> (r -> b)

My C++ version is:

template<class A, class B, class R>
B fmap(const std::function<B(A)> &funcA, const std::function<A(R)> &funcR) {
    return funcA(funcR());
}

std::string function_1(int n);
double function_2(std::string s);

fmap(function_2, function_1);

The error is:

note: candidate template ignored: could not match 'function<type-parameter-0-1 (type-parameter-0-0)>' against 'double (std::__1::basic_string<char>)'

B fmap(const std::function<B(A)> &funcA, const std::function<A(R)> &funcR) {

What is the correct way to implement fmap function?

Answer 1

You can do this with a neat template conversion trick from Template type deduction with std::function

#include <functional>
#include <iostream>
#include <string>
using namespace std;

template<class T>
struct AsFunction
    : public AsFunction<decltype(&T::operator())>
{};

template<class ReturnType, class... Args>
struct AsFunction<ReturnType(Args...)> {
    using type = std::function<ReturnType(Args...)>;
};

template<class ReturnType, class... Args>
struct AsFunction<ReturnType(*)(Args...)> {
    using type = std::function<ReturnType(Args...)>;
};


template<class Class, class ReturnType, class... Args>
struct AsFunction<ReturnType(Class::*)(Args...) const> {
    using type = std::function<ReturnType(Args...)>;
};

template<class F>
auto toFunction(F f) -> typename AsFunction<F>::type {
    return { f };
}

template<class A, class B, class R>
B fmap(const std::function<B(A)>& funcA, const std::function<A(R)>& funcR, R value) {
    return funcA(funcR(value));
}

template <class T>
auto ToFunction(T t) {
    return t;
}

std::string function_1(int n) {
    return ""s;
}

double function_2(std::string s) {
    return 0.0;
}

int main() {
    fmap(toFunction(function_2), toFunction(function_1), 5);
    return 0;
}