A function with no arguments is just a value in Haskell. You don't call it, you just use it. Since there are no side effects, there is no observable difference.
In OCaml there are simply no parameter-less functions, to get something like that you need to add a dummy unit argument.
Not so in C++. C++, unlike Haskell and OCaml, maintains clear difference between f
and f()
. bind
gives you the former because you can always turn it into the latter by adding ()
. You can write your own wrapper for bind
that does just that quite easily. Going the other way around would be a tad more difficult.
Here's a possible implementation of such wrapper:
#include <functional>
#include <utility>
#include <iostream>
template <typename T>
struct is_noargs_callable {
private:
typedef char(&yes)[1];
typedef char(&no)[2];
template<typename U>
static yes test(decltype((std::declval<U>())())*);
template<typename>
static no test(...);
public:
static const bool value = sizeof(test<T>(0)) == sizeof(yes);
};
template <typename T>
struct is_noargs_callable<T()> {
static const bool value = true;
};
template <typename T>
struct is_noargs_callable<T(...)> {
static const bool value = true;
};
template <typename T>
auto call_me_if_you_can(T t) -> typename std::enable_if<is_noargs_callable<T>::value, decltype(t())>::type
{
return t();
}
template <typename T>
auto call_me_if_you_can(T t) -> typename std::enable_if<!is_noargs_callable<T>::value, T>::type
{
return t;
}
template <typename... Args>
auto apply(Args&&... args) -> decltype(call_me_if_you_can(std::bind(args...))) {
return call_me_if_you_can(std::bind(args...));
}
// testing
void foo(int a, int b, int c) { std::cout << "foo(" << a << "," << b << "," << c << ")"; }
int main ()
{
using namespace std::placeholders;
std::cout << "zero : " ; apply(foo, _1, _2, _3); std::cout << " : " ; apply(foo, _1, _2, _3)(1,2,3); std::cout << std::endl;
std::cout << "one : " ; apply(foo, 1, _1, _2); std::cout << " : " ; apply(foo, 1, _1, _2)(2,3); std::cout << std::endl;
std::cout << "two : " ; apply(foo, 1, 2, _1); std::cout << " : " ; apply(foo, 1, 2, _1)(3); std::cout << std::endl;
std::cout << "three: " ; apply(foo, 1, 2, 3); std::cout << " : "; /* nothing to test here */ std::cout << std::endl;
}
However, killing the difference between f
and f()
just in this one place does not IMHO contribute to the overall consistency of C++ programming. If you don't like the distinction, kill it everywhere (or just use you a Haskell for great good).