List comprehension in C++ using functional patterns, C++ (clang)

List comprehension in C++ using functional patterns

//clang 3.8.0

#include <iostream>
#include <list>
#include <iterator>
#include <type_traits>
#include <algorithm>
#include <iostream>

// example of applying the monad pattern to create a list comprehension like structure in C++
/*
  The list monad
*/

//The unit list containing 'a'
/*
  let unit : 'a -> 'a t = fun a -> [a]
*/
template <class A> 
std::list<A> unit (A const& a) { return std::list<A> (1u, a); }

//The 'bind' operator
/*
  let rec ( * ) : 'a t -> ('a -> 'b t) -> 'b t =
    fun l -> fun k ->
      match l with | [] -> [] | (h :: tl) -> k h @ tl * k
*/
template <class A, class F>
typename std::result_of<F(A)>::type 
operator * (std::list<A> a, F k) {
  typedef typename std::result_of<F(A)>::type result_t;

if (a.empty ())
    return result_t ();

result_t res = k (a.front ());
  a.pop_front ();
  res.splice (res.end (), a * k);

return res;
}

int main()
{
    
  //l = [1, 2, 3]
  std::list<int> l = {1, 2, 3};
  
  //m = [1, 4, 9]
  auto m = l * [](int x) { return unit (float (x * x)); };
  for (int n : m) {
     std::cout << n << '\n';
  }
}


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