MkList.cpp

#include <functional>
#include <list>

#include <boost/any.hpp>

using boost::any;
using std::list;

// list_cast :: list<any> -> list<T>
template<class T>
list<T> list_cast(list<any> l) {
  list<T> result;
  for(any elem: l) {
    result.push_back(any_cast<T>(elem));
  }
  return result;
};

class MkList {
  std::function<std::list<any>(any)> f;

public:
  // MkList constructor instantiates a type-erased version
  // of any polymorphic (template) function object
  template <class INPUT_FUNC_T>
  MkList(INPUT_FUNT_T in_f): {
    f = [in_f](any arg) { return in_f(arg); };
  }

  // MkList() type erases an arg, passes to f and type-casts the result
  template <class T>
  std::list<T> operator()(T arg) {
      any type_erased_arg = arg;
      std::list<any> type_erased_result = f(type_erased_arg);
      return list_cast<T>(type_erased_result);
  }
};

// Now suppose we have \x -> [x] translated to C++ as
struct to_list {
  template <class T>
  list<T> operator()(T arg) {
    list<T> result;
    result.push_back(arg);
    return result;
  }
}

// or as (C++14)
auto to_list2 = [](auto arg) {
  list<decltype(arg)> result;
  result.push_back(arg);
  return result;
}

// We can define singleton as:
auto singleton = MkList(to_list());

// or
auto singleton2 = MkList(to_list2);