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 | Posted by Dennis Ritchie |  Permalink Reply |
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Dennis Ritchie 
| Hi,
Is it possible to write on D recursion using std.variant?
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#include <boost/variant.hpp>
#include <iostream>
struct Nil {};
auto nil = Nil{};
template <typename T>
struct Cons;
template <typename T>
using List = boost::variant<Nil,
boost::recursive_wrapper<Cons<T>>>;
template <typename T>
struct Cons {
Cons(T val, List<T> list) : head(val), tail(list) {}
T head;
List<T> tail;
};
template <typename T>
class length_visitor : public boost::static_visitor<size_t> {
public:
int operator()(Nil) const {
return 0;
}
int operator()(const Cons<T>& c) const {
return 1 + length(c.tail);
}
};
template <typename T>
auto cons(T head, List<T> tail) {
return List<T>(Cons<T>(head, tail));
}
template <typename T>
auto cons(T head, Nil) {
return List<T>(Cons<T>(head, List<T>(Nil{})));
}
template <typename T>
size_t length(const List<T>& list) {
return boost::apply_visitor(length_visitor<T>(), list);
}
int main() {
auto l = cons(3, cons(2, cons(1, nil)));
std::cout << length(l) << std::endl; // prints 3
return 0;
}
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http://ideone.com/qBuOvJ
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