lazy_fibs.cxx

#include <iostream>
using namespace std;

#define LAZY(expression) Lazy<Cons>{[=]{ return expression.get(); }}
#define LAZY_REF(expression) Lazy<Cons>{[&]{ return expression.get(); }}

template<typename T>
class Lazy {
    private:
        mutable shared_ptr<T> value;
        function<shared_ptr<T>()> thunk;

    public:
        Lazy(shared_ptr<T> value) : value(value) {}
        Lazy(function<shared_ptr<T>()> thunk) : thunk(thunk) {}

        shared_ptr<T> get() const {
            if (not value)
                value = thunk();
            return value;
        }

        shared_ptr<T> operator->() const { return get(); }
};

struct Cons {
    intmax_t head;
    Lazy<Cons> tail;
    Cons(intmax_t head, Lazy<Cons> tail) : head(head), tail(tail) {}
};

Lazy<Cons> cons(intmax_t head, Lazy<Cons> tail) {
    return make_shared<Cons>(head, tail);
}

Lazy<Cons> tail(Lazy<Cons> c) { return LAZY(c->tail); }

Lazy<Cons> zipWithPlus(Lazy<Cons> xs, Lazy<Cons> ys) {
    return cons(xs->head + ys->head, LAZY(zipWithPlus(tail(xs), tail(ys))));
}

int main() {
    Lazy<Cons> const fibs =
        cons(0, cons(1, LAZY_REF(zipWithPlus(fibs, tail(fibs)))));

    shared_ptr<Cons> it = fibs.get();
    for (int i = 0; i < 90; ++i, it = it->tail.get())
        cout << it->head << endl;

    return 0;
}