2005年08月10日

#include <iostream>
using namespace std;
 
#define cons(x, y)      Cons< Int< x >, y >
#define nil             Nil
 
// nil  or []
struct Nil {};
 
// cons h t  or h:t
template <typename H, typename T>
struct Cons {
    typedef H head;
    typedef T tail;
};
 
// element of lists
template<int n>
struct Int{
    enum {val = n};
    static void print() { cout << n; }
};
 
// printer
template <typename L>
struct ListPrinter {
    static void print(){
        cout << '[';
        L::head::print();
        ListPrinter<typename L::tail>::print2();
        cout << ']';
    }
    static void print2(){
        cout << ',';
        L::head::print();
        ListPrinter<typename L::tail>::print2();
    }
};
template <>
struct ListPrinter<Nil> {
    static void print(){
        cout << '[' << ']';
    }
    static void print2(){
    }
};
 
// foldr function
// foldr f e [] = e
// foldr f e (h:t) = f h (foldr f e t)
// cons case:
template<template<typename,typename> class F, typename E, typename L >
struct Foldr{
    typedef typename F<typename L::head, typename  Foldr<F,E,typename L::tail>::val >::val val;
};
// nil case:
template<template<typename,typename> class F, typename E>
struct Foldr<F,E,Nil>{
    typedef E val;
};
 
// map function
// map f [] = []
// map f (h:t) = f h:(map f t)
// cons case:
template<template<typename> class F, typename L >
struct Map{
    typedef Cons< typename F<typename L::head>::val , typename Map<F, typename L::tail>::val> val;
};
// nil case:
template<template<typename> class F>
struct Map<F,Nil>{
    typedef Nil val;
};
 
//addition operator
template <typename X, typename Y>
struct Add {
    typedef Int<X::val + Y::val> val;
};
 
// zero
typedef Int<0> Zero;
 
//douoblring operator
template <typename X>
struct Double{
    typedef Int<X::val + X::val> val;
};
 
int main(int argc, char *argv[])
{
    typedef cons(4, cons(3, cons(2, cons(1, nil)))) l;
    typedef cons(5, l) l2;
    ListPrinter< l >::print();  cout << endl;
    ListPrinter< l2 >::print(); cout << endl;
     typedef Foldr<Add, Zero, l2>::val sum;
    sum::print(); cout << endl;
     typedef Map<Double, l2>::val l3;
    ListPrinter< l3 >::print(); cout << endl;
}