gistfile1.hs

data Natural = Zero | Succ Natural

add :: Natural -> Natural -> Natural
add Zero b = b
add (Succ a) b = add a (Succ b)

mul :: Natural -> Natural -> Natural
mul Zero _ = Zero
mul _ Zero = Zero
mul (Succ a) b = add b (mul a b)

gistfile2.cpp

struct Zero {};

template <typename a>
struct Succ {};

// determine if a type is a natural number
template <typename a>
struct Natural;

template <>
struct Natural<Zero> { 
    using type = true_t;
};

template <typename a>
struct Natural<Succ<a>> {
    using type = Natural<a>::type;
};

// addition
template <typename a, typename b>
struct add;

template <typename b, typename = Natural<b>>
struct add<Zero, b> {
    using type = b;
};

template <typename a, typename b,
    typename = Natural<a>, typename = Natural<b>>
struct add<Succ<a>, b> {
    using type = add<a, Succ<b>>::type;
};

// multiplication
template <typename a, typename b>
struct mul;

template <typename a, typename = Natural<a>>
struct mul<Zero, a> {
    using type = Zero;
};

template <typename a, typename = Natural<a>>
struct mul<a, Zero> {
    using type = Zero;
};

template <typename a, typename b, 
    typename = Natural<a>, typename = Natural<b>>
struct mul<Succ<a>, b> {
    using type = add<b, mul<a, b>::type>::type;
}