error: no viable conversion from 'tuple<[...], std::__1::tuple<unsigned long long, unsigned long long>>' to 'const tuple<[...], uint_type>'

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Suhaib

I'm trying to implement Ackerman function using a tuple of unsinged long long

This is the line that i'm trying to create

A(m, n) = A(m - 1, A(m, n - 1))

So here is my code:

    uint_type calculate(tuple_type const& args) const {

    uint_type retval;

        uint_type retval;

        auto second = make_tuple(m - 1, calculate(make_tuple(m, n - 1)));
        auto first = make_tuple(m - 1, second);
        retval = calculate(first);
        return retval;
       }
   };

and here is the error:

error: no viable conversion from 'tuple<[...], std::__1::tuple>' to 'const tuple<[...], uint_type>' retval = calculate(firstTuple);

^~~~~~~~~~emphasized text

Dietrich Epp

Type errors are sometimes hard to find if you use a lot of auto and template functions like make_tuple. Here's how you'd write it with less use of auto and make_tuple:

uint_type calculate(tuple_type const& args) const {
    const uint_type m = get<0>(args);
    const uint_type n = get<1>(args);

    uint_type retval;

    tuple_type second{m - 1, calculate(tuple_type{m, n - 1})};
    tuple_type first{m - 1, second};
    //                      ^^^^^^ error
    retval = calculate(first);
    return retval;
}

You can see how adding a couple type annotations moved the error message exactly to where the error is.

However, that's not the Ackermann function. This is the Ackermann function:

uint_type calculate(tuple_type args) const {
    const uint_type m = get<0>(args);
    const uint_type n = get<1>(args);
    if (m == 0)
        return n + 1;
    if (n == 0)
        return calculate(tuple_type{m - 1, 1});
    return calculate(tuple_type{m - 1, calculate(tuple_type{m, n - 1})});
}

Note that the tuple_type can be omitted:

uint_type calculate(tuple_type args) const {
    const uint_type m = get<0>(args);
    const uint_type n = get<1>(args);
    if (m == 0)
        return n + 1;
    if (n == 0)
        return calculate({m - 1, 1});
    return calculate({m - 1, calculate({m, n - 1})});
}

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