# Barriers to New Developers

**URL:** <https://discourse.mc-stan.org/t/barriers-to-new-developers/163>\
**Category:** Developers\
**Created:** [December 15, 2016, 8:12pm UTC](https://discourse.mc-stan.org/t/barriers-to-new-developers/163 "2016-12-15T20:12:56Z")\
**Posts on this page:** 1\
**Showing post:** 39

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**Author:** ![Jonas\_K](https://avatars.discourse-cdn.com/v4/letter/j/bbe5ce/32.png) [@Jonas\_K](https://discourse.mc-stan.org/u/Jonas_K)\
**Post date:** [December 11, 2017, 3:50pm UTC](https://discourse.mc-stan.org/t/barriers-to-new-developers/163/39 "2017-12-11T15:50:37Z")

</div>

Hi Stan Developers,

I would like add to this discussion from my own experience. Information on Stan is too scattered, often not clear, and sometimes even conflicting. In the post [Multivariate Function with Known Gradients - RStan](http://discourse.mc-stan.org/t/multivariate-function-with-known-gradients-rstan/2704) I was asking for help to implement the a simple function that is callable from a Stan program and uses pre-calculated gradients. I have tried to follow the approach in [https://github.com/stan-dev/math/wiki/Adding-a-new-function-with-known-gradients](https://github.com/stan-dev/math/wiki/Adding-a-new-function-with-known-gradients) to add my function, but without success. Additionally, I have tried to write my function in the same way as the `log_determinant` in the Stan Math Library Reference:

```
namespace stan {
        namespace math {

        inline var vecToDoubleTwo(const Eigen::Matrix<var, Eigen::Dynamic, 1>& m) {
            using Eigen::Matrix;

            Matrix<double, Eigen::Dynamic, 1> m_d(m.rows());
            for (int i = 0; i < m.size(); ++i)
                m_d(i) = m(i).val();

            double val = 0;
            for (int i=0; i<m.size(); ++i) val += m_d(i) * m_d(i);

            vari** varis
            = ChainableStack::memalloc_.alloc_array<vari*>(m.size());
            for (int i = 0; i < m.size(); ++i)
                varis[i] = m(i).vi_;

            double* gradients
            = ChainableStack::memalloc_.alloc_array<double>(m.size());
            for (int i = 0; i < m.size(); ++i)
                gradients[i] = 2 * m_d(i);

            return var(new precomputed_gradients_vari(val, m.size(),
                                                      varis, gradients));
        }

    }
}

```

This was also without success.

In both occasions I could use the function from a C++ program with the intended interface of the Stan Math Library. However, as soon as the function should be called from a Stan program, there are issues with variable types. The function works and Stan can find it. There is no problem to print output with it, e.g.

```
print(vecToDoubleTwo(x));

```

but

```
target += vecToDoubleTwo(x);

```

results in an error message saying that a `var` can’t be assign to a `double` (according to the Stan Math Library Reference, a `var` should never be assigned to `double`). It works nevertheless for the `log_determinant` and I can’t find the correct information to fill in the missing links on how to implement the function correctly.

I will appreciate any help.

Regards,  
Jonas

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