Re: [eigen] Using custom scalar types

```On 6/16/12 10:55 AM, Brad Bell wrote:
```
```Looking at the example in
http://eigen.tuxfamily.org/dox/TopicCustomizingEigen.html#CustomScalarType
one sees
... snip ...
IsInteger = 0,
IsSigned,
I suspect that IsSigned should be 1 for this example ?
```
```
That's how we did it -- IsSigned = 1.  There are examples for
scalars and complex in Eigen itself in Core/NumTraits.h
that we followed.

We have a pretty extensive integration of our
reverse-mode auto-dif with Eigen in the Stan
project for MCMC sampling in Bayesian models (we
use Hamiltonian Monte Carlo, which requires gradients).
The basic auto-dif is a more OO approach to the basic

Here's the definitions for matrices:

For Eigen, we also defined scalar_product_traits<var,double>
and <double,var>, which gives us some mixed-operation
capability, but not enough through all the expression
templates.

We wound up short-circuiting all of the expression
templates with a very expensive conversion of double
values to auto-dif vars for mixed matrix ops.  You see
this in the to_var functions (available for double and
var so we could still write relatively templated matrix ops).
We're now looking into whether we can add an implicit constructor
for Matrix<var,...> based on a Matrix<double,...>.

We have had no problem at all using Eigen with all
var type inputs.  We've used the LLT solvers, determinants,
inverses, Eigenvalues/vectors, dot products, norms, etc. etc.
We've validated the derivatives versus known matrix derivatives.
We found the Magnus and Neudecker book and The Matrix Cookbook
really useful in this regard as they have complementary sets
of formulas for matrix derivatives.

Compile times got so long with header-only use in the
combination of auto-dif and Eigen that we eventually broke out a
..cpp file for the functions that was really slowing us down.
We could move more from the .hpp to the .cpp.

The real win performance-wise was vectorizing the
derivatives for dot products (we're mainly using this
for statistical models like regressions where there's
a lot of dot products -- also matrix products are
just bunches of dot products).  This unfortunately circumvents
all the expression templates in Eigen, so it's not the
optimal solution -- that would be to do template specializations
for auto-dif for all the expression templates.

```
```Furthermore, one also sees
... snip ...
... snip ...
Is this a typo, or should the argument y be passed by value instead of by const reference ?
```
```
We've had no problem just using argument-dependent lookup.
So we never defined any internal:: versions of special
functions.

- Bob Carpenter
Columbia Uni, Dept of Statistics

```

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