Change from split() to vector_split()

[MaurusGubser]

I use Birch for implementing a HMM for a model of two legs in a course of motion. Basically, I have a state transition model, which is linear and Gaussian with a non-diagonal covariance and an observation model, which is non-linear with Gaussian noise. Thus, I use a vector expression for the state variable, which I split up in a vector of expressions to compute the observation variable.

Previously, I used split in my code. After the most recent update of Birch, split is deprecated. I guess I should use _vectorsplit, but it is not clear to me, how a VectorSplitExpression is used. With split, I could define a new variable x (vector of expressions), whose elements I could access using the bracket operator, see code fragment below.

function state_to_observation(u:Random<Real[_]>) -> Array<Expression<Real>> {
  let x <- split<Real>(u);
  y:Array<Expression<Real>>;

  y.insert(1, x[15]*cst_0 + x[13]*cos(x[3]) + (x[14] + g)*sin(x[3]));
  ...
  return y;
}

How can I use _vectorsplit and how can I access the elements of the vector after using _vectorsplit?

Not sure if this is the right place to ask a question, but I did not find any other place to do so.

[lawmurray]

Thanks for the question. The aim has been to make the use of split unnecessary in circumstances such as this, so that you can just access elements directly with a new function element(). Can you try something like this?

function state_to_observation(u:Random<Real[_]>) -> Array<Expression<Real>> {
  y:Array<Expression<Real>>;
  y.insert(1, element(u, 15)*cst_0 + element(u, 13)*cos(element(u, 3)) + (element(u, 14) + g)*sin(element(u, 3));
  ...
  return y;
}

On the to-do list is to overload the square brackets again so that e.g. element(u, 15) can be replaced with (the nicer) u[15] again.