[Stan 2.33] Expose functions with tuple return type or argument type

[adamgorm]

Description

expose_functions fails for cmdstan models where at least one user-defined function either returns a tuple or takes a tuple as argument.

It would be perfect if we could for instance use R lists as the interface to Stan tuples, such that

• functions that return a tuple return a list when called from R,
• functions that take a tuple as argument take a list as argument when called from R.

If this is not possible, then it would also be very useful to be able to expose all other functions than those with tuple return/argument type (so that one can make utility functions returning tuples / taking tuples as arguments, while still being able to expose all other functions).

Reproducible examples

1: Fails

We can't expose a function that returns a tuple.

functions {
  tuple(int, vector)
  f(int a)
  {
    tuple(int, vector[5]) x;
    x.1 = a * 2;
    x.2 = rep_vector(a, 5);
    return x;
  }
}

2: Fails

We can't expose a function that takes a tuple as argument.

functions {
  vector
  f(tuple(int, vector) x)
  {
    return x.2;
  }
}

3: Works

However, note that we can expose a function with a tuple in the function body.

functions {
  vector
  f(int a)
  {
    tuple(int, vector[5]) x;
    x.1 = a * 2;
    x.2 = rep_vector(a, 5);
    return x.2;
  }
}

[andrjohns]

Thanks for flagging!

It looks like Rcpp doesn't support tuples out-of-the box, I'll have a look at adding the wrappers that we need

[jgabry]

This is fixed now by @andrjohns' PR #860 and I just checked that exposing the functions in the examples above works without error now. I'm going close this issue for now, but @adamgorm if you run into further trouble with this we can reopen the issue. Thanks for opening this.