hack stan::io::var_context for container variables with dimension of size 0

[mitzimorris]

Summary:

Currently validate_dims is a non-virtual function on class stan::io::var_context. Make this class virtural and move implementation to classes stan::io::dump and cmdstan::io::json.

Description:

In JSON notation, multi-dim arrays with leading dim 0 are indistinguishable from single-dim arrays of size 0, e.g., 3d array 0 x 0 x 0 is represented in JSON as '[ ]'. Therefore, for JSON notation, we need to hack the function validate_dims.

Rdump notation allows this - e.g.

d_m_ar <- structure(c(), .Dim = c(0, 0, 0))

defines a 3d array, dims 0 x 0 x 0

in JSON, this is represented as

    "d_m_ar" : [ ]

while [ [ [ ] ] ] describes a 1 x 1 x 0 array.

Reproducible Steps:

Please report steps to reproduce the issue. If it's not possible to reproduce, please include a description of how you discovered the issue.

If you have a reproducible example, please include it.

Current Output:

Currently, CmdStan and by extension CmdStanPy and CmdStanR cannot accept JSON inputs is there are any 0-size multi-dim container objects.

Expected Output:

Stan program:

data { 
  int d;
  vector[d] d_v1;
  vector[d] d_v2;
  row_vector[d] d_rv1;
  row_vector[d] d_rv2;
  vector[d] d_v_ar[d];
  row_vector[d] d_rv_ar[d];
  matrix[d, d] d_m;
} 

should run (and do nothing) for following JSON data file:

{
    "flag" : 0,
    "d" : 0,
    "d_v1" : [],
    "d_v2" : [],
    "d_rv1" : [],
    "d_rv2" : [],
    "d_v_ar" : [],
    "d_rv_ar" : [],
    "d_m" : [],
    "d_m_ar" : []
}

Additional Information:

see stan-dev/cmdstan#775

Current Version:

v2.21.0