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18.2 Functions as statements

In some cases, it makes sense to have functions that do not return a value. For example, a routine to print the lower-triangular portion of a matrix can be defined as follows.

functions {
  void pretty_print_tri_lower(matrix x) {
    if (rows(x) == 0) {
      print("empty matrix");
      return;
    }
    print("rows=", rows(x), " cols=", cols(x));
    for (m in 1:rows(x))
      for (n in 1:m)
        print("[", m, ",", n, "]=", x[m, n]);
  }
}

The special symbol void is used as the return type. This is not a type itself in that there are no values of type void; it merely indicates the lack of a value. As such, return statements for void functions are not allowed to have arguments, as in the return statement in the body of the previous example.

Void functions applied to appropriately typed arguments may be used on their own as statements. For example, the pretty-print function defined above may be applied to a covariance matrix being defined in the transformed parameters block.

transformed parameters {
  cov_matrix[K] Sigma;
  ... code to set Sigma ...
  pretty_print_tri_lower(Sigma);
  ...