Issue specifying a custom ordinal family

Interfaces brms
,
1

I am trying to specify a hurdle adjacent category shift model, but whenever specials = "ordinal" is supplied, brm will generate a block of unfinished function - as a custom family is not recognized in stan_ordinal_lpmf(). Are there any alternatives?

For example:

hurdle_acat <- custom_family(
    "hurdle_acat", 
    dpars       = c("mu", "hu", "disc"),
    links       = c("logit", "logit", "log"),
    type        = "int", 
    specials    = "ordinal",
    threshold   = "flexible"
)

stan_funs <- "
    // acat
    real acat_logit_lpmf(int y, real mu, real disc, vector thres) {
        int nthres = num_elements(thres);
        vector[nthres + 1] p = append_row(0, cumulative_sum(disc * (mu - thres)));
        return p[y] - log_sum_exp(p);
    }

    // hurdle acat 
    real hurdle_acat_logit_lpmf(int y, real mu, real hu, real disc, vector thres) {
        if (y == 0) {
            return bernoulli_lpmf(1 | hu);
        } else {
            return bernoulli_lpmf(0 | hu) +
                acat_logit_lpmf(y | mu, disc, thres);
        }
    }

    real hurdle_acat_logit_merged_lpmf(int y, real mu, real hu, real disc, vector thres, array[] int j) {
        return hurdle_acat_logit_lpmf(y | mu, hu, disc, thres[j[1]:j[2]]);
    }
"
stanvars <- stanvar(scode = stan_funs, block = "functions")
make_stancode(formula = formula, data = df, family = hurdle_acat, stanvars = stanvars)

It returns:

// generated with brms 2.23.0
functions {
  /* custom-logit log-PDF for a single response
   * Args:
   *   y: response category
   *   mu: latent mean parameter
   *   disc: discrimination parameter
   *   thres: ordinal thresholds
   * Returns:
   *   a scalar to be added to the log posterior
   */
   real custom_logit_lpmf(int y, real mu, real disc, vector thres) {
  /* custom-logit log-PDF for a single response and merged thresholds
   * Args:
   *   y: response category
   *   mu: latent mean parameter
   *   disc: discrimination parameter
   *   thres: vector of merged ordinal thresholds
   *   j: start and end index for the applid threshold within 'thres'
   * Returns:
   *   a scalar to be added to the log posterior
   */
   real custom_logit_merged_lpmf(int y, real mu, real disc, vector thres, array[] int j) {
     return custom_logit_lpmf(y | mu, disc, thres[j[1]:j[2]]);
   }

  
    // acat
    real acat_logit_lpmf(int y, real mu, real disc, vector thres) {
        int nthres = num_elements(thres);
        vector[nthres + 1] p = append_row(0, cumulative_sum(disc * (mu - thres)));
        return p[y] - log_sum_exp(p);
    }

    // hurdle acat 
    real hurdle_acat_logit_lpmf(int y, real mu, real hu, real disc, vector thres) {
        if (y == 0) {
            return bernoulli_lpmf(1 | hu);
        } else {
            return bernoulli_lpmf(0 | hu) +
                acat_logit_lpmf(y | mu, disc, thres);
        }
    }

    real hurdle_acat_logit_merged_lpmf(int y, real mu, real hu, real disc, vector thres, array[] int j) {
        return hurdle_acat_logit_lpmf(y | mu, hu, disc, thres[j[1]:j[2]]);
    }

}