I’m interested in trying out a few different link functions for regression on the mixture proportion simplex. This is how my custom family is currently set up:

```
ei_binomial <- custom_family(
"ei_binomial",
dpars = c("mu", "po", "pm"),
links = rep("identity", 3),
lb = c(NA, 0, 0), ub = c(NA, 1, 1),
type = "int", vars = c("yo[n]", "ym[n]", "trials[n]")
)
ei_binomial_funs <- "
real ei_binomial_lpmf(int y, real mu, real po, real pm, int yo, int ym, int T) {
return log(yo*po + ym*pm + (1-po-pm)*exp(binomial_logit_lpmf(y|T, mu)));
}
int ei_binomial_rng(real mu, real po, real pm, int T) {
int which_component = categorical_rng([po, pm, (1-po-pm)]');
if (which_component == 1) {
return 0;
}
if (which_component == 2) {
return T;
}
return binomial_rng(T, inv_logit(mu));
}"
```

The resulting fits are consistent with a model I previously specified directly in Stan that uses a proper simplex for the proportions, but the joint posterior of the mixing proportions doesn’t constrain the sum to 1 so it behaves badly when trying to do posterior checks.

I was wondering whether it’s possible to specify a simplex parameter for a custom family or if there’s some other approach that might work. I know that I could use unrestricted parameters and transform them inside the pmf but I was hoping to be able to use a non-linear specification to easily try out different link functions from R itself.

- Operating System: Ubuntu 18.04.1 LTS
- brms Version: 2.7.0