EDIT2: The main problem is that brms does not support changing the declaration of parameters. Dirichlet prior by itself is no problem, but with a non-simplex parameter set it is useless.

Here is a workaround together with a similar dataset to make it work:

```
pacman::p_load(pacman, tidyverse, rio, magrittr, janitor, hablar, skimr, brms, pxweb, cmdstanr)
```

Data loading and preparation:

```
# PXWEB query
pxweb_query_list <-
list("Region"=c("0114","0115","0117","0120","0123","0125","0126","0127",
"0128","0136","0138","0139","0140","0160","0162","0163",
"0180","0181","0182","0183","0184","0186","0187","0188",
"0191","0192","0305","0319","0330","0331","0360","0380",
"0381","0382","0428","0461","0480","0481","0482","0483",
"0484","0486","0488","0509","0512","0513","0560","0561",
"0562","0563","0580","0581","0582","0583","0584","0586",
"0604","0617","0642","0643","0662","0665","0680","0682",
"0683","0684","0685","0686","0687","0760","0761","0763",
"0764","0765","0767","0780","0781","0821","0834","0840",
"0860","0861","0862","0880","0881","0882","0883","0884",
"0885","0980","1060","1080","1081","1082","1083","1214",
"1229","1230","1231","1233","1256","1257","1260","1261",
"1262","1263","1264","1265","1266","1267","1270","1272",
"1273","1275","1276","1277","1278","1280","1281","1282",
"1283","1284","1285","1286","1287","1290","1291","1292",
"1293","1315","1380","1381","1382","1383","1384","1401",
"1402","1407","1415","1419","1421","1427","1430","1435",
"1438","1439","1440","1441","1442","1443","1444","1445",
"1446","1447","1452","1460","1461","1462","1463","1465",
"1466","1470","1471","1472","1473","1480","1481","1482",
"1484","1485","1486","1487","1488","1489","1490","1491",
"1492","1493","1494","1495","1496","1497","1498","1499",
"1715","1730","1737","1760","1761","1762","1763","1764",
"1765","1766","1780","1781","1782","1783","1784","1785",
"1814","1860","1861","1862","1863","1864","1880","1881",
"1882","1883","1884","1885","1904","1907","1960","1961",
"1962","1980","1981","1982","1983","1984","2021","2023",
"2026","2029","2031","2034","2039","2061","2062","2080",
"2081","2082","2083","2084","2085","2101","2104","2121",
"2132","2161","2180","2181","2182","2183","2184","2260",
"2262","2280","2281","2282","2283","2284","2303","2305",
"2309","2313","2321","2326","2361","2380","2401","2403",
"2404","2409","2417","2418","2421","2422","2425","2460",
"2462","2463","2480","2481","2482","2505","2506","2510",
"2513","2514","2518","2521","2523","2560","2580","2581",
"2582","2583","2584"),
"Partimm"=c("S"),
"ContentsCode"=c("ME0104B6","ME0104B7"),
"Tid"=c("2002","2006","2010","2014","2018"))
# Download data
px_data <-
pxweb_get(url = "http://api.scb.se/OV0104/v1/doris/sv/ssd/ME/ME0104/ME0104C/ME0104T3",
query = pxweb_query_list)
# Convert to data.frame
px_data_frame <- as.data.frame(px_data, column.name.type = "text", variable.value.type = "text")
px_data_frame %>%
as_tibble() %>%
remove_constant() %>%
clean_names() %>%
rename(municipality = region, year = valar, proportion = andel_roster_av_giltiga_roster, count = antal_roster) %>%
pivot_longer(cols=c(count, proportion)) %>%
pivot_wider(names_from = c(name, year), values_from = value) %>%
select(-count_2002, -count_2006, -count_2010, -count_2014) %>%
mutate(across(starts_with("proportion_"), ~.x/100),
population_2018 = round(count_2018/proportion_2018)) %>%
select(-proportion_2018) -> s_data
```

The most important part:

```
stanvar(block = "parameters",
scode = "simplex[K_c] b_c;") ->
s_code
bf(count_2018|trials(population_2018) ~ a + b*c)+
lf(a ~ 1)+
lf(b ~ 1)+
lf(c ~ 0 + proportion_2002 + proportion_2006 + proportion_2010 + proportion_2014)+
set_nl(TRUE)+
binomial() ->
bf_formula
set_prior("normal(0, 1.5)", nlpar = "a")+
set_prior("normal(0, 1.5)", nlpar = "b")+
set_prior("dirichlet(rep_vector( 0.5, K_c ))", nlpar = "c", class="b") ->
priors
make_standata(formula = bf_formula,
prior = priors,
data = s_data,
stanvars = s_code) ->
stan_data
NULL -> class(stan_data)
make_stancode(formula = bf_formula,
prior = priors,
data = s_data,
stanvars = s_code) %>%
str_remove("vector\\[K_c\\] b_c\\;") %>% # remove the old declaration
write_stan_file() %>%
cmdstan_model() ->
cmdstanr_model
cmdstanr_model$sample(data = stan_data,
parallel_chains = 4) ->
cmdstanr_fit
cmdstanr_fit$output_files() %>%
rstan::read_stan_csv() ->
rstan_fit
cmdstanr_model ->
attributes(rstan_fit)$CmdStanModel
brm(formula = bf_formula,
prior = priors,
backend = "cmdstanr",
data = s_data,
empty = TRUE) ->
weighted_mean_model
rstan_fit ->
weighted_mean_model$fit
rename_pars(weighted_mean_model) ->
weighted_mean_model
summary(weighted_mean_model)
```

EDIT: Restructured the code a bit.