warmup draws omitted from stanfit objects

[wds15]

Right now, the conversion of stanfit objects into the draws format relies on the as.array function from Rstan. It would be great to document this somewhere and I would actually expect the function to convert all draws in the stanfit object, but the function drops the warmup samples without a way to influence this as the following example demonstrates:

library(rstan)
library(posterior)

ex_model_code <- '
  parameters {
    real alpha[2,3];
    real beta[2]; 
  } 
  model {
    for (i in 1:2) for (j in 1:3) 
      alpha[i, j] ~ normal(0, 1); 
    for (i in 1:2) 
      beta ~ normal(0, 2); 
  } 
'

## fit the model 
fit <- stan(model_code = ex_model_code, chains = 4) 

## we sampled and kept the warmup samples
fit

draws_post <- as_draws(fit)

## but converting to draws will omit the warmup draws! There is no way
## to get the warmup draws, since "as.array" is called. The as.array
## call will by default use "extract(x, permuted = FALSE, inc_warmup =
## FALSE)" such that warmup is always dropped.
draws_post

## I think, the default should be to return the warmup draws like so:
draws_post_alt <- as_draws(rstan::extract(fit, permuted = FALSE, inc_warmup = TRUE))

draws_post_alt