RStan error passing issue with parallel chains

rstan_error_hiding_chains_exampe.R

library(rstan)

# simple model with a bug
model_code <- "
parameters{
    real a;
}
model{
    a ~ normal(0,1);
}
generated quantities{
    real buggy; // undefined, so will throw an error after sampling
}
"

# running with parallel cores hides undefined variable message
# instead get crypted message about slot "mode"
m1 <- stan( model_code=model_code , chains=2 , cores=2 )

# now run with 1 chain (or 1 core) to see problem with buggy variable
m2 <- stan( model_code=model_code , chains=1 )