rstan::extract() fails when a parameter is renamed with a space

Summary:

The rstan::extract() function fails when used on a renamed parameter with a space in its name.

Description:

I often rename parameters inside of a stanfit object e.g. replace beta[1-4] with the names of the predictors that the coefficients correspond with. I do this so that when I create a regression table or plot from the stanfit, the result will already have the correct (and meaningful) parameter names. However, the rstan::extract() function fails when used with a parameter that has a custom name with a space in it. For example, if I have renamed a parameter foo bar, then extract(fit, pars = 'foo bar') produces the following error: Error in check_pars(allpars, pars) : no parameter foo bar.

Is this functionality that could be added, or will this never work because you can't name a parameter foo bar directly in a stan model due to the space?

Reproducible Steps:

I'm including an example of this behavior based on the help page for rstan::extract().

 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 <- stan(model_code = ex_model_code, chains = 4) 

 names(fit)[grep('beta', names(fit))] <- c('Beta1', 'Beta 2')

 extract(fit, pars = 'beta')
 extract(fit, pars = 'Beta1')
 extract(fit, pars = 'Beta 2')

RStan Version:

rstan_2.17.3

R Version:

R version 3.4.4 (2018-03-15)

Operating System:

OS X 10.13.3

Start with the reproducible stan model and the three rstan::extract() calls in the issue, then inspect extract()'s parameter-name handling. Done means extracting by a renamed parameter such as "Beta 2" succeeds while the existing beta and Beta1 selections continue to work.