Inability to parse within formula variable transformations

First of all, thanks for this tremendous work. This is a huge step forward in increasing the accessibility of building Bayesian mixed-effect models efficiently and effectively.

I'm not sure that it is necessary to support the feature of within formula variable transformation, but it should be documented that inline variable transformations do not always compile as expected. Here is a simple example:

library(lme4)
library(rstan)
library(glmer2stan)
data(sleepstudy) # built into lme4

# fit with lme4
m1_lme4 <- lmer( Reaction ~ Days + (Days | Subject), sleepstudy, REML=FALSE )

# construct subject index --- glmer2stan forces you to manage your own cluster indices
sleepstudy$subject_index <- as.integer(as.factor(sleepstudy$Subject))

# fit with glmer2stan
m1_g2s <- lmer2stan( Reaction ~ Days + (Days | subject_index), data=sleepstudy )

# add squared terms

m2_lme4 <- lmer( Reaction ~ Days + I(Days^2) + (Days | Subject), sleepstudy, REML=FALSE )

m2_g2s <- lmer2stan( Reaction ~ Days + I(Days^2) + (Days | subject_index), data=sleepstudy )

In this case, m2_g2s fails to compile:

Starting Stan model

TRANSLATING MODEL 'Reaction ~ Days + I(Days^2) + (Days | subject_index) [gaussian]' FROM Stan CODE TO C++ CODE NOW.
Error in stanc(file = file, model_code = model_code, model_name = model_name,  : 
  failed to parse Stan model 'Reaction ~ Days + I(Days^2) + (Days | subject_index) [gaussian]' with error message:
EXPECTATION FAILURE LOCATION: file=input; line=5, column=15

    real IDays^2[N];
              ^-- here

DIAGNOSTIC(S) FROM PARSER:
Parser expecting: ";"

And here is the sessionInfo(), though I have also tested on a 64bit SLES platform and found the same results:

R version 3.0.2 (2013-09-25)
Platform: i386-w64-mingw32/i386 (32-bit)

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rstan_2.0.1      glmer2stan_0.994 inline_0.3.13    Rcpp_0.10.6      devtools_1.4.1  
[6] lme4_1.0-5       Matrix_1.1-0     lattice_0.20-24 

loaded via a namespace (and not attached):
 [1] codetools_0.2-8 digest_0.6.4    evaluate_0.5.1  grid_3.0.2      httr_0.2       
 [6] MASS_7.3-29     memoise_0.1     minqa_1.2.1     nlme_3.1-113    parallel_3.0.2 
[11] RCurl_1.95-4.1  splines_3.0.2   stats4_3.0.2    stringr_0.6.2   tools_3.0.2    
[16] whisker_0.3-2

I don't think this is a bug -- just a fact that should be documented at some point.

rmcelreath / glmer2stan

Inability to parse within formula variable transformations #3