---
title: "RStan: the R interface to Stan"
author: "Stan Development Team"
date: "`r Sys.Date()`"
output: 
  rmarkdown::html_vignette:
    toc: true
bibliography: rstan.bib
vignette: >
  %\VignetteIndexEntry{RStan}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include=FALSE}
library(rstan)
knitr::opts_chunk$set(
  echo = TRUE,
  comment = NA,
  fig.align = "center",
  fig.height = 5,
  fig.width = 7
  )
```

In this vignette we present RStan, the R interface to Stan. Stan is a C++
library for Bayesian inference using the No-U-Turn sampler (a variant of
Hamiltonian Monte Carlo) or frequentist inference via optimization. We
illustrate the features of RStan through an example in
@GelmanCarlinSternRubin:2003.


## Introduction 

Stan is a C++ library for Bayesian modeling and inference that primarily uses
the No-U-Turn sampler (NUTS) [@hoffman-gelman:2012] to obtain posterior
simulations given a user-specified model and data. Alternatively, Stan can
utilize the LBFGS optimization algorithm to maximize an objective function, such
as a log-likelihood. The R package __rstan__ provides RStan, the R interface to
Stan. The __rstan__ package allows one to conveniently fit Stan models from R
[@rprj] and access the output, including posterior inferences and intermediate
quantities such as evaluations of the log posterior density and its gradients.

In this vignette we provide a concise introduction to the functionality included
in the __rstan__ package. Stan's website [mc-stan.org](http://mc-stan.org) has 
additional details and provides up-to-date information about how to operate both
Stan and its many interfaces including RStan. See, for example, _RStan Getting 
Started_ [@rstangettingstarted2012].


## Prerequisites

Stan has a modeling language, which is similar to but not identical to that of
the Bayesian graphical modeling package BUGS [@WinBUGS]. A parser translates a
model expressed in the Stan language to C++ code, whereupon it is compiled to an
executable program and loaded as a Dynamic Shared Object (DSO) in R which can
then be called by the user.

A C++ compiler, such as [`g++`](http://gcc.gnu.org) or 
[`clang++`](http://clang.llvm.org), is required for this process. For
instructions on installing a C++ compiler for use with RStan see 
[RStan-Getting-Started](https://github.com/stan-dev/rstan/wiki/RStan-Getting-Started#prerequisites).

The __rstan__ package also depends heavily on several other R packages:

* __StanHeaders__  (Stan C++ headers)
* __BH__ (Boost C++ headers)
* __RcppEigen__ (Eigen C++ headers)
* __Rcpp__ (facilitates using C++ from R)
* __inline__ (compiles C++ for use with R)

These dependencies should be automatically installed if you install the 
__rstan__ package via one of the conventional mechanisms.

## Typical Workflow

The following is a typical workflow for using Stan via RStan for Bayesian
inference.

1. Represent a statistical model by writing its log posterior density (up to an 
normalizing constant that does not depend on the unknown parameters in the 
model) using the Stan modeling language. We recommend using a separate file with
a `.stan` extension, although it can also be done using a character string
within R.
2. Translate the Stan program to C++ code using the `stanc` function.
3. Compile the C++ code to create a DSO (also called a dynamic link library
(DLL)) that can be loaded by R.
4. Run the DSO to sample from the posterior distribution.
5. Diagnose non-convergence of the MCMC chains.
6. Conduct inference based on the posterior sample (the MCMC draws from the
posterior distribution).

Conveniently, steps 2, 3, and 4, above, are all performed implicitly by a single
call to the `stan` function.


## Example

Throughout the rest of the vignette we'll use a hierarchical meta-analysis model
described in section 5.5 of @GelmanCarlinSternRubin:2003 as a running example. A
hierarchical model is used to model the effect of coaching programs on college
admissions tests. The data, shown in the table below, summarize the results of
experiments conducted in eight high schools, with an estimated standard error
for each. These data and model are of historical interest as an example of full
Bayesian inference [@Rubin1981]. For short, we call this the _Eight
Schools_ examples.

School | Estimate ($y_j$) | Standard Error ($\sigma_j$)
------ | -------- | --------------
A      | 28       | 15
B      | 8        | 10
C      | -3       | 16
D      | 7        | 11
E      | -1       | 9
F      | 1        | 11
G      | 18       | 10
H      | 12       | 18

We use the Eight Schools example here because it is simple but also represents a
nontrivial Markov chain simulation problem in that there is dependence between
the parameters of original interest in the study --- the effects of coaching in
each of the eight schools --- and the hyperparameter representing the variation
of these effects in the modeled population.  Certain implementations of a Gibbs
sampler or a Hamiltonian Monte Carlo sampler can be slow to converge in this
example.

The statistical model of interest is specified as

$$
\begin{aligned} 
y_j &\sim \mathsf{Normal}(\theta_j, \sigma_j), \quad j=1,\ldots,8 \\
\theta_j &\sim \mathsf{Normal}(\mu, \tau), \quad j=1,\ldots,8 \\
p(\mu, \tau) &\propto 1,
\end{aligned}
$$

where each $\sigma_j$ is assumed known.


### Write a Stan Program

RStan allows a Stan program to be coded in a text file
(typically with suffix `.stan`) or in a R character vector (of length one). We
put the following code for the Eight Schools model into the file `schools.stan`:

```{r, echo=FALSE, comment=""}
cat(readLines("schools.stan"), sep = "\n")
```

The first section of the Stan program above, the `data` block, specifies the 
data that is conditioned upon in Bayes Rule: the number of schools, $J$, the 
vector of estimates, $(y_1, \ldots, y_J)$, and the vector of standard errors of
the estimates $(\sigma_{1}, \ldots, \sigma_{J})$. Data are declared as integer
or real and can be vectors (or, more generally, arrays) if dimensions are 
specified. Data can also be constrained; for example, in the above model $J$ has
been restricted to be at least $1$ and the components of $\sigma_y$ must all be 
positive.

The `parameters` block declares the parameters whose posterior distribution is
sought. These are the the mean, $\mu$, and standard deviation, $\tau$, of the
school effects, plus the _standardized_ school-level effects $\eta$. In this
model, we let the unstandardized school-level effects, $\theta$, be a
transformed parameter constructed by scaling the standardized effects by
$\tau$ and shifting them by $\mu$ rather than directly declaring $\theta$ as a
parameter. By parameterizing the model this way, the sampler runs more 
efficiently because the resulting multivariate geometry is more amendable to 
Hamiltonian Monte Carlo [@Neal:2011].

Finally, the `model` block looks similar to standard statistical notation.
(Just be careful:  the second argument to Stan's normal$(\cdot,\cdot)$
distribution is the standard deviation, not the variance as is usual in
statistical notation). We have written the model in vector notation, which
allows Stan to make use of more efficient algorithmic differentiation (AD). It
would also be possible --- but less efficient --- to write the model by
replacing `normal_lpdf(y | theta,sigma)` with a loop over the $J$ schools,

```
for (j in 1:J) 
  target += normal_lpdf(y[j] | theta[j],sigma[j]);
```

Stan has versions of many of the most useful R functions for statistical 
modeling, including probability distributions, matrix operations, and various
special functions. However, the names of the Stan functions may differ from
their R counterparts and, more subtly, the parameterizations of probability
distributions in Stan may differ from those in R for the same distribution. To
mitigate this problem, the `lookup` function can be passed an R function or
character string naming an R function, and RStan will attempt to look up the
corresponding Stan function, display its arguments, and give the page number in
@StanManual where the function is discussed.

```{r, lookup}
lookup("dnorm")
lookup(dwilcox)   # no corresponding Stan function
```

If the `lookup` function fails to find an R function that corresponds to a 
Stan function, it will treat its argument as a regular expression and attempt to
find matches with the names of Stan functions.

### Preparing the Data

The `stan` function accepts data as a named list, a character vector of object
names, or an `environment`. Alternatively, the `data` argument can be omitted
and R will search for objects that have the same names as those declared in the
`data` block of the Stan program. Here is the data for the Eight Schools
example:

```{r, schools-data}
schools_data <- list(
  J = 8,
  y = c(28,  8, -3,  7, -1,  1, 18, 12),
  sigma = c(15, 10, 16, 11,  9, 11, 10, 18)
)
```

It would also be possible (indeed, encouraged) to read in the data from a file
rather than to directly enter the numbers in the R script.


### Sample from the Posterior Distribution

Next, we can call the `stan` function to draw posterior samples:

```{r, callstan, results="hide"}
library(rstan)
fit1 <- stan(
  file = "schools.stan",  # Stan program
  data = schools_data,    # named list of data
  chains = 4,             # number of Markov chains
  warmup = 1000,          # number of warmup iterations per chain
  iter = 2000,            # total number of iterations per chain
  cores = 2,              # number of cores (using 2 just for the vignette)
  refresh = 1000          # show progress every 'refresh' iterations
  )
```

The `stan` function wraps the following three steps: 

* Translate a model in Stan code to C++ code 
* Compile the C++ code to a dynamic shared object (DSO) and load the DSO
* Sample given some user-specified data and other settings

A single call to `stan` performs all three steps, but they can also be executed 
one by one (see the help pages for `stanc`, `stan_model`, and `sampling`), which
can be useful for debugging. In addition, Stan saves the DSO so that when the 
same model is fit again (possibly with new data and settings) we can avoid
recompilation. If an error happens after the model is compiled but before
sampling (e.g., problems with inputs like data and initial values), we can still
reuse the compiled model.

The `stan` function returns a stanfit object, which is an S4 object of class 
`"stanfit"`. For those who are not familiar with the concept of class and S4 
class in R, refer to @chambers2010software. An S4 class consists of some
attributes (data) to model an object and some methods to model the behavior of
the object. From a user's perspective, once a stanfit object is created, we are
mainly concerned about what methods are defined.

If no error occurs, the returned stanfit object includes the sample drawn from 
the posterior distribution for the model parameters and other quantities defined
in the model. If there is an error (e.g. a syntax error in the Stan program),
`stan` will either quit or return a stanfit object that contains no posterior
draws.

For class `"stanfit"`, many methods such as `print` and `plot` are defined for
working with the MCMC sample. For example, the following shows a summary of the
parameters from the Eight Schools model using the `print` method:

```{r, print}
print(fit1, pars=c("theta", "mu", "tau", "lp__"), probs=c(.1,.5,.9))
```

The last line of this output, `lp__`, is the logarithm of the (unnormalized)
posterior density as calculated by Stan.  This log density can be used in
various ways for model evaluation and comparison (see, e.g., @Vehtari2012).

#### Arguments to the `stan` Function

The primary arguments for sampling (in functions `stan` and `sampling`) include 
data, initial values, and the options of the sampler such as `chains`, `iter`, 
and `warmup`. In particular, `warmup` specifies the number of iterations that 
are used by the NUTS sampler for the adaptation phase before sampling begins. 
After the warmup, the sampler turns off adaptation and continues until a total 
of `iter` iterations (including `warmup`) have been completed. There is no 
theoretical guarantee that the draws obtained during warmup are from the
posterior distribution, so the warmup draws should only be used for diagnosis
and not inference. The summaries for the parameters shown by the `print` method
are calculated using only post-warmup draws.

The optional `init` argument can be used to specify initial values for the
Markov chains. There are several ways to specify initial values, and the details
can be found in the documentation of the `stan` function. The vast majority of
the time it is adequate to allow Stan to generate its own initial values
randomly. However, sometimes it is better to specify the initial values for at
least a subset of the objects declared in the `parameters` block of a Stan
program.

Stan uses a random number generator (RNG) that supports parallelism. The 
initialization of the RNG is determined by the arguments `seed` and `chain_id`. 
Even if we are running multiple chains from one call to the `stan` function we 
only need to specify one seed, which is randomly generated by R if not 
specified.

#### Data Preprocessing and Passing

The data passed to `stan` will go through a preprocessing procedure. The details
of this preprocessing are documented in the documentation for the `stan` 
function. Here we stress a few important steps. First, RStan allows the user to
pass more objects as data than what is declared in the `data` block (silently 
omitting any unnecessary objects). In general, an element in the list of data passed
to Stan from R should be numeric and its dimension should match the declaration
in the `data` block of the model. So for example, the `factor` type in R is not
supported as a data element for RStan and must be converted to integer codes via
`as.integer`. The Stan modeling language distinguishes between integers and
doubles (type `int` and `real` in Stan modeling language, respectively). The
`stan` function will convert some R data (which is double-precision usually) to
integers if possible.

The Stan language has scalars and other types that are sets of scalars, e.g. 
vectors, matrices, and arrays. As R does not have true scalars, RStan treats
vectors of length one as scalars. However, consider a model with a `data` block
defined as

```
data {                
  int<lower=1> N;      
  real y[N];
} 
```

in which `N` can be $1$ as a special case. So if we know that `N` is always 
larger than $1$, we can use a vector of length `N` in R as the data input for 
`y` (for example, a vector created by `y <- rnorm(N)`). If we want to prevent 
RStan from treating the input data for `y` as a scalar when $N`$ is $1$, we
need to explicitly make it an array as the following R code shows:

```
y <- as.array(y)
```

Stan cannot handle missing values in data automatically, so no element of the 
data can contain `NA` values. An important step in RStan's data preprocessing is
to check missing values and issue an error if any are found. There are, however,
various ways of writing Stan programs that account for missing data (see
@StanManual).


### Methods for the `"stanfit"` Class

The other vignette included with the __rstan__ package discusses stanfit objects
in greater detail and gives examples of accessing the most important content 
contained in the objects (e.g., posterior draws, diagnostic summaries). Also, a
full list of available methods can be found in the documentation for the 
`"stanfit"` class at `help("stanfit", "rstan")`. Here we give only a few 
examples.

The `plot` method for stanfit objects provides various graphical overviews of 
the output. The default plot shows posterior uncertainty intervals (by default 
80% (inner) and 95% (outer)) and the posterior median for all the parameters as 
well as `lp__` (the log of posterior density function up to an additive 
constant):

```{r, stanfit-plot}
plot(fit1)
```

The optional `plotfun` argument can be used to select among the various 
available plots. See `help("plot,stanfit-method")`.

The `traceplot` method is used to plot the time series of the posterior draws. 
If we include the warmup draws by setting `inc_warmup=TRUE`, the background
color of the warmup area is different from the post-warmup phase:

```{r, stanfit-traceplot}
traceplot(fit1, pars = c("mu", "tau"), inc_warmup = TRUE, nrow = 2)
```

To assess the convergence of the Markov chains, in addition to visually 
inspecting traceplots we can calculate the split $\hat{R}$ statistic. Split 
$\hat{R}$ is an updated version of the $\hat{R}$ statistic proposed in
@GelmanRubin:1992 that is based on splitting each chain into two halves. See the
Stan manual for more details. The estimated $\hat{R}$ for each parameter is 
included as one of the columns in the output from the `summary` and `print`
methods.

```{r, stanfit-print}
print(fit1, pars = c("mu", "tau"))
```

Again, see the additional vignette on stanfit objects for more details.


### Sampling Difficulties

The best way to visualize the output of a model is through the ShinyStan 
interface, which can be accessed via the 
[__shinystan__](https://cran.r-project.org/package=shinystan) R package. 
ShinyStan facilitates both the visualization of parameter distributions and
diagnosing problems with the sampler. The documentation for the __shinystan__
package provides instructions for using the interface with stanfit objects.

In addition to using ShinyStan, it is also possible to diagnose some 
sampling problems using functions in the __rstan__ package. The
`get_sampler_params` function returns information on parameters
related the performance of the sampler:

```{r, get_sampler_params}
# all chains combined
sampler_params <- get_sampler_params(fit1, inc_warmup = TRUE)
summary(do.call(rbind, sampler_params), digits = 2)

# each chain separately
lapply(sampler_params, summary, digits = 2)
```

Here we see that there are a small number of divergent transitions, which are 
identified by `divergent__` being $1$. Ideally, there should be no divergent 
transitions after the warmup phase. The best way to try to eliminate divergent 
transitions is by increasing the target acceptance probability, which by default
is $0.8$. In this case the mean of `accept_stat__` is close to $0.8$ for all 
chains, but has a very skewed distribution because the median is near $0.95$. We
could go back and call `stan` again and specify the optional argument 
`control=list(adapt_delta=0.9)` to try to eliminate the divergent transitions. 
However, sometimes when the target acceptance rate is high, the stepsize is very
small and the sampler hits its limit on the number of leapfrog steps it can take
per iteration. In this case, it is a non-issue because each chain has a 
`treedepth__` of at most $7$ and the default is $10$. But if any `treedepth__` 
were $11$, then it would be wise to increase the limit by passing 
`control=list(max_treedepth=12)` (for example) to `stan`. See the vignette on
stanfit objects for more on the structure of the object returned by
`get_sampler_params`.

We can also make a graphical representation of (much of the) the same 
information using `pairs`. The "pairs"" plot can be used to get a sense of
whether any sampling difficulties are occurring in the tails or near the mode:

```{r, pairs-plot}
pairs(fit1, pars = c("mu", "tau", "lp__"), las = 1)
```

In the plot above, the marginal distribution of each selected parameter is 
included as a histogram along the diagonal. By default, draws with below-median 
`accept_stat__` (MCMC proposal acceptance rate) are plotted below the diagonal 
and those with above-median `accept_stat__` are plotted above the diagonal (this
can be changed using the `condition` argument). Each off-diagonal square 
represents a bivariate distribution of the draws for the intersection of the 
row-variable and the column-variable. Ideally, the below-diagonal intersection 
and the above-diagonal intersection of the same two variables should have 
distributions that are mirror images of each other. Any yellow points would 
indicate transitions where the maximum `treedepth__` was hit, and red points 
indicate a divergent transition.


## Additional Topics

### User-defined Stan Functions

Stan also permits users to define their own functions that can be used 
throughout a Stan program. These functions are defined in the `functions` block.
The `functions` block is optional but, if it exists, it must come before any
other block. This mechanism allows users to implement statistical distributions
or other functionality that is not currently available in Stan. However, even if
the user's function merely wraps calls to existing Stan functions, the code in
the `model` block can be much more readible if several lines of Stan code that
accomplish one (or perhaps two) task(s) are replaced by a call to a user-defined
function.

Another reason to utilize user-defined functions is that RStan provides the 
`expose_stan_functions` function for exporting such functions to the R global
environment so that they can be tested in R to ensure they are working
properly. For example,

```{r, expose_stan_functions}
model_code <-
'
functions {
  real standard_normal_rng() {
    return normal_rng(0,1);
  }
}
model {}
'
expose_stan_functions(stanc(model_code = model_code))
standard_normal_rng(seed = 1)
```


### The Log-Posterior (function and gradient)

Stan defines the log of the probability density function of a posterior 
distribution up to an unknown additive constant. We use `lp__` to represent the 
realizations of this log kernel at each iteration (and `lp__` is treated as an 
unknown in the summary and the calculation of split $\hat{R}$ and effective 
sample size).

A nice feature of the __rstan__ package is that it exposes functions for 
calculating both `lp__` and its gradients for a given stanfit object. These two 
functions are `log_prob` and `grad_log_prob`, respectively. Both take parameters
on the _unconstrained_ space, even if the support of a parameter is not the 
whole real line. The Stan manual [@StanManual] has full details on the 
particular transformations Stan uses to map from the entire real line to some 
subspace of it (and vice-versa).

It maybe the case that the number of unconstrained parameters might be less than
the total number of parameters. For example, for a simplex parameter of length 
$K$, there are actually only $K-1$ unconstrained parameters because of the 
constraint that all elements of a simplex must be nonnegative and sum to one. 
The `get_num_upars` method is provided to get the number of unconstrained 
parameters, while the `unconstrain_pars` and `constrain_pars` methods can be 
used to compute unconstrained and constrained values of parameters respectively.
The former takes a list of parameters as input and transforms it to an 
unconstrained vector, and the latter does the opposite. Using these functions, 
we can implement other algorithms such as maximum a posteriori estimation of 
Bayesian models.

### Optimization in Stan

RStan also provides an interface to Stan's optimizers, which can be used to
obtain a point estimate by maximizing the (perhaps penalized) likelihood 
function defined by a Stan program. We illustrate this feature using a very
simple example: estimating the mean from samples assumed to be drawn from a
normal distribution with known standard deviation. That is, we assume

$$y_n \sim \mathsf{Normal}(\mu,1), \quad n = 1, \ldots, N. $$

By specifying a prior $p(\mu) \propto 1$, the maximum a posteriori estimator for
$\mu$ is just the sample mean. We don't need to explicitly code this prior for
$\mu$, as $p(\mu) \propto 1$ is the default if no prior is specified.

We first create an object of class `"stanmodel"` and then use the `optimizing`
method, to which data and other arguments can be fed.

```{r, optimizer, results="hide"}
ocode <- "
  data {
    int<lower=1> N;
    real y[N];
  } 
  parameters {
    real mu;
  } 
  model {
    target += normal_lpdf(y | mu, 1);
  } 
"

sm <- stan_model(model_code = ocode)
y2 <- rnorm(20)
```

```{r}
mean(y2)
optimizing(sm, data = list(y = y2, N = length(y2)), hessian = TRUE)
```


### Model Compilation

As mentioned earlier in the vignette, Stan programs are written in the Stan
modeling language, translated to C++ code, and then compiled to a dynamic shared
object (DSO). The DSO is then loaded by R and executed to draw the posterior
sample. The process of compiling C++ code to DSO sometimes takes a while. When
the model is the same, we can reuse the DSO from a previous run. The `stan`
function accepts the optional argument `fit`, which can be used to pass an
existing fitted model object so that the compiled model is reused. When reusing
a previous fitted model, we can still specify different values for the other
arguments to `stan`, including passing different data to the `data` argument.

In addition, if fitted models are saved using functions like `save` and 
`save.image`, RStan is able to save DSOs, so that they can be used across R 
sessions. To avoid saving the DSO, specify `save_dso=FALSE` when calling the 
`stan` function.

If the user executes `rstan_options(auto_write = TRUE)`, then a serialized 
version of the compiled model will be automatically saved to the hard disk in 
the same directory as the `.stan` file or in R's temporary directory if the Stan
program is expressed as a character string. Although this option is not enabled 
by default due to CRAN policy, it should ordinarily be specified by users in 
order to eliminate redundant compilation.

Stan runs much faster when the code is compiled at the maximum level of 
optimization, which is `-O3` on most C++ compilers. However, the default value 
is `-O2` in R, which is appropriate for most R packages but entails a slight 
slowdown for Stan. You can change this default locally by following the 
instructions at 
[CRAN - Customizing-package-compilation](https://cran.r-project.org/doc/manuals/r-release/R-admin.html#Customizing-package-compilation).
However, you should be advised that setting `CXXFLAGS = -O3` may cause adverse 
side effects for other R packages.

See the documentation for the `stanc` and `stan_model` functions for more
details on the parsing and compilation of Stan programs.

### Running Multiple Chains in Parallel

The number of Markov chains to run can be specified using the `chains` argument 
to the `stan` or `sampling` functions. By default, the chains are executed 
serially (i.e., one at a time) using the parent R process. There is also an 
optional `cores` argument that can be set to the number of chains (if the 
hardware has sufficient processors and RAM), which is appropriate on most 
laptops. We typically recommend first calling 
`options(mc.cores=parallel::detectCores())` once per R session so that all
available cores can be used without needing to manually specify the `cores`
argument.

For users working with a different parallelization scheme (perhaps with a remote
cluster), the __rstan__ package provides a function called `sflist2stanfit` for
consolidating a list of multiple stanfit objects (created from the same Stan
program and using the same number of warmup and sampling iterations) into a
single stanfit object. It is important to specify the same seed for all the
chains and equally important to use a different chain ID (argument `chain_id`),
the combination of which ensures that the random numbers generated in Stan for
all chains are essentially independent. This is handled automatically
(internally) when $`cores` > 1$.


## Working with CmdStan

The __rstan__ package provides some functions for creating data for and reading 
output from CmdStan, the command line interface to Stan.

First, when Stan reads data or initial values, it supports a subset of the 
syntax of R dump data formats. So if we use the `dump` function in base R to 
prepare data, Stan might not be able to read the contents. The `stan_rdump` 
function in __rstan__ is designed to dump the data from R to a format that is 
supported by Stan, with semantics that are very similar to the `dump` function.

Second, the `read_stan_csv` function creates a stanfit object from reading the 
CSV files generated by CmdStan. The resulting stanfit object is compatible with
the various methods for diagnostics and posterior analysis.


## See Also

* The other vignette for the __rstan__ package, which shows how to access the
contents of stanfit objects.
* The very thorough [Stan manual](http://mc-stan.org/documentation/) [@StanManual].
* The `stan_demo` function, which can be used to fit many of the example models
in the manual.
* The [__shinystan__](https://cran.r-project.org/package=shinystan) R
package, which provides the ShinyStan GUI for exploring MCMC output.
* The [__loo__](https://cran.r-project.org/package=loo) R
package, which is very useful for model comparison using stanfit objects.
* The [__rstanarm__](https://cran.r-project.org/package=rstanarm) R
package, which provides a `glmer`-style interface to Stan.


