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## 18.7 Documenting Functions

Functions will ideally be documented at their interface level. The Stan style guide for function documentation follows the same format as used by the Doxygen (C++) and Javadoc (Java) automatic documentation systems. Such specifications indicate the variables and their types and the return value, prefaced with some descriptive text.

For example, here’s some documentation for the prediction matrix generator.

/**
* Return a data matrix of specified size with rows
* corresponding to items and the first column filled
* with the value 1 to represent the intercept and the
* remaining columns randomly filled with unit-normal draws.
*
* @param N Number of rows corresponding to data items
* @param K Number of predictors, counting the intercept, per
*          item.
* @return Simulated predictor matrix.
*/
matrix predictors_rng(int N, int K) {
...

The comment begins with /**, ends with */, and has an asterisk (*) on each line. It uses @param followed by the argument’s identifier to document a function argument. The tag @return is used to indicate the return value. Stan does not (yet) have an automatic documentation generator like Javadoc or Doxygen, so this just looks like a big comment starting with /* and ending with */ to the Stan parser.

For functions that raise exceptions, exceptions can be documented using @throws.29

For example,

 ...
* @param theta
* @throws If any of the entries of theta is negative.
*/
real entropy(vector theta) {
...

Usually an exception type would be provided, but these are not exposed as part of the Stan language, so there is no need to document them.

1. As of Stan 2.9.0, the only way a user-defined producer will raise an exception is if a function it calls (including sampling statements) raises an exception via the reject statement.