1 Void Functions
Stan does not technically support functions that do not return values. It does support two types of statements, one printing and one for rejecting outputs.
Although print and reject appear to have the syntax of functions, they are actually special kinds of statements with slightly different form and behavior than other functions. First, they are the constructs that allow a variable number of arguments. Second, they are the the only constructs to accept string literals (e.g., "hello world") as arguments. Third, they have no effect on the log density function and operate solely through side effects.
The special keyword void is used for their return type because they behave like variadic functions with void return type, even though they are special kinds of statements.