## 35.3 File extensions

The recommended file extension for Stan model files is .stan. Files which contain only function definitions (intended for use with #include) should be given the .stanfunctions extension. A .stanfunctions file only includes the function definition and does not require the functions{} block wrapped around the function. A simple example of usage where the function is defined and saved in the file foo.stanfunctions:

real foo(real x, real y) {
return sqrt(x * log(y));
}

The function foo can be accessed in the Stan program by including the path to the foo.stanfunctions file as:

functions {
#include foo.stanfunctions;
}
// ...body...

For Stan data dump files, the recommended extension is .R, or more informatively, .data.R. For JSON output, the recommended extension is .json.