In the first line, :int is a "guard" that constrains the argument and result of the function. A guard is not quite the same thing as a type declaration; guards are optional and can specify constraints. The first :int ensures that the body of the function will only have to handle an integer argument. Without the second :int above, the function would not be able to return a value. Being able to see up front that information escapes out of the function is helpful for security auditing.