When defining functions, one or more arguments can be given a default value such as f(arg = val). If no parameter is supplied for arg, then val is taken as the value of arg. The position of these arguments in the function's input is important, just as it is for normal arguments; that's why they are called optional positional arguments. Here is an example of a f function with an optional argument b:

# code in chapter 3\arguments.jl:
f(a, b = 5) = a + b

If f(1), then it returns 6, f(2, 5) returns 7, and f(3) returns 8. However, calling it with f() or f(1,2,3) returns an error, because there is no matching function f with zero or three arguments. These arguments are still only defined by position: calling f(2, b = 5) raises an error as ERROR: function f does not accept keyword arguments.

Until now, arguments were only defined by position. For code clarity, it can be useful to explicitly call the arguments by name, so they are called optional keyword arguments. Because...