An optional argument lets a Ruby method be called without a value for every parameter. A default-valued parameter uses the value in its definition when the caller omits that argument; a supplied value takes its place. A starred parameter (`*args`) is optional in a different way: it collects any extra positional arguments into an array, which is empty if there are none. section 2.4.1
A default versus a catch-all
def report(name, count=1, *notes) p name, count, notes end report("Ada") # "Ada", 1, [] report("Ada", 3, "urgent") # "Ada", 3, ["urgent"] `count` has a fallback value. `notes` has no fallback expression: its starred parameter receives an array of whatever arguments remain. section 2.4.1 section 2.4.3
Argument binding gets more interesting when required parameters appear on both sides of optional ones. Ruby assigns the required parameters first, including those at the right edge. Of the arguments left in the middle, a default-valued parameter can receive a value; a starred parameter collects the rest. If no value remains for a default-valued parameter, its default applies. Omitting a required argument still raises `ArgumentError`. section 2.4.2
>> args_unleashed(1,2,3,4,5) 1
1
2
[3]
4
5
=> [1, 2, [3], 4, 5]
>> args_unleashed(1,2,3,4) 2
1
2
[]
3
4
=> [1, 2, [], 3, 4]
>> args_unleashed(1,2,3) 3
1
1
[]
2
3
=> [1, 1, [], 2, 3]
>> args_unleashed(1,2,3,4,5,6,7,8) 4
1
2
[3, 4, 5, 6]
7
8
=> [1, 2, [3, 4, 5, 6], 7, 8]
>> args_unleashed(1,2) 5
wrong number of arguments (given 2, expected 3+) (ArgumentError)
Compare calls with three, four, and five arguments: the right-hand required parameters keep their values, while the default-valued parameter and catch-all array change. section 2.4.2
Parameter order must make that assignment possible. Required positional parameters can be on the left, the right, or both, with optional positional parameters between them. A starred catch-all cannot come before a default-valued positional parameter: after the catch-all collected the remaining arguments, there would be no sensible way to decide whether the later parameter should receive one. section 2.4.3
Keyword arguments have their own optional form. In `def options(a: 1, b: 2)`, calling `options` uses both defaults; calling `options(b: 10)` replaces only `b`. Unlike positional arguments, keywords need not be supplied in parameter order. A keyword written without a default, such as `b:`, is required. section 9.3.8
