In the previous example, it might be reasonable to assume that most of the time, the quantity argument is going to be 1. It would be convenient if messages that had a quantity of 1 didn’t even have to write it. For example:
ring-up-item cash-register price 17.29.
If you run the expression above, a debugger will appear with the error message “Using an undefined value” and q selected. This is because the arguments to the message didn’t give a value to q, so its value is undefined. This is the same error as using a local variable before you’ve assigned it a value.
Scripts can handle missing values rather than just causing an error: They can default the value of the argument. In essence, we want assign 1 to q if the message didn’t give it a value in the first place. There is a form of assignment, ?=, just for this case. It is called defaulting. Change the ring-up-item script to:
$ price p, quantity q.
$ cost.
q ?= 1.
cost := p * q.
sub-total := sub-total + cost.
Now, if argument quantity is part of the message, then q will be set by the message, otherwise it will be set to 1.
» This is actually the only way that you can assign to the variables for arguments. Argument variables, like p and q, cannot be assigned to using :=. Remember: argument variables are assigned by the message, local variables are assigned by your script.
