Learning Perl 6

Chapter 2. Number Guessing

You’re about to be thrown in the deep end. There are some basic things you need to know to write useful programs, and you’ll meet a lot of them in this chapter so you can write a number-guessing program by the end. It’s quite a bit to take in all at once but it should make the rest of the chapters more interesting.

Binding and Assignment

You read a little about variables in Chapter 1. To store a value in a variable you assign to it. The item assignment operator, =, stores a single thingy for you. $number is a scalar variable; it can store exactly one thingy. This is item assignment because there’s one thingy. This “sets” the value:

my $number = 2;

If you decide that you don’t want that value you can replace it:

$number = 3;

Sometimes you want a value that you can’t change (more likely a value you don’t want another part of your program to change). Instead of the assignment operator you can use the binding operator, :=, to set the value:

my $sides-of-a-square := 4;
$sides-of-a-square = 5

When you try to change the value you get an error:

Cannot assign to an immutable value

It’s not the binding operator that makes the variable immutable. It merely makes the thingy on the left the same as the one on the right. In this case, $sides-of-square is actually 4 and not just a variable that happens to store 4. You can’t assign to 4, so you can’t assign to $sides-of-a-square.

If you first assign to a scalar variable then bind to that variable you end up with two names for the ...