One-line comments

One-line comments are separated from the program source code with the # character. Everything after the # symbol is skipped by the compiler until the end of the current line.

In the following example, we take the 'Hello, World!' program from Chapter 1, What is Perl 6?, and add a one-line comment to it:

say 'Hello, World!'; # Prints 'Hello, World!'

This is a fully correct Perl 6 program. It has one call of the built-in say function and a comment about what it does. The part of the string, starting from the # character, is a one-line comment.

If you run that program, it prints Hello, World! and nothing more. It works exactly the same as the program with no comments at all.

One-line comments may also occupy a separate line of code. For example, let's add more comments to the same program:

# This is a program in Perl 6.
# It prints the 'Hello, World!' string.
# To run it, install Rakudo Star and 
# run it from the command-line:
# perl6 ./hello.pl
 
say 'Hello, World!'; # it prints the string
 
# The program ends here.

This is also a completely valid Perl 6 program. From the perspective of business logic, the program did not change, as with the previous example, but from the perspective of future maintenance, it became much better as it explains what it does.

Another common practice of using one-line comments is to temporarily hide pieces of code. For example, during the debugging process, you want to disable some actions. Instead of removing the line of code, you can comment it out by adding the # character at the beginning of the line. Consider the following lines of code as an example:

say 'Hello, World!';
# print "Hello, World!\n";

Sometimes, you do the opposite—you add additional printing instructions to the program to see the values of different variables, and comment those instructions before making the code production-ready.

Here is an example of how you print the values that were passed to the function. I have used the addition example from Chapter 1, What is Perl 6?:

sub add($x, $y) {
    say "x = $x, y = $y";
    return $x + $y;
}

say add(4, 5); # 9

This program simply prints the result of the addition, but if you uncomment the first line of the subroutine code, it will also print the values of the $x and $y variables in a human-readable format which is shown in the following code:

x = 4, y = 5
9