Course of Raku / Objects, I/O, and exceptions / Input and output / File handles / Exercises / Pass a handle to a function

Solution: Pass a handle to a function

Here is a possible solution to the task.

Code

sub log-line($fh, $message) {
    $fh.say($message);
}

my $fh = open 'log.txt', :w;
log-line($fh, 'started');
log-line($fh, 'working');
log-line($fh, 'done');
$fh.close;

print slurp 'log.txt';

🦋 You can find the source code in the file pass-a-handle.raku.

Output

started
working
done

Comments

  1. The handle returned by open is just a value held in $fh, so it can be passed to log-line as an argument like any string or number. Inside the subroutine, $fh.say writes through that same open handle.

  2. Because the handle stays open across all three calls, each log-line appends another line to the same file. close then flushes everything, and slurp reads the three lines back.

  3. This is what makes handles composable: a function can accept a handle and write to (or read from) it without caring which file it points at — the caller decides that when it opens the file.

Course navigation

← Pass a handle to a function   |   Quiz — Reading lines →