Date Created: 2024-06-24
Date Modified: 2024-06-24

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EVAL LOOP
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                              INTRODUCTION
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Today, we're exploring an unconventional way to create a loop in Perl using
more self-modifying code. This technique, while not practical for everyday
use, showcases Perl's flexibility and power. Let's dive into this intriguing
code snippet:

    #!/usr/bin/env perl

    use feature qw|say|;

    $_ = <<'EOF';
    10;
    s~(\d+)(?{ say qq($1) })~$1-1~e;
    sleep 1;
    $1 ? eval : say q(Countdown complete!);
    EOF

    eval;

This code creates a countdown from 10 to 1 without using any traditional
loop constructs. Let's break it down.


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PART 1: THE HEREDOC SETUP
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We start by assigning a heredoc to the $_ variable. This heredoc contains
the code that will be repeatedly evaluated.

The first line in the heredoc sets our starting number: 10.


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PART 2: THE SUBSTITUTION OPERATOR
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The substitution operator s~~~ is the heart of our loop simulation:

    s~(\d+)(?{ say qq($1) })~$1-1~e;

This line does several things:

  - It matches a number (\d+)
  - It prints the matched number using a code block (?{ say qq($1) })
  - It replaces the number with itself minus one ($1-1)
  - The 'e' flag at the end tells Perl to evaluate the replacement as code


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PART 3: SLOWING IT DOWN
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We add a sleep 1; to slow down the countdown for visual effect.


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PART 4: THE EXIT CONDITION
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The final line in the heredoc is our exit condition:

    $1 ? eval : say q(Countdown complete!);

If $1 (our number) is truthy (non-zero), it evaluates the code again.
Otherwise, it prints "Countdown complete!"


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PART 5: KICKING IT OFF
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Finally, outside the heredoc, we have a single eval; statement. This kicks
off the whole process by evaluating the code in $_.


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PART 6: THE RESULT
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When run, this script will count down from 10 to 1, pausing for a second
between each number, and then print "Countdown complete!"


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                              CONCLUSION
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While this example is more of a curiosity than a practical coding technique,
it illustrates Perl's flexibility and the creative ways we can bend the
language to our will. It's a reminder of why Perl is often described as a
"Swiss Army chainsaw" of programming languages!


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