Day 12 – I just felt a disturbance in the switch

So I said “I’m going to advent post about the spec change to given/when earlier this year, OK?”

And more than one person said “What spec change?”

And I said, “Exactly.”

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We speak quite proudly about “whirlpool development” in Perl 6-land. Many forces push on a particular feature and force it to converge to an ideal point: specification, implementation, bugs, corner cases, actual real-world usage…

Perl 6 is not alone in this. By my count, the people behind the HTML specification have now realized at least twice that if you try to lead by specifying, you will find to your dismay that users do something different than you expected them to, and that in the resulting competition between reality and specification, reality wins out by virtue of being real.

I guess my point is: if you have a specification and a user community, often the right thing is to specify stuff that eventually ends up in implementations that the user community benefits from. But information can also come flowing back from actual real-world usage, and in some of those cases, it’s good to go adapting the spec.

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Alright. What has happened to given/when since (say) some clown described them in an advent post six years ago?

To answer that question, first let me say that everything in that post is still true, and still works. (And not because I went back and changed it. I promise!)

There are two small changes, and they are both of the kind that enable new behavior, not restrict existing behavior.

First small change: we already knew (see the old post) that the switching behavior of when works not just in a given block, but in any “topicalizer” block such as a for loop or subroutine that takes $_ as a parameter.

given $answer {
    when "Atlantis" { say "that is CORRECT" }
    default { say "BZZZZZZZZZT!" }
}

for 1..100 {
    when * %% 15 { say "Fizzbuzz" }
    when * %% 3 { say "Fizz" }
    when * %% 5 { say "Buzz" }
    default { say $_ }
}

sub expand($_) {
    when "R" { say "Red Mars" }
    when "G" { say "Green Mars" }
    when "B" { say "Blue Mars" }
    default { die "Unknown contraction '$_'" }
}

But even subroutines that don’t take $_ as a parameter get their own lexical $_ to modify. So the rule is actually less about special topicalizer blocks and more about “is there something nice in $_ right now that I might want to switch on?”. We can even set $_ ourselves if we want.

sub seek-the-answer() {
    $_ = (^100).pick;
    when 42 { say "The answer!" }
    default { say "A number" }
}

In other words, we already knew that when (and default) were pretty separate from given. But this shows that the rift is bigger than previously thought. The switch statement logic is all in the when statements. In this light, given is just a handy topicalizer block for when we temporarily want to set $_ to something.

Second small change: you can nest when statements!

I didn’t see that one coming. I’m pretty sure I’ve never used it in the wild. But apparently people have! And yes, I can see it being very useful sometimes.

when * > 2 {
    when 4 { say 'four!' }
    default { say 'huge' }
}
default {
    say 'little'
}

You might remember that a when block has an implicit succeed statement at the end which makes the surrounding topicalizer block exit. (Meaning you don’t have to remember to break out of the switch manually, because it’s done for you by default.) If you want to override the succeed statement and continue past the when block, then you write an explicit proceed at the end of the when block. Fall-through, as it were, is opt-in.

given $verse-number {
    when * >= 12 { say "Twelve drummers drumming"; proceed }
    when * >= 11 { say "Eleven pipers piping"; proceed }
    # ...
    when * >= 5 { say "FIIIIIIVE GOLDEN RINGS!"; proceed }
    when * >= 4 { say "Four calling birds"; proceed }
    when * >= 3 { say "Three French hens"; proceed }
    when * >= 2 {
        say "Two turtle doves";
        say "and a partridge in a pear tree";
    }
    say "a partridge in a pear tree";
}

All that is still true, save for the word “topicalizer”, since we now realize that when blocks show up basically anywhere. The new rule is this: when makes the surrounding block exit. If you’re in a nested-when-type situation, “the surrounding block” is taken to be the innermost block that isn’t a when block. (So, usually a given block or similar.)

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It’s nice to see spec changes happening due to how things are being used in practice. This particular spec change came about because jnthn tried to implement a patch in Rakudo to detect topicalizer blocks more strictly, and he came across userland cases such as the above two, and decided (correctly) to align the spec with those cases instead of the other way around.

The given/when spec has been very stable, and so it’s rare to see a change happen in that part of the synopses. I find the change to be an improvement, and even something of a simplification. I’m glad Perl 6 is the type of language that adapts the spec to the users instead of vice versa.

Just felt like sharing this. Enjoy your Advent.