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masak (6289)

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Been programming Perl since 2001. Found Perl 6 somewhere around 2004, and fell in love. Now developing November (a Perl 6 wiki), Druid (a Perl 6 board game), pls (a Perl 6 project installer), GGE (a regex engine), and Yapsi (a Perl 6 implementation). Heavy user of and irregular committer to Rakudo.

Journal of masak (6289)

Sunday July 11, 2010
04:55 PM

Iterating your way to happiness with Perl 6

[ #40442 ]

I thought I'd have an easy time today, just regurgitating what S04 says about "Loop statements". Perl 5 already got this part pretty right already. Actually, even C got it pretty right. So what new does Perl 6 have to offer? That's what this post is about.

So, nothing much has changed about the while and until loops that we know and love.

while EXPR { ... }
until EXPR { ... }

Then there's the kind of loop when you want to test the condition *after* the block has run, rather than before. In Perl 5, that looks like this:

do { ... } while EXPR;
do { ... } until EXPR;

This construct tends to cause fresh Perl programmers a lot of grief, since do isn't really a loop construct. There's some wording about this in perldoc perlsyn:

Note also that the loop control statements described later will NOT work in this construct, because modifiers don’t take loop labels. Sorry.

Perl 6 solves this by

  • recognizing and disallowing while and until after do blocks, and
  • introducing the repeat block.

So now you write it like this instead:

repeat { ... } while EXPR;
repeat { ... } until EXPR;

And you get two bonus features from this: first, since the while or until is mandatory, you can put it on its own line. Generally, closing line-ending curlies act like they have implicit semicolons after them in Perl 6, but here the parser is smart enough to expect a while or until, so it doesn't put one in.

Second, you're allowed to put the condition up front if you want:

repeat while EXPR { ... }
repeat until EXPR { ... }

Even though the condition is before the loop here, it'll still not be run until after each iteration, because it's a repeat loop, and they work like that.

Then there's the loop construct that loops forever, aptly named loop:

loop { ... }

In C, we'd have that as for (;;) { ... }. And, symmetrically, you can also write it like this in Perl 6:

loop (;;) { ... }

Or, more generally, you can do any C-style for loop, if you just spell it loop:

loop (EXPR; EXPR; EXPR) { ... }

And what was known alternately in Perl 5 as for and foreach becomes just for in Perl 6.

The syntax for for in Perl 6 is what you'd expect:

for EXPR { ... }

But it packs a lot more power underneath. Or rather, the whole language is geared towards packing for with a lot more power. Some examples:

  • If you want to name the item you're currently looping over, just prefix the block with -> $a. (If you don't, you'll find the item in $_, as usual.)
  • If you want to loop two items at a time, prefix the block with -> $a, $b.
  • Neither of the above are special syntaxes belonging to the for construct as such; rather, the -> arrows belong to the block, making it, in the prevailing terminology, a "pointy block". In fact, all of the loop constructs I've brought up (except the C-style loop) can be given pointy blocks; the expression will then be bound to the variable and usable from within the block. You can do it with if statements, too! It's quite useful.
  • If you want to loop over two lists simultaneously, you can use the infix:<Z> operator to interleave the lists. Loop one item at a time, and you'll get alternating elements from the two lists. Loop two at a time, and $a (or whatever) will always be from the first list, and $b from the second. Gone are the days when you had to do manual trickery with indexes and stuff because the for loop wasn't powerful enough.
  • You can even eliminate nested loops sometimes with the infix:<X> operator, which does a Cartesian product (known in SQL circles as a "cross join") of two lists. So if you planned to do for @a { for @b { ... } } anyway, you might as well do for @a X @b { ... } and save yourself one level of indentation.
  • All of the above is lazy, so with a sensible Perl 6 implementation, there's no huge memory waste from building up all these big lists; it's all generated on-the-fly. (This, incidentally, means that we also read files with for loops in Perl 6, rather than with a magical while construct as in Perl 5.)

That's it for today. I forgot to mention the looping construct that only loops over one item... but you can look that one up yourself. Oh, and Perl 5.10 also has it.

Actually, I got to thinking about all this, since I figured out the other day how to do the FIRST and LAST phasers in Yapsi. But this blog post felt like a natural precursor to the one I wanted to write. Hopefully soon. 哈哈

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