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 donââ¬â¢t take loop labels. Sorry.
Perl 6 solves this by
while and until after do blocks, andrepeat 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:
-> $a. (If you don't, you'll find the item in $_, as usual.)-> $a, $b.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.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.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.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
Re:Excellent!
masak on 2010-07-12T10:16:23
Gracias; fixed.
Re:Awesome!
masak on 2010-07-12T10:25:40
I'll take that as a sign that posts about aspects of Perl 6 that I take for granted nowadays can be interesting to others.
:-) Thanks for the feedback. I'll see if I can gear my output more towards these explanatory posts.