Terrence Brannon wrote:
I do not believe that is possible.I gave a talk on Perl 6 Junctions at the Thousand Oaks Perl Mongers meeting last night
http://www.hcoop.net/~terry/perl/talks/p6-junctions/index.html
and two questions/desires came out of it:
1: will it be possible to know which element of a junction is currently being used? E.g.:
my @first_set = qw(1 1); my @new_set = qw(1 1.4 1 1 8 1 1 1 0.8);
my $any_new_set = any(@new_set); my $any_first_set = any(@first_set);
if ( (abs($any_first_set - $any_new_set)) > 0.5) { "a variation in the readings is too large".say; printf "we we examining %d and %d when it happened", $any_new_set.current, $any_first_set.current ; # desired feature
}
This is the "filtering" or "unification" behavior that people keep wanting junctions to have, which they do not.
A given junction always has all of the values it was made with. No more, no less. If you want something else, you have to make a new junction. Consider that it's been decided that :
$j = 11|0; 10 < $j < 1
Is true. $j retains the 0 even after the 0 failed a test.
As for the "current" value, there is only a current value during threading. In this example, the threading is fully contained in C< (abs($any_first_set - $any_new_set)) > 0.5 >. By the time the printf comes, the threading is long past.
If you wish to change the behavior, you're welcome to put out some proposals. But I'll warn you from experience that Damian is rather stubborn about the current behavior. =)
2: Unless the array of values can be specified lazily, it will not be practical to use Perl 6 Junctions on large datasets. For example I
might like to be able to specify a sub ref/closure whose execution yields a
new array value or undef when no more values. I.e.:
sub mynext { my($age) = $sth->fetchrow_array; $age }
my $junction = any(\&mynext) ;
You now have a junction whose only value is a coderef.
I do not believe that you can create a 'lazy junction'. But I don't recall the topic coming up before, so we'll have to wait for Damian to come back unless someone else knows for certain.
3: Do junctions short circuit? I.e., whenever the condition is met,
does it continue immediately. Using the example from point #1, can we assume
that the body of the "then" branch will fire when 8 of @new_set is
encountered?
Yes, they short circuit.
However, your second statement might be a bit misleading. When the 8 is encountered, the evaluation of the junctions terminates, and then processing moves on to the next statement, in this case the say. What you said might be construed as the junctions were still being threaded when the the say and printf occurred.
HTH, -- Rod Adams
