Perl5 -> Perl 6: about the topic variable

The topic variable $_ is a well known variable that is used as a global variable when nothing is provided. In Perl 5 the rule of thumb is that a lot of operators can modify/act the topic varaible when nothing more is provided. As an example, consider the following piece of code:

my @bands = ( 'Foo Fighters', 'Soundgarden', 'Puddle of Mudd', 'Depeche Mode' );

for ( @bands ){
    say "I like $_ !";
}

my %albums_of = map { $_ => int( rand( 10 ) ) } @bands;

for ( keys %albums_of ){
    say "I've got $albums_of{ $_ } albums of $_ !";
}

that produces the following output:

I like Foo Fighters !
I like Soundgarden !
I like Puddle of Mudd !
I like Depeche Mode !
I've got 2 albums of Puddle of Mudd !
I've got 1 albums of Foo Fighters !
I've got 1 albums of Depeche Mode !
I've got 1 albums of Soundgarden !

As you can see, the for loop as well as map operates on $_ (topic variable) by default. Let’s see this in Perl 6:

my @bands = 'Foo Fighters', 'Soundgarden', 'Puddle of Mudd', 'Depeche Mode';

for @bands { "I like $_".say; }

my %albums_of = @bands.map: { $_ => Int( 10.rand ); }
for %albums_of.keys { say "I've got %albums_of{ $_ } albums of $_ !"; }

Nothing really impressive here: operators we were used to work with the topic variable remain pretty much the same. What is new in Perl 6 is that the topic variable can be omitted, so that you can find method calls on no-object at all, imagine the topic variable is there. Let’s see this in action:

for @bands { .say; }

# it is the same as
# for @bands { $_.say; }

As another example consider the following:

my $when = Date.today( formatter => { "%02d / %02d / %04d".sprintf: .day, .month, .year ; } );

$when.say;

where within the block for the formatter the default object refence is $_ since none has been specified as prototype. You can name the topic variable as you want by either place it explicitly or use a named variable (signature):

my $when = Date.today( formatter => -> $o { "%02d / %02d / %04d".sprintf: $o.day, $o.month, $o.year ; } );

The article Perl5 -> Perl 6: about the topic variable has been posted by Luca Ferrari on September 30, 2017

Tags: raku , perl , programming