CODEWITHSUNDEEP
perl
Dan Walmsley
Dan Walmsley

Reputation: 2821

What is the shift command doing in this example perl code for FSA::Rules?

I am not very skilled in Perl, and I am trying to use FSA::Rules. What is the shift doing in this code? 

my $fsa = FSA::Rules->new(
 ping => {
     do => sub {
         print "ping!\n";
         my $state = shift;
         $state->result('pong');
         $state->machine->{count}++;
     },
     rules => [
         game_over => sub { shift->machine->{count} >= 20 },
         pong      => sub { shift->result eq 'pong' },
     ],
 },

 pong => {
     do => sub { print "pong!\n" },
     rules => [ ping => 1, ], # always goes back to ping
 },
 game_over => { do => sub { print "Game Over\n" } }
);

$fsa->start;
$fsa->switch until $fsa->at('game_over');

Upvotes: 1

Views: 231

Answers (3)

Ted Hopp
Ted Hopp

Reputation: 234795

The shift command is accessing the next argument to each subroutine.

The arguments to FSA::Rules->new is a hash with three keys: ping, pong, and game_over. Let's take ping: it maps to another hash containing two keys: do and rules. The do key maps to a subroutine:

sub {
    print "ping!\n";
    my $state = shift;
    $state->result('pong');
    $state->machine->{count}++;
}

This method expects to be called with an FSA::State object as the argument. The other subroutines follow similar ideas. Consult the FSA::Rules docs for how to use all this.

Upvotes: 3

swestrup
swestrup

Reputation: 4209

By default shift removes and returns the first element of the @_ array. Since that is the parameter array that is passed to subroutines, its getting the first subroutine argument. For example

sub { print shift; }

is an anonymous subroutine that just consumes and prints its first argument.

Upvotes: 4

Victor Bruno
Victor Bruno

Reputation: 1043

Shift removes and returns the first element of an array. When the array is not specified, @_ is assumed. @_ in that context is the list of arguments passed to the subroutine.

Upvotes: 2

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