In Perl, a closure is a subroutine that can access variables defined outside its own scope. Here is an example of a closure in Perl:
sub counter {
my $count = 0;
return sub {
return ++$count;
};
}
my $increment = counter();
print $increment->(); # outputs 1
print $increment->(); # outputs 2
print $increment->(); # outputs 3
In this example, counter() returns an anonymous subroutine that increments a $count variable and returns it. The $count variable is defined within the scope of counter(), but it is accessed by the anonymous subroutine returned by counter(). When the anonymous subroutine is called, it increments the $count variable and returns it. Each time it is called, the $count variable retains its value between calls due to the closure.
Another example of using closures is in building a memoizing function, which is a function that caches its results in memory to avoid recomputing them for the same input. Here is an example implementation:
sub memoize {
my $func = shift;
my %cache;
return sub {
my $arg = shift;
if (exists $cache{$arg}) {
return $cache{$arg};
}
my $result = $func->($arg);
$cache{$arg} = $result;
return $result;
};
}
sub fibonacci {
my $n = shift;
if ($n < 2) {
return $n;
}
return fibonacci($n - 1) + fibonacci($n - 2);
}
my $memoized_fibonacci = memoize(\&fibonacci);
print $memoized_fibonacci->(10); # outputs 55
In this example, memoize() returns an anonymous subroutine that takes an argument and calls the original function ($func) with that argument, storing the result in a cache hash so that subsequent calls with the same argument can be returned from cache instead of recomputing the result. The original function fibonacci() recursively calculates the Fibonacci number for a given input with no caching, but memoize() allows for its results to be cached and returned from cache for the same input.