Talk:Closure (computer programming)

In perl at least, a closure doesn't have to be a function definition. It's more or less anything between a pair of braces, that serves as it's own variable scope. In Damian Conway's Object Oriented Perl, I believe he uses closures to implement the private variables of an object, so it might be misleading to say that the closure itself is an object or acts like one. In any case, something needs to be said about scope here. Wesley

How could one use a closure that's not a function definition?

Regarding the "acts as an object" comment, I was thinking of a closure like this:

sub counter {
    my $x = shift;
    return sub { print "$x\n"; $x--; };
}

$h = counter(4);
$g = counter(7);

&$h();  # gives 4
&$h();  # gives 3
&$g();  # gives 7
&$h();  # gives 2

AxelBoldt

A closure can be used as a way to simulate static variables. For instance consider the following (slow) function:

sub fib {
  my $n = shift;
  if ($n <= 1) {
    return 1;
  }
  else {
    return fib($n-1) + fib($n-2);
  }
}

It works, but calculating (say) fib(40) may take a while. Now let's add a private closure in which we cache results (this kind of caching is known as memoizing):

{
  my %cache;
  sub fib {
    my $n = shift;
    if ($n <= 1) {
      return 1;
    }
    elsif (exists $cache{$n}) {
      return $cache{$n};
    }
    else {
      return $cache{$n} = fib($n-1) + fib($n-2);
    }
  }
}

Now the function runs much more quickly. If we had static variables (the "my $foo if 0;" bug does not count in my books!), that would be the right way to do this, but closures can do it as well.

BTW I hope that the example I gave is not too complex. I wanted to provide something that was accessible but gave an idea of why someone might choose to use closures.

bjt

---

IMO the example was too complex. Also, Perl is a heinous language for explaining computer science concepts (e.g., you have to use "shift" to peel off arguments --- utterly obscure --- not to mention all the other random syntactic noise that comes with Perl). I replaced it with a more basic explanation that just conveys the idea of lexical scope capture. I used ML, which has a far more concise and pseudocode-like syntax than Perl.

If you want to do a more involved programming example, IMO you should create an article for first-class function or higher-order function and write it there; or, better still add it to functional programming. The term "closure", as commonly used by language designers/implementors, refers specifically to the data structure with the code pointer and environment pointer. Programming using higher-order functions is a broader idea, and belongs in a broader article. k.lee 00:44, 24 Oct 2003 (UTC)