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Re^2: Lexical closures

by spurperl (Priest)
on Oct 25, 2008 at 08:12 UTC ( #719477=note: print w/replies, xml ) Need Help??

in reply to Re: Lexical closures
in thread Lexical closures

Nope, lambda does the same. Python function definitions also create closures, BTW. And it still doesn't explain why the same happens in Common Lisp and Javascript, both of which support closures.

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Re^3: Lexical closures
by Corion (Pope) on Oct 25, 2008 at 08:27 UTC

    Well, it depends on what the closures really close over, respectively when a new instance of the loop variable/value gets created. It seems that in Perl, at least when you use a lexical loop variable, you get a new copy each iteration, while in the other languages, you don't. To test this theory, create a new lexical variable within the loop body in each language and see if that's different, that is, use the local equivalent of the following Perl code to create the closures:

    for (0..2) { my $i = $_; push @funcs, sub { $i * $_[0] }; };

    You want to force allocation of a new instance of the lexical variable so your subroutine references get a new instance on each way around.

    Of course, most of the languages have map, so using it would be more apt.

Re^3: Lexical closures
by mpeever (Friar) on Oct 25, 2008 at 17:40 UTC

    The real question is, what do your Scheme and Common Lisp code samples look like?

    The Perl behaviour is precisely what I would expect. It generates three closures, each capturing a value of $i. So your first foreach block reduces to:

    my @flist = ( sub {0 * $_[0]}, sub {1 * $_[0]}, sub {2 * $_[0]});
    It's obvious, then, that 0 2 4 is the correct output of
    foreach my $f (@flist) { print $f->(2), "\n"; }

      OK, I took some time to mock out some Scheme: here are both behaviours you've seen:

      (define cclosures (lambda (values) (cond ((null? values) '()) (else (cons (lambda (x) (* (car values) x)) (cclosures (cdr values))))))) (define cclosures2 (let ((val -1)) (lambda (values) (cond ((null? values) '()) (else (begin (set! val (car values)) (cons (lambda (x) (* val x)) (cclosures2 (cdr values))))))))) (define clprint (lambda (closures) (map (lambda (fn) (fn 2)) closures))) > (clprint (cclosures '(0 1 2))) (0 2 4) > (clprint (cclosures2 '(0 1 2))) (4 4 4)

      cclosures uses the equivalent ofdeclaring my $i inside the foreach loop: it defines a new variable called val for every iteration.

      cclosures2 uses the equivalent of declaring my $i outside the foreach loop: val gets reassigned with every iteration.

      Notice the closure closes over the variable, not the value. So when the variable is reassigned, the value inside the closure changes too.


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