As you might know Perl version 5.10 has introduce a keyword called state that allows the creation of so-called state variables. These are very similar to the static variables of C. You declare them insied a function. They are scoped to the function so you cannot reach them from the outside, but they never get destroyed and they are initialized only once.

Dave Horsfall suggested to use this feature to detect unwanted recursion. Or calling the same function twice.

The idea is to add a small construct to every function: 

confess 'recursion' if state $recursed++;

The code will look like this:

examples/detect_recursion.pl

use strict;
use warnings;
use 5.010;

use Carp qw(confess);

sub a {
    #say 'enter a';
    confess 'recursion' if state $recursed++;
    b();
}
sub b {
    #say 'enter b';
    confess 'recursion' if state $recursed++;
    c();
}

sub c {
    #say 'enter c';
    confess 'recursion' if state $recursed++;
    a();
}

a();

Then if we run the script we get the following exception: 

recursion at examples/detect_recursion.pl line 9.
    main::a() called at detect_recursion.pl line 21
    main::c() called at detect_recursion.pl line 15
    main::b() called at detect_recursion.pl line 10
    main::a() called at detect_recursion.pl line 25

That is, when a() called again deep down the call stack, the state variable already has a value and this Perl will call confess that will raise an exception with a nice stack-trace.

False alarms

While this idea can be used in some of the cases, in other it might generate false alarms.

In this case we call the same function twice. One after the other, but not in a recursion.

examples/detect_recursion_falsly.pl

use strict;
use warnings;
use 5.010;

use Carp qw(confess);

sub a {
    confess 'recursion' if state $recursed++;
}

a();
a();



recursion at detect_recursion_falsly.pl line 8.
    main::a() called at detect_recursion_falsly.pl line 12

Comments

If all of the exits from a function do a

$recursed--

then I *think* the false alarm goes away.

sub a { #say 'enter a'; confess 'recursion' if state $recursed++; b(); $recursed--; } sub b { #say 'enter b'; confess 'recursion' if state $recursed++; c(); $recursed--; }

sub c { #say 'enter c'; confess 'recursion' if state $recursed++; a(); $recursed--; }

Also, I thought that declaring and modifying the state variable in the

state

declaration was outlawed recently. Of was that just for

my

?