Yary has asked for the wisdom of the Perl Monks concerning the following question:

I have this memory of Perl limiting how deep subroutine recursion could go, but the following test suggests not: 
perl -E 'sub f{say $_[0] unless $_[0] % 10000;exit if $_[0] > 30000;f(
+$_[0]+1)};f(1)'

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on perl 5.12 and 5.20, on windows and NetBSD, they both run (and without apparent limit, if you remove the "exit")

I got bit when debugging a recursive sub, and my perl process hard-froze my Windows 7 machine. The mouse moved but nothing else worked, not ctrl-alt-del, not "Fn-Sleep". I waited about 10 minutes, thinking it would run out of memory and exit, but the machine just stayed frozen :-(

Just curious if there was a recursion limit, and if so, when was it lifted?

Next day addendum - Thanks for all the replies! I've encountered the runtime warning, and the regexp recursion limit, and the debugger warnings all in the past, and I must have confabulated them. You all got ++ votes from me for your reminders.

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Re: No recursion depth limit?
by Athanasius (Archbishop) on Sep 02, 2015 at 03:29 UTC

    Hello Yary,

    It depends what you mean by “limit.” AFAICT, Perl has never prevented subroutine recursion to arbitrary depth; but if warnings are in use, it has always warned about Deep recursion when the recursion depth reaches a certain pre-set limit. From perl5101delta#New-or-Changed-Diagnostics:

    Deep recursion on subroutine "%s"
    • It is now possible to change the depth threshold for this warning from the default of 100, by recompiling the perl binary, setting the C pre-processor macro PERL_SUB_DEPTH_WARN to the desired value.

    On my Strawberry Perl, this limit is still set to the default:

    13:24 >perl -wE "$|++; say ''; sub f { print qq($_[0] ); if ($_[0] >= 
+99) { say 'Bye!'; } else { f($_[0] + 1); } } f(1);"

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 2
+7 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 5
+0 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 7
+3 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 9
+6 97 98 99 Bye!

13:24 >perl -wE "$|++; say ''; sub f { print qq($_[0] ); if ($_[0] >= 
+100) { say 'Bye!'; } else { f($_[0] + 1); } } f(1);"

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 2
+7 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 5
+0 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 7
+3 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 9
+6 97 98 99 Deep recursion on subroutine "main::f" at -e line 1.
100 Bye!

13:24 >perl -v

This is perl 5, version 22, subversion 0 (v5.22.0) built for MSWin32-x
+64-multi-thread

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    Hope that helps,

    Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: No recursion depth limit?
by kcott (Archbishop) on Sep 02, 2015 at 03:31 UTC

    G'day Yary,

    "I have this memory of Perl limiting how deep subroutine recursion could go, ..."

    Perl only warns when a certain recursion depth is reached; it doesn't prevent you from recursing deeper.

    "... but the following test suggests not:"

    If you ask for warnings, you get them: 

    $ perl -E 'sub f{say $_[0] unless $_[0] % 10000;exit if $_[0] > 30000;
+f($_[0]+1)};f(1)'
10000
20000
30000

$ perl -wE 'sub f{say $_[0] unless $_[0] % 10000;exit if $_[0] > 30000
+;f($_[0]+1)};f(1)'
Deep recursion on subroutine "main::f" at -e line 1.
10000
20000
30000

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    "Just curious if there was a recursion limit, ..."

    The limit is 100, i.e. on the 100th recursion, a warning is emitted. 

    $ perl -wE 'sub f{say $_[0] unless $_[0] % 33;exit if $_[0] >= 99;f($_
+[0]+1)};f(1)'
33
66
99
ken@ganymede: ~/tmp
$ perl -wE 'sub f{say $_[0] unless $_[0] % 33;exit if $_[0] >= 100;f($
+_[0]+1)};f(1)'
33
66
99
Deep recursion on subroutine "main::f" at -e line 1.

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    Search for "Deep recursion on subroutine" in perldiag for more details and additional information.

    Also, if you're using the warnings pragma, which I strongly recommend, you can selectively turn off warning messages.

    — Ken

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Re: No recursion depth limit?
by Anonymous Monk on Sep 02, 2015 at 03:02 UTC
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Re: No recursion depth limit?
by stevieb (Canon) on Sep 02, 2015 at 13:45 UTC

    I didn't see anyone post about this warning while using the debugger (sorry if I missed it). You can change the default of 100 in the debugger very easily for testing/troubleshooting by setting $DB::deep...

    Default:

    $ perl -d rec.pl

main::(rec.pl:10):      f(1);

  DB<1> c
main::f(rec.pl:6):          $_[0] unless $_[0] % 10000;

100 levels deep in subroutine calls!

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    Set to 500:

    $ perl -d rec.pl

main::(rec.pl:10):      f(1);

  DB<1> $DB::deep = 500

  DB<2> c
main::f(rec.pl:6):          $_[0] unless $_[0] % 10000;

500 levels deep in subroutine calls!

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Re: No recursion depth limit?
by jf1 (Beadle) on Sep 10, 2015 at 15:06 UTC

    You can get a hard recursion limit and thus similar behaviour as e.g. in python with:

     use warnings FATAL => 'recursion';

    Just wondering: Do people here actually use this statement?

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      I almost always use strictures; which makes warnings fatal. So, the answer is "yes" in a slightly roundabout way; see strictures. Oh, say… I just "saw" the documentation and while recursion is a fatal in strictures v1, which I'm still using in production, it is a plain warning in v2. Hmmmm, not sure how I feel about that. I see the rationale is that it will cause an error if it's going to so why stop it at some early, semi-arbitrary point. Well, alllllllrighty then.

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