Re: exit this way

Further to Your Mother's reply: Because Perl has no main() function and allows intermixing function calls and definitions, strange things can happen. Better by far, IMHO, to have someone put in a function call and scratch their head (even for quite a while) about why it is never executed than to have the code compile and apparently run correctly for hours/days/weeks/... before realizing that, hey, something ain't right! Here's an (admittedly rather contrived) example of goofy behavior:

c:\@Work\Perl\monks>perl -wMstrict -le
"S(3);
 T();
 S(9);
 T();
 ;;
 exit_here_to_avoid_weirdness();
 ;;
 my $x = 42;
 sub S { $x = $_[0];  printf qq{in S: x == $x  };  T(); }
 ;;
 sub T { ++$x;  print qq{in T: x == $x}; }
 ;;
 sub exit_here_to_avoid_weirdness { ;;; }
 T();
"
in S: x == 3  in T: x == 4
in T: x == 5
in S: x == 9  in T: x == 10
in T: x == 11
in T: x == 43
[download]

Is that 43 result from the last call to T() correct? Maybe better not to make it at all:

c:\@Work\Perl\monks>perl -wMstrict -le
"S(3);
 T();
 S(9);
 T();
 ;;
 exit_here_to_avoid_weirdness();
 ;;
 my $x = 42;
 sub S { $x = $_[0];  printf qq{in S: x == $x  };  T(); }
 ;;
 sub T { ++$x;  print qq{in T: x == $x}; }
 ;;
 sub exit_here_to_avoid_weirdness { exit; }
 T();
"
in S: x == 3  in T: x == 4
in T: x == 5
in S: x == 9  in T: x == 10
in T: x == 11
[download]

(Of course, it would be nice to have a "code not reachable" message, but that's life.)

Give a man a fish: <%-{-{-{-<

Comment on Re: exit this way Select or Download Code

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Re^2: exit this way by 1nickt (Canon) on Sep 01, 2015 at 02:32 UTC
it would be nice to have a "code not reachable" message ~~There isn't such a check for post-`exit()`, but Perl::Critic definitely has a rule for pre-`use strict;`. I guess one could write a rule that ignored subroutines...~~ I guess there is! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^3: exit this way by Anonymous Monk on Sep 01, 2015 at 05:54 UTC
it would be nice to have a "code not reachable" message There isn't such a check for post-exit(), but Perl::Critic definitely has a rule for pre-use strict;. I guess one could write a rule that ignored subroutines... Hmm, Perl::Critic::Policy::ControlStructures::ProhibitUnreachableCode - Don't write code after an unconditional die, exit, or next. Perl::Critic::Policy::Modules::ProhibitExcessMainComplexity - Minimize complexity in code that is outside of subroutines.	[reply]
Re^4: exit this way by 1nickt (Canon) on Sep 01, 2015 at 07:11 UTC
Oof. Updated. I ran a little test script but my `.perlcriticrc` had got borked somehow so `perlcritic` ran with `-5` (why is that the default, anyway?), and the rule has a severity of 4. I didn't notice anything wrong because "Code before strictures enabled" has a severity of 5, so it did throw an error. Thanks for the correction. (When I first went to answer the question I thought I remembered there was a rule!) `#!/usr/bin/perl use warnings; my $foo = 'bar'; use strict; exit(0); my $baz = 'quux';` [download] `$ perlcritic 1140661.pl Code before strictures are enabled at line 3, column 1. See page 429 +of PBP. (Severity: 5)` [download] $ perlcritic -4 1140661.pl Code before strictures are enabled at line 3, column 1. See page 429 +of PBP. (Severity: 5) Unreachable code at line 9, column 1. Consider removing it. (Severit +y: 4) $ [download] The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re^2: exit this way by ExReg (Priest) on Sep 01, 2015 at 04:50 UTC
This will probably become a standards issue, and I will probably start putting the exits in. I hate it when I have to abide by standards that I get after others about. I know I would never leave stray bits of code roaming around after the subs start. ;^)	[reply]