I'm running ActiveState's 32 bit ActivePerl 5.14.2 on Windows 7. I wanted to mess around with a Git pre-commit hook to detect programs being checked in with syntax errors. (Somehow I just managed to do such a bad commit.) So as a test program I randomly jotted this:
use strict;
use warnings;
Syntax error!
exit 0;
However, it compiles and executes with no warnings, and errorlevel is zero on exit. How is this valid syntax?
Perl has a syntax called "indirect method notation". It makes the following two lines equivalent:
new Foo $bar # Indirect
Foo->new( $bar ) # Direct
So that means the following two lines are equivalent:
Syntax error ! exit 0; # Indirect
error->Syntax( ! exit 0 ); # Direct
Finally, let's cleanup the argument list.
error->Syntax( !exit( 0 ) );
Not only is it valid syntax, it doesn't result in a run-time error because the first thing executed is exit( 0 ).
This "feature" can be disabled using any of the following:
no feature qw( indirect ); # Perl 5.32+
use v5.36; # Perl 5.36+
Alternatively, you can get warnings using the following:
no indirect; # CPAN module
!exit(0) can no more be a type error than !$x since neither are typed.new Class and print $fh ... instead of Class->new(...) and $fh->print(...). I will grant you that it causes a weird error messages, thoughI don't know why, but this is what Perl makes of it:
perl -MO=Deparse -w yuck
BEGIN { $^W = 1; }
use warnings;
use strict 'refs';
'error'->Syntax(!exit(0));
yuck syntax OK
It seems that the parser thinks you're calling the method Syntax on the error-object... Strange indeed!
new Class instead of Class->new(). To call the method Syntax, the exit function is executed, so the run-time error never occures.
The reason you do not get an error is that the first executed code is
exit(0);
Because you did not have a semicolon on the first line:
Syntax error!
The compiler will guess (incorrectly) that this is a subroutine call with a not operator ! thrown in. It will then execute the arguments to this subroutine, which happens to be exit(0), at which point the program exits and sets errorlevel to 0. Nothing else is executed, so no more runtime errors are reported.
You will notice that if you change exit(0) to something like print "Hello world!" you do get an error:
Can't locate object method "Syntax" via package "error" ...
and your error level will be set:
> echo %errorlevel%
255
>The compiler will guess (incorrectly) The compiler can't do anything incorrectly.
As noted above this is caused by the indirect method calling notation. You can warn on this:
use strict;
use warnings;
no indirect;
Syntax error!
exit 0;
Produces:
Indirect call of method "Syntax" on object "error" at - line 5.
This requires the indirect CPAN module.
You can also use no indirect "fatal"; to cause the program to die (this is what I do)
use v.5:32; no feature 'indirect';Try Perl 6, it seems to fulfill your expectations more readily:
===SORRY!=== Error while compiling synerror.p6
Negation metaoperator not followed by valid infix
at synerror.p6:1
------> Syntax error!⏏<EOL>
expecting any of:
infix
infix stopper
no indirectto stop those ones from happeningwhatever / 25 ; # / ; die "this dies!";