Why is this program valid? I was trying to create a syntax error

514
votes

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?

6
perl
Did you just prove that typing random words into perl produces working programs??!?!?!?! – Peter M
@PeterM Hardly random words. I proved I don't know enough about Perl syntax. Now I know a bit more. – Bill Ruppert
You probably want no indirect to stop those ones from happening – LeoNerd
This is the most famous perl question ever. Even better as Schwartz's snippet: whatever / 25 ; # / ; die "this dies!"; – jm666
Written by linguist Larry Wall, Perl allows authors a lot of creative space. There is a sub-category in perl programming called Perl Poetry, valid Perl expressing stuff beyond computer interpretation: perlmonks.org/?node_id=1111395 – bbaassssiiee

6 Answers

563
votes

Perl has a syntax called "indirect method notation". It allows

Foo->new($bar)

to be written as

new Foo $bar

So that means

Syntax error ! exit 0;

is the same as

error->Syntax(! exit 0);

or

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).

116
votes

I 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!

55
votes

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
35
votes

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)

7
votes

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
3
votes

In this paper, we aim to answer a long-standing open problem in the programming languages community: is it possible to smear paint on the wall without creating valid Perl?

TLDR; Hardly