Omitting parens around arguments is fraught with peril. You've just encountered yet another situation where the parser gets "confused".

It's my understanding that the constants created using constant and using functions with the empty prototype are not identical anymore, and that using constant is the recommended method.

I thought the recommended advice was to use Readonly instead of use constant. Is this wrong, or if not, could you explain the circumstances when you should use each one?

cheers
why_bird
........
Those are my principles. If you don't like them I have others.
-- Groucho Marx
.......

Not sure if I would recommend Readonly over constant — at least some people have been bitten by its subtle peculiarities. As an example, see this thread.

BTW, there's also Scalar::Readonly, which is directly manipulating the readonly-flag of scalars (instead of working with tied magic)...

___

Update:  (...it was only a matter of time until someone would post a benchmark, so... :)

use strict;
use warnings;
use Benchmark 'cmpthese';

use Readonly;
use Scalar::Readonly ':all';
use constant NL => "\012";
Readonly my $NL => "\012";
my $nl = "\012";  readonly_on($nl);
my $Nl = "\012";

cmpthese 2000000 => {
    constant  => sub { my $s = "abc".NL."def".NL."ghi"; },
    constant2 => sub { my $s = "abc${\NL}def${\NL}ghi"; },
    Readonly  => sub { my $s = "abc${NL}def${NL}ghi";   },
    ScalarRO  => sub { my $s = "abc${nl}def${nl}ghi";   },
    scalarRW  => sub { my $s = "abc${Nl}def${Nl}ghi";   },
    direct    => sub { my $s = "abc\012def\012ghi";     },
};

__END__

               Rate  Readonly constant2  scalarRW  ScalarRO  constant 
+   direct
Readonly    77851/s        --      -84%      -85%      -85%      -96% 
+     -96%
constant2  484262/s      522%        --       -5%       -5%      -75% 
+     -75%
scalarRW   510204/s      555%        5%        --       -0%      -74% 
+     -74%
ScalarRO   511509/s      557%        6%        0%        --      -74% 
+     -74%
constant  1960784/s     2419%      305%      284%      283%        -- 
+      -0%
direct    1960784/s     2419%      305%      284%      283%        0% 
+       --
[download]

(as running the benchmark multiple times suggests, differences up to around 5% are insignificant)

___

BTW, I think I remember PBP somewhere saying it's not meant to be treated as a bible...

Use Readoonly if you your need to program to run more slowly, otherwise: Don't.

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

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Perl Best Practices recommends Readonly. While constants with use constant are replaced at compile time, Readonly variables are normal variables, just set read-only.
The problem with use constant is that the constants are actual functions which don't interpolate in strings!

I recently needed the line feed ("\012") as constant (Note: "\n" is platform dependent!).
Compare:


use Readonly;
Readonly $NL => "\012";
[...]
print FILE "header1${NL}header2${NL}";


use constant NL => "\012";
[...]
print FILE "header1" . NL . "header2" . NL;
[download]

The speed penalty for Readonly is not meaningful in my case. However if you have large calculations which only dependent on constants, maybe inside a loop, then use constant is better because perl can optimise it at compile time.

Is there some meaning to unary * that I don't know about (and couldn't find in perlop) that makes this parse this way? Or do unprototyped functions just gobble up anything that vaguely looks like a term?

I tend to use parens zealously and have avoided learning about this kind of thing in the past, I guess it's time to head to the docs and read up.

Edit: Good point Fletch, totally forgot about that. Thanks!

Its the sigil for GLOBs (e.g. *STDOUT). How's the compiler supposed to know that $r didn't have a reference to a GLOB in it (somewhere else there was a my $r = \*STDOUT)?

The cake is a lie.
The cake is a lie.
The cake is a lie.

In Perl 6, you can declare a constant simply by using constant instead of my. And you can declare one with or without a sigil. So you could have:

constant �π = 2*atan2(1,0);
constant $pi ::= �π;

Thing is, constant is lexical, like a read-only version of my. So how do you export one?

You could also declare subs that act like unprototyped Perl 5 subs, or declare to take no arguments, or you can declare a 'term' directly.

—John