Re: replacing literals with constants

Deparsing the code shows what Perl sees (in tmp.pl):

#!/usr/bin/perl -w
use strict;

sub ten { 10 }

print "9 .. 10 - 1 with constant\n";
for my $i (9 .. ten - 1) {
  print $i, "\n";
}
print "-> unexpected\n\n";
[download]

gives when deparsed:

corion@outerlimits:~/Projekte$ perl -MO=Deparse tmp.pl 
BEGIN { $^W = 1; }
use strict;
sub ten {
    10;
}
print "9 .. 10 - 1 with constant\n";
foreach my $i (9 .. ten(-1)) {
    print $i, "\n";
}
print "-> unexpected\n\n";
tmp.pl syntax OK
[download]

Since you don't have a prototype on your constant, the -1 gets interpreted as an argument to your subroutine call ten. The fix is to use a prototype, or to use the constant module, which does the same:

sub ten() { 10 };
[download]

... or ...

use constant ten => 10;
[download]

Comment on Re: replacing literals with constants Select or Download Code

Replies are listed 'Best First'.
Re^2: replacing literals with constants by hexcoder (Curate) on May 14, 2022 at 09:20 UTC
Thanks a lot! So, I will always use prototypes for constants from now on.	[reply]
Re^3: replacing literals with constants by kcott (Archbishop) on May 14, 2022 at 13:17 UTC
G'day hexcoder, It is a convention, but not a requirement, to use all uppercase for constants. This makes it easier to identify constants in your code: `TEN` stands out more than `ten`. See the examples in constant. "So, I will always use prototypes for constants from now on." For simple values (e.g. numbers, strings, etc.) I tend to use constant. For example: `use constant { ONE => 1, TWO => 2, ... NINE => 9, TEN => 10, };` [download] This is a lot simpler than coding: `sub ONE () { 1; } sub TWO () { 2; } ... sub NINE () { 9; } sub TEN () { 10; }` [download] I do use subroutines with an empty prototype on occasions. This would be when more complicated code is required. Consider the following example that creates a singleton object. `{ my %constructor_args; BEGIN { # ... code to populate %constructor_args ... } sub OBJ () { Some::Module::->new(\%constructor_args); } }` [download] Now, `OBJ` can be used (read-only) wherever the singleton instance is required, but cannot be modified; and `%constructor_args` cannot be accessed anywhere else in the code. That's a somewhat contrived example; however, I hope it gives you an idea of where a subroutine with an empty prototype might be used for a constant (where use of the `constant` pragma might be unwieldy or impractical). Here (in the spoiler) is an actual implementation of that. It's still rather contrived, as there are much better ways of doing this, but it does show the process. In this example, a single Text::CSV object is used as the value of a constant which processes two CSV (pipe-separated) files. (You might want to see Inline::Files if you're unfamiliar with that module.) <Reveal this spoiler or all in this thread> — Ken	[reply] [d/l] [select]
Re^3: replacing literals with constants by LanX (Saint) on May 14, 2022 at 22:35 UTC
why not just use `constant` which is self describing? many maintainers - like you - will struggle to understand the "magic" of empty prototypes. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply]