Re^4: Importing constans and variables when "require"ing

Thanks for the added clarifications. Being quite lazy I never searched into the manuals for what a Perl constant is. I just assumed it would be the same as C. And that's why I use it often: more for (thinking) being lighter than a variable and much less for its read-only property. I did see smoke signals in forums saying that they never really use Perl constants, there is no point, but I ignored them. And so a constant is a sub with constant return value which ideally would be optimised by Perl to become more-or-less C #define.

That also explains the fact that there is no complain if I replace what Perl sees as a bareword "XYZ::CONST" with its sub equivalent "XYZ::CONST()" something like what Perlbotics suggested but without the use const.

So a Perl constant is a glorious sub and would be lighter than a variable if Perl eventually optimises it to a constant realising that it has a constant return value. And it's read-only. Great!

Now, why does it complain with Name "XYZ::PI" used only once: possible typo when I print the foreign variable (print $XYZ::PI). Doesn't the definition of "use" include write AS WELL as READ?

bw, bliako

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Re^5: Importing constans and variables when "require"ing by soonix (Canon) on Feb 24, 2019 at 12:47 UTC
`used only once` doesn't refer to useing, more to using/defining by "mentioning" it. And since you only `require XYZ` (the module but not the variable `XYZ::PI`), you are mentioning `XYZ::PI` only once. The compilation of `XYZ.pm`, where `XYZ::PI` is defined, happens only later, during runtime of `example.pl` , when the `require` ist actually executed. If you `use XYZ` instead, Perl sees the declaration of `XYZ::PI` early enough to avoid the warning.	[reply] [d/l] [select]
Re^6: Importing constans and variables when "require"ing by bliako (Monsignor) on Feb 24, 2019 at 13:09 UTC
OK thanks. I will confess that I found it weird that the problem is "possible typo" and not complaining because of unknwon variable. Eventually I paid more attention to the so-frequent error message `Global symbol "$x" requires explicit package name (did you forget to declare "my $x"?) at...` I realised that it says "requires explicit package name" whereas I have usually been concentrating on "did you forget to declare?" part. Combined with your message and LanX's `Another effect of strict (vars) is to enforce pre declaration of unqualified variables with my or our.` I get to understand it, hopefully fully. So, evidently, one can get away with declaring variables using my/our completely if it uses fully-qualified variables. So `use strict; $main::x = 42;` will succeed whereas `use strict; $x = 42;` will fail. thanks, bw, bliako	[reply] [d/l] [select]
Re^7: Importing constans and variables when "require"ing by soonix (Canon) on Feb 24, 2019 at 18:16 UTC
Yes, but be aware that `$main::x` is a global variable (as in the first error message), which means it can be accessed and modified from anywhere, even outside the corresponding module. So, if once upon a time its value differs from what you expect, you'll have a huge space to search for the culprit. OTOH, if you use scoped (i.e. my) variables, you'll need to search only the correspondig scope. The error message `Global symbol "$x" requires explicit package name` [download] actually means `Do you really want "$x" to be a global symbol?` [download]	[reply] [d/l] [select]
Re^5: Importing constans and variables when "require"ing by LanX (Saint) on Feb 24, 2019 at 15:09 UTC
In short: Strict and warnings add a lot of compile time checks to help you catch "potential" problems. This usually works well if you don't try to tweak the normal procedures like you do by switching to runtime effects. Either ignore or disable these messages or start to read what Perl really does. ;) Hint: Perl 5 is mostly compatible to Perl 4 which didn't have strict ... edit ... and does dynamic binding of vars and subs unlike C. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re^5: Importing constants and variables when "require"ing by Your Mother (Archbishop) on Feb 24, 2019 at 16:42 UTC
Readonly might be closer to what you’d imagine you’d like. Check the whole doc though.	[reply]
Re^6: Importing constants and variables when "require"ing by bliako (Monsignor) on Feb 25, 2019 at 11:39 UTC
I am reading it right now and finding lots more of caveats regarding constants...	[reply]


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Re^4: Importing constans and variables when "require"ing

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