Ampersand prefix for package constants ?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Ampersand prefix for package constants ? by jonadab (Parson) on Sep 30, 2003 at 19:23 UTC
The & is the sigil for code. &foo is for subroutines what $foo is for scalars and @foo is for arrays and so on. &POSIX::O_RDWR is probably defined in package POSIX as a `sub O_RDWR { ... }` In Perl5, the & does not necessarily take a reference to the subroutine; it could be calling the routine, depending on several things. In Perl6 this will get cleared up, and the & sigil will always take a coderef. `$;=sub{$/};@;=map{my($a,$b)=($_,$;);$;=sub{$a.$b->()}} split//,".rekcah lreP rehtona tsuJ";$\=$ ;->();print$/` [download]	[reply] [d/l] [select]
Re: Ampersand prefix for package constants ? by liz (Monsignor) on Sep 30, 2003 at 20:17 UTC
In Perl 5, constants _are_ subroutines: `use constant FOO => 42;` [download] is basically syntactic sugar for: `sub FOO { 42 }` [download] Normally, you wouldn't need to use the & sigil, but many of the POSIX constants are basically AUTOLOADed subroutines. So I bet you have something like: `use POSIX ();` [download] at the start of your program. Because the subroutines have not been encountered at compile time, the compiler needs a hint that it is a subroutine (otherwise it will assume it is a "bareword"). Observe: `$ perl -mPOSIX -e '$a = POSIX::O_RDWR; print "$a\n"' POSIX::O_RDWR $ perl -mPOSIX -e '$a = &POSIX::O_RDWR; print "$a\n"' 2` [download] An alternate way is to export the constant your own namespace so that you can refer to it directly without the POSIX:: prefix: `use POSIX qw(O_RDWR);` [download] or, as a one liner: `$ perl -mPOSIX=O_RDWR -e '$a = O_RDWR; print "$a\n"' 2` [download] Hope this helps. Liz	[reply] [d/l] [select]
Re: Ampersand prefix for package constants ? (don't) by tye (Sage) on Sep 30, 2003 at 21:57 UTC
Calling subroutines with an ampersand but without parentheses is a special invocations that preserves @_ and should only be used in special advanced situations. This is not one of those. You'd be better off writing `POSIX::O_RDWR()` or (if you used strict) `POSIX::O_RDWR`. Some information related to such choices is summarized in (tye)Re: A question of style. Update: For those players at home confused by this answer (thinking I got my advice backward as far as strict.pm is concerned, one of whom was kind enough to /tell me about it)... The point of strict.pm is pretty much to catch simple mistakes, including typos. Let's take two simple mistakes as examples: 1) you mispelt "O_RDWR" or 2) forgot to "use POSIX". Now consider these correct usages: `perl -le " use POSIX; print 'x',POSIX::O_RDWR();" perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR();" perl -le " use POSIX; print 'x',POSIX::O_RDWR;" perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR;"` [download] which gives us results similar to the following: `> perl -le " use POSIX; print 'x',POSIX::O_RDWR();" x2 > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR();" x2 > perl -le " use POSIX; print 'x',POSIX::O_RDWR;" x2 > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWR;" x2` [download] Now consider these mistakes: `perl -le " print 'x',POSIX::O_RDWR();" perl -le " use POSIX; print 'x',POSIX::O_RDWT();" perl -le "use strict; print 'x',POSIX::O_RDWR();" perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT();" perl -le "use strict; print 'x',POSIX::O_RDWR;" perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT;"` [download] which are all caught for us as follows: > perl -le " print 'x',POSIX::O_RDWR();" Undefined subroutine &POSIX::O_RDWR called at -e line 1. > perl -le " use POSIX; print 'x',POSIX::O_RDWT();" O_RDWT is not a valid POSIX macro at -e line 1 > perl -le "use strict; print 'x',POSIX::O_RDWR();" Undefined subroutine &POSIX::O_RDWR called at -e line 1. > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT();" O_RDWT is not a valid POSIX macro at -e line 1 > perl -le "use strict; print 'x',POSIX::O_RDWR;" Bareword "POSIX::O_RDWR" not allowed while "strict subs" in use at -e +line 1. Execution of -e aborted due to compilation errors. > perl -le "use strict; use POSIX; print 'x',POSIX::O_RDWT;" Bareword "POSIX::O_RDWT" not allowed while "strict subs" in use at -e +line 1. Execution of -e aborted due to compilation errors. [download] and finally consider these mistakes: `perl -le " print 'x',POSIX::O_RDWR;" perl -le " use POSIX; print 'x',POSIX::O_RDWT;"` [download] which are not caught for us: `> perl -le " print 'x',POSIX::O_RDWR;" xPOSIX::O_RDWR > perl -le " use POSIX; print 'x',POSIX::O_RDWT;" xPOSIX::O_RDWT` [download] and you see why I say you should only write `POSIX::O_RDWR` if you used strict (because if you try to write `POSIX::O_RDWR` without using strict.pm, you aren't protected from a simple mistake going unreported). - tye	[reply] [d/l] [select]