use constant behavior
in [Perl]
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From: ccc31807 on 27 Apr 2010 17:28 use constant DIR => 'log_files'; mkdir DIR unless -e DIR; This places a subdirectory in my current working directory named log_files, creating it if it doesn't exist. print_log(); sub print_log { open LOG, '>', "DIR/log.csv" or die "Cannot open LOG, $!"; ... } This throws a DOES NOT EXIST error. However, print_log(DIR); sub print_log { my $dir = shift; open LOG, '>', "$dir/log.csv" or die "Cannot open LOG, $!"; ... } works just fine. What am I missing? Thanks, CC.
From: Uri Guttman on 27 Apr 2010 17:35 >>>>> "c" == ccc31807 <cartercc(a)gmail.com> writes: c> use constant DIR => 'log_files'; c> mkdir DIR unless -e DIR; c> This places a subdirectory in my current working directory named c> log_files, creating it if it doesn't exist. c> print_log(); c> sub print_log c> { c> open LOG, '>', "DIR/log.csv" or die "Cannot open LOG, $!"; c> ... c> } c> This throws a DOES NOT EXIST error. However, perl constants are really subs with a prototype of no arguments. they are converted at compile to their value (and constant folded if possible). "DIR" won't work because subs don't directly interpolate. you can work around that with DIR . '/log.csv' or the @{[DIR]} interpolation trick. another way is to use the ReadOnly module which declares regular variables which can't be modified. then you can use $DIR in either situation. uri -- Uri Guttman ------ uri(a)stemsystems.com -------- http://www.sysarch.com -- ----- Perl Code Review , Architecture, Development, Training, Support ------ --------- Gourmet Hot Cocoa Mix ---- http://bestfriendscocoa.com ---------
From: ccc31807 on 28 Apr 2010 07:40 On Apr 27, 5:35 pm, "Uri Guttman" <u...(a)StemSystems.com> wrote: > perl constants are really subs with a prototype of no arguments. they > are converted at compile to their value (and constant folded if > possible). Uri, thanks for your explanation. I would then assume that - use constant DIR => 'log_files'; would be more or less equivalent to - sub DIR { return 'log_files'; } Is this assumption correct, more or less? Thanks, CC.
From: Tad McClellan on 28 Apr 2010 08:16 ccc31807 <cartercc(a)gmail.com> wrote: > use constant DIR => 'log_files'; > open LOG, '>', "DIR/log.csv" or die "Cannot open LOG, $!"; perldoc constant Constants defined using this module cannot be interpolated into strings like variables. -- Tad McClellan email: perl -le "print scalar reverse qq/moc.liamg\100cm.j.dat/" The above message is a Usenet post. I don't recall having given anyone permission to use it on a Web site.
From: Uri Guttman on 28 Apr 2010 13:09
>>>>> "c" == ccc31807 <cartercc(a)gmail.com> writes: c> On Apr 27, 5:35�pm, "Uri Guttman" <u...(a)StemSystems.com> wrote: >> perl constants are really subs with a prototype of no arguments. they >> are converted at compile to their value (and constant folded if >> possible). c> Uri, thanks for your explanation. c> I would then assume that c> - use constant DIR => 'log_files'; c> would be more or less equivalent to c> - sub DIR { return 'log_files'; } sub DIR() { 'log_files' } the important difference is the prototype of () so it can be compile time converted to a constant. the return isn't needed (i dunno if it affects it becoming a proper constant but i doubt it). uri -- Uri Guttman ------ uri(a)stemsystems.com -------- http://www.sysarch.com -- ----- Perl Code Review , Architecture, Development, Training, Support ------ --------- Gourmet Hot Cocoa Mix ---- http://bestfriendscocoa.com --------- |