Using constants. What am I doing wrong?

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

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Re: Using constants. What am I doing wrong? by kvale (Monsignor) on Feb 22, 2005 at 04:01 UTC
As with all "use" directives, defining a constant happens at compile time, before the code is run. At compile time, $Base has no value. That is why this works: `#!/usr/bin/perl -w use strict; use constant BASE => 'mybase'; print "BASE: ", BASE, "\n";` [download] and your code does not. Update: If you want to set a 'symbolic constant' to a variable, you could use a variable and stick to the convetion that all caps means constant: `!/usr/bin/perl -w use strict; my $BASE = 'mybase'; print "BASE: $BASE\n";` [download] This also has the advantage of allowing variable interpolation. -Mark	[reply] [d/l] [select]
Re: Using constants. What am I doing wrong? by dws (Chancellor) on Feb 22, 2005 at 04:05 UTC
Why doesn't the code below print "mybase"? In Perl, execution of a script doesn't necessarily happen top-to-bottom. Certain constructs, notably `BEGIN { }` [download] blocks, are executed as they are encountered. The `use` keyword is also processed when encountered, so that definitions it pulls in are available as the rest of the script is compiling. In your script, this means that `use constant BASE => $Base;` [download] is being executed before `$Base` has been assigned a value. Since the purpose of the `constant` module is to establish constants, simply rewriting that line to `use constant BASE => 'mybase';` [download] is sufficient.	[reply] [d/l] [select]
Re: Using constants. What am I doing wrong? by saskaqueer (Friar) on Feb 22, 2005 at 04:50 UTC
Others have told you why you get the result you get; here's a couple of ways to get around it: `# this: BEGIN { use vars '$Base'; $Base = 'mybase'; } use constant BASE => $Base; # or this: my $Base = 'mybase'; sub BASE () { $Base }` [download]	[reply] [d/l]
Re^2: Using constants. What am I doing wrong? by dragonchild (Archbishop) on Feb 22, 2005 at 14:29 UTC
The correct solution you're looking for is: `my $Base; BEGIN { $Base = 'mybase' } use constant BASE => $Base;` [download] Being right, does not endow the right to be rude; politeness costs nothing. Being unknowing, is not the same as being stupid. Expressing a contrary opinion, whether to the individual or the group, is more often a sign of deeper thought than of cantankerous belligerence. Do not mistake your goals as the only goals; your opinion as the only opinion; your confidence as correctness. Saying you know better is not the same as explaining you know better.	[reply] [d/l]
Re^3: Using constants. What am I doing wrong? by saskaqueer (Friar) on Feb 23, 2005 at 06:54 UTC
Strange. I could have sworn I'd tried that solution first, but found it to not work. Just tried it again before I was going to post saying 'that does not work', but all of a sudden it works. Strange world :)	[reply]
Re^2: Using constants. What am I doing wrong? by Random_Walk (Prior) on Feb 22, 2005 at 11:34 UTC
The second one does not make a constant `perl -le'my $BASE="are belong to us"; sub BASE () { $BASE }; $BASE="fred"; print "all your base ", BASE' all your base fred` [download] Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply] [d/l]
Re: Using constants. What am I doing wrong? by rlb3 (Deacon) on Feb 22, 2005 at 04:02 UTC
Hello, A "use" statment is called at compile time, before any other code is evaluated. So when your constant is looking for $Base and it is not yet initialized. It might work if you put the $Base in a BEGIN block before the "use". rlb3	[reply]
Re^2: Using constants. What am I doing wrong? by Random_Walk (Prior) on Feb 22, 2005 at 11:20 UTC
update I am dumb, the one below does not work 'cos I put a my in the BEGIN block, when good habits turn bad! Without the my it works a charm `>perl -le'BEGIN{$BASE="are belong to us"} use constant BASE => $BASE; print "all your base ", BASE' all your base are belong to us >` [download] original message I though that would work too but it does not for me `> perl -le'BEGIN{$BASE="are belong to us"} use constant BASE => $BASE; print "all your base ", BASE' all your base > # but this does >perl -le'use constant BASE=>"are belong to us"; print "all your base ", BASE' all your base are belong to us >` [download] But I guess the OP was after something more flexible Cheers, R. Pereant, qui ante nos nostra dixerunt!	[reply] [d/l] [select]

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