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Subject Author Date
retrieving a constant from a dynamically loaded module olivier.grant 10-26-2005
Posted by olivier.grant on October 26, 2005, 3:29 am
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Hi All,

I need to be able to dynamically load a module and retrieve a constant
from it. Simply put, i want to allow a user to specify a module
HisModule.pm containing the package HisModule and get
HisModule::VERSION.

in his file :
[code]
package HisModule;
use constant VERSION;
1;
[/code]

in my code :
[code]
$module = shift @ARGV;
print " Using module v".$module::VERSION."\n";
[/code]

I know this is not the right syntax, but I can't seem to find it .. Is
it even possible to do this ?

Thanks,

Olivier



Posted by Gunnar Hjalmarsson on October 26, 2005, 12:36 pm
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olivier.grant@gmail.com wrote:
> I need to be able to dynamically load a module and retrieve a constant
> from it. Simply put, i want to allow a user to specify a module
> HisModule.pm containing the package HisModule and get
> HisModule::VERSION.
>
> in his file :
> [code]
> package HisModule;
> use constant VERSION;
> 1;
> [/code]
>
> in my code :
> [code]
> $module = shift @ARGV;

chomp $module;
require $module;

> print " Using module v".$module::VERSION."\n";
> [/code]

--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl


Posted by olivier.grant on October 26, 2005, 5:12 am
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I got that part, but the problem is that as standard, each module and
script has a constant called "TSD_VERSION", so I need to be able to
specify that the TSD_VERSION constant i'm looking for is the one from
the module specifyed by the user, so the syntax should look something
like this :

$module::TSD_VERSION;

but that doesn't work. wrapping this statement in an eval will work,
but several problems pop in :
the following line of code

eval("print ".$module."::TSD_VERSION;");
eval("print(\"module version
v\".".$ScriptData."::TSD_VERSION);");

will print the correct version number, but if you add a bit more to the
line to output doesn't work :

eval("Logger::Print(\"Using core module \".$ScriptData.\"
v\".".$ScriptData.'::TSD_VERSION'.".\"
(\".".$ScriptData.'::TSD_LAST_UPDATE'.".\")\n\");");

(sorry for the obscure code)



Posted by harryfmudd [AT] comcast [DOT] on October 26, 2005, 11:03 am
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olivier.grant@gmail.com wrote:

[snip!]

> the following line of code
>
> eval("print ".$module."::TSD_VERSION;");
> eval("print(\"module version
> v\".".$ScriptData."::TSD_VERSION);");
>
> will print the correct version number, but if you add a bit more to the
> line to output doesn't work :
>
> eval("Logger::Print(\"Using core module \".$ScriptData.\"
> v\".".$ScriptData.'::TSD_VERSION'.".\"
> (\".".$ScriptData.'::TSD_LAST_UPDATE'.".\")\n\");");
>
> (sorry for the obscure code)
>

Well, there's something to be said for the K.I.S.S. principle. Why build
a big complicated string if you can call Logger::Print directly, and
just eval() the part that needs eval()-ing? Something like

Logger::Print ("Using core module '$ScriptData' v" .
eval ::TSD_VERSION"} .
eval ::TSD_LAST_UPDATE} . "\n");

seems clearer to me.

If you must build a big complicated string, have you printed it before
eval()-ing it, to see what you get? You might also consider using the
"qq{}" construct, so you don't have to escape all the embedded quotes.
Note that in my cold-coded simplification I changed your output by using
the "'" character. If you _must_ have '"', consider Logger::Print
(qq{Using core module "$ScriptData v" ...

Tom Wyant


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