Use with variable

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

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Re: Use with variable by borisz (Canon) on Sep 08, 2004 at 13:11 UTC
Yes, maybe you need to call import yourself. `$mod = "Digest::MD5"; eval "require $mod";` [download] Boris	[reply] [d/l]
Re^2: Use with variable by edan (Curate) on Sep 08, 2004 at 13:31 UTC
If you're already going to `eval`, might as well just do: `my $mod = "Digest::MD5"; eval "use $mod";` [download] and avoid calling import yourself... And don't forget to check `$@`... -- edan	[reply] [d/l]
Re^3: Use with variable by Aristotle (Chancellor) on Sep 08, 2004 at 22:11 UTC
Actually, you don't need to interpolate the variable, and it's much safer not to: `my $module = "Digest::MD5"; eval 'use $module'; # note single quotes` [download] That's because `use $var` actually works. You just have to somehow populate `$var` before the compile-time effect of use takes place. A `BEGIN` block lets you do that. Silly example: `my $module; BEGIN { $module = "Digest::MD5"; } use $module;` [download] Note that the snippets of course aren't equivalent — the first loads the module at runtime, the second loads it at compile time. Makeshifts last the longest.	[reply] [d/l] [select]
Re: Use with variable by Corion (Patriarch) on Sep 08, 2004 at 13:08 UTC
`perldoc -f use` tells you the following: Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to `BEGIN { require Module; import Module LIST; }` [download] except that Module must be a bareword. So, you can easily do it like the following: `my $module = "CGI"; $module->require; $module->import;` [download] Update: Fixed the ugly bug I introduced. Thanks to tomhukins for spotting it. Update 2: I should test before I promise broken things as fixed. See my reply to edan further down	[reply] [d/l] [select]
Re^2: Use with variable by tomhukins (Curate) on Sep 08, 2004 at 13:18 UTC
Module must be a bareword `my $module = "CGI"; require $module;` [download] That's an interesting bareword you use. ;-)	[reply] [d/l]
Re^2: Use with variable by edan (Curate) on Sep 08, 2004 at 13:34 UTC
`my $module = "CGI"; $module->require; $module->import;` [download] `Can't locate object method "require" via package "CGI" (perhaps you forgot to load "CGI"?) at t.pl line 2.` -- edan	[reply] [d/l]
Re^3: Use with variable by Corion (Patriarch) on Sep 08, 2004 at 13:39 UTC
That's what I get for not testing my stuff :-(( So it seems the only actually working way is the ugly way of converting the module name to the file name and then requiring that file : `my $module = "CGI"; my $filename = $module; $filename =~ s!::!/!g; $filename .= ".pm"; require $filename or die "$module did not return a true value"; $module->import;` [download] (this time tested it ...)	[reply] [d/l]
Re^4: Use with variable by edan (Curate) on Sep 08, 2004 at 13:42 UTC
Re^5: Use with variable by Corion (Patriarch) on Sep 08, 2004 at 13:48 UTC
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Re: Use with variable by radiantmatrix (Parson) on Sep 08, 2004 at 14:36 UTC
Others have answered (the eval route is the way I'd go); but I have a question. Was your query merely curiosity, or have you actually found a use for this technique? I've never encountered a problem where this was the solution, and I'm just curious from a "hey, that's really unusual" point-of-view. Would you mind sharing more about what you're doing? ^{-- $me = rand($hacker{perl});}	[reply]
Re^2: Use with variable by edan (Curate) on Sep 08, 2004 at 15:09 UTC
It's easy to think of applications for this technique. Any sort of "plug-in" or "load-on-request" situation. Or "drivers", for instance. Ever use DBI? Notice how you only have to "`use DBI;`" and then the appropriate "DBD::*" module is loaded according to the datasource in your `DBI->connect()` call? That magic is done by dynamically loading a module that is contained in, yes, you guessed it, a variable! Actually, the eval method is used there: `# --- load the code my $driver_class = "DBD::$driver"; eval qq{package # hide from PAUSE DBI::_firesafe; # just in case require $driver_class; # load the driver }; if ($@) { #....` [download] -- edan	[reply] [d/l]
Re^3: Use with variable by radiantmatrix (Parson) on Sep 08, 2004 at 15:34 UTC
Hey, neat. I've never written anything that could benefit from plugins, so I never encountered the need for that before. I definitely need to file that one away for later. Thanks! ^{-- $me = rand($hacker{perl});}	[reply]


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