checking for preloaded modules

ben-at-geeklair has asked for the wisdom of the Perl Monks concerning the following question:

I'm writing a custom error-handling module for a steaming pile of CGIs that should rollback any active database handles, disconnect 'em, and choke out a nice, friendly message before dying. I'd like to be able to take advantage of HTML::Template with a default error template if possible, but to just spit out an HTML page if HTML::Template hasn't been loaded by the script that calls my module. Does anyone know of a reliable way to determine (from within the module) if the calling script has loaded another module? Is this the kind of thing for which I should just use eval()?
I'm also currently passing any active database handles manually into the module's functions. Is there a smart way to get a list of all active handles, so I don't have to trust myself to pass them to the error function?

Thanks much,

Ben

Comment on checking for preloaded modules

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Re: checking for preloaded modules by davorg (Chancellor) on Apr 18, 2001 at 12:08 UTC
You could probably use the `%INC` hash. This contains a list of all of the modules that have been loaded. The keys are the names of the modules and the values are the paths that the modules were loaded from. `if ($INC{'HTML/Template.pm'}) { # do HTML::Template stuff } else { # do vanilla HTML stuff }` [download] Update: Corrected hash key. Thanks to jmcnamara for pointing it out (twice!) -- <http://www.dave.org.uk> "Perl makes the fun jobs fun and the boring jobs bearable" - me	[reply] [d/l]
Re: checking for preloaded modules by nardo (Friar) on Apr 18, 2001 at 01:13 UTC
`if(defined(%HTML::Template::)) { #HTML::Template has been loaded } else { #HTML::Template has not been loaded }` [download]	[reply] [d/l]
Re: checking for preloaded modules by sachmet (Scribe) on Apr 18, 2001 at 01:20 UTC
To answer your first question, you could always check to see if the symbol table entries exist. For example: `> perl print "Data::Dumper exists\n" if %Data::Dumper::; > perl -MData::Dumper print "Data::Dumper exists\n" if %Data::Dumper::; Data::Dumper exists` [download] So you could do: `if (%HTML::Template::) { # HTML::template stuff } else { # Regular HTML stuff }` [download] Keep in mind, however, there is a caveat to that: if you load HTML::Template::othermodule without loading HTML::Template, that may fail. Instead, you can do: `if (exists ${%HTML::Template::}{'import'}) { ... }` [download] which checks for the existance of the import subroutine. import is in every package by default, and that can be used to see if it exists (and hence if a module has been loaded by that name).	[reply] [d/l] [select]
Re: checking for preloaded modules by arturo (Vicar) on Apr 18, 2001 at 01:16 UTC
Smudged (not too dirty, not too clean) hack is the one you suggest: wrap an `eval` block around the HTML::Template version, if the module's not loaded, `$@` will be set, and you can just generate the 'plain ole' HTML page by hand. But since any CPAN modules worth its salt (IMO) sets $VERSION, you should be able to test `defined ($HTML::Template::VERSION)` and achieve similar result. But in a way, that's dirtier than the `eval {}` solution because a module can be perfectly functional without setting `$VERSION`, so I can see an otherwise excellent coder leaving that out. Philosophy can be made out of anything. Or less -- Jerry A. Fodor	[reply] [d/l] [select]
Re: checking for preloaded modules by princepawn (Parson) on Apr 18, 2001 at 02:26 UTC
This wasn't your main question, but CGI::Application makes use of HTML::Template internally and it must export a re-useable error-handling routine. If you aren't sure, msg AgentM or join the support mailing list.	[reply]