[Canberra-pm] how to tell if a library is available?

[Kim Holburn]

Thanks, that looks good, I'll try it.

On 2008/Jun/17, at 10:39 AM, Paul Fenwick wrote:

> G'day Kim / p5p,
>
> Kim Holburn wrote:
>
>> It obviously works at run time as in the eval but what exactly it  
>> does  then I'm not sure.  I think from the behaviour I'm seeing  
>> that the use  in the eval is not determined at compile time because  
>> at compile time  it's just a string.  Therefore references to  
>> variables in the library  are not linked at compile time unless a  
>> naked "use" or "require"  statement appears somewhere.
>
> Ah!  You may want something like this:
>
> BEGIN {
> 	eval "use Text::Autoformat";
> 	if ($@) {
> 		# Holy smokes Batman!  Text::Autoformat isn't
> 		# available!  Do something to set up the variables,
> 		# subroutines, and other things that Text::Autoformat
> 		# would normally provide for us
> 	}
> }
>
> By wrapping the code inside a BEGIN block, you force it to be  
> executed at compile-time, before Perl tries to resolve the rest of  
> the subroutines and variables in your code.  If Text::Autoformat  
> exists, then it gets loaded at compile time, and everything is  
> fine.  If it doesn't exist, then you'll need to do *something* to  
> ensure that parts of your code trying to use Text::Autoformat have  
> something to play with.  I can't be more specific without seeing the  
> warnings that's being generated; perhaps a piece of sample code may  
> help here?
>
> > I guess what I'd like are some conditional compile directives but  
> they
> > don't exist.
>
> If you want code that's simply not compiled at all if a module isn't  
> installed, you can do something like this:
>
> BEGIN {	eval "use Text::Autoformat"; }
>
> use constant AUTOFORMAT => $INC{'Text/Autoformat.pm'};
>
> # Later, in your main code...
>
> if (AUTOFORMAT) {
> 	# Do something that uses Text::AutoFormat...
> } else {
> 	# Do something else.
> }
>
> What we're doing here is checking for the module at compile-time,  
> setting a constant based upon whether it's been found and loaded,  
> and then conditionally compiling code based upon that constant.   
> Because Perl knows that constants can't change, it can completely  
> prune unwanted code from the parse tree at compile-time.  The text  
> that was pruned won't give you warnings, because as far as Perl is  
> concerned, it wasn't there. ;)
>
> Of course, if you have to start pulling tricks like this, you'll  
> probably find it becomes *much* easier to simply insist the modules  
> you want are installed, or using PAR (the Perl Archiver) to bundle  
> them up with your program.  You can find our tutorial on that at:
>
> 	http://perltraining.com.au/tips/2008-05-23.html
>
> Cheerio,
>
> 	Paul
>
> -- 
> Paul Fenwick <pjf at perltraining.com.au> | http://perltraining.com.au/
> Director of Training                   | Ph:  +61 3 9354 6001
> Perl Training Australia                | Fax: +61 3 9354 2681

--
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