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

[Andres Kievsky]

Kim,

As you noticed, the problem is that perl is still compiling the code inside the if() block. I don't know of any way to stop perl from compiling it; It has to be valid perl, and when it's checked, the autoformat() sub has not been defined. You can circunvent this problem by adding parenthesis:

autoformat($data. {...});

instead of

autoformat $data. {...};

However, I think it'd be nicer to wrap the AUTOFORMAT sections in a plug-in that hides all this complexity and provides a single entry point for all code that might need to get its data formatted. Or why not just bundle Autoformat.pm? From what I can see, it's a single .pm file that can be copied to your source tree.

- ank
----- Original Message ----
From: Kim Holburn <kim at holburn.net>
To: Paul Fenwick <pjf at perltraining.com.au>
Cc: canberra-pm at pm.org
Sent: Wednesday, 18 June, 2008 7:26:03 PM
Subject: Re: [Canberra-pm] how to tell if a library is available?

Hi Paul,

Thanks very much for your reply.
I still have a niggle.

I tried your constant solution.  It works fine when it can find the  
library but produces a strange error when it can't.  So there still  
appears to be some compiling going on.

$ perl test3.pl
Useless use of single ref constructor in void context at test3.pl line  
19.
no

Here is my script:

#!/usr/bin/perl -w

BEGIN { eval "use Text::Autoformat"; }
use constant AUTOFORMAT => $INC{'Text/Autoformat.pm'};

my $data = <<'EOM';

>> That bundle of copyrights relates to the very considerable
>> investment required to make a succession of feature films.

It's a kind of circular argument.  People make an investment in a
movie in order to make a profit because they know they have these
copyrights and a bunch of people who earn their living helping them
protect those copyrights.

EOM
if (AUTOFORMAT) {
   my $width = 72;
   $data = autoformat $data, { right=>$width } ;
   print "yes\n";
   print $data;
}
else { print "no\n"; }



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

--
Kim Holburn
IT Network & Security Consultant
Ph: +39 06 855 4294  M: +39 3494957443
mailto:kim at holburn.net  aim://kimholburn
skype://kholburn - PGP Public Key on request

Democracy imposed from without is the severest form of tyranny.
                           -- Lloyd Biggle, Jr. Analog, Apr 1961



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