use module question.

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

Replies are listed 'Best First'.
Re: use module question. by jeffa (Bishop) on Aug 04, 2005 at 15:33 UTC
use statements are always loaded first, regarless of what run time conditions you throw on them. For example: `if (0) { use Data::Dumper; } print Dumper [1..10];` [download] Try using require instead. But i would actually use an Object Oriented approach and instantiate the appropriate object instead. In fact, this is good case for using Class::Factory jeffa L-LL-L--L-LL-L--L-LL-L-- -R--R-RR-R--R-RR-R--R-RR B--B--B--B--B--B--B--B-- H---H---H---H---H---H--- (the triplet paradiddle with high-hat)	[reply] [d/l]
Re^2: use module question. by Miyah (Initiate) on Aug 04, 2005 at 17:04 UTC
Thanks everyone! `require` did the trick.	[reply] [d/l]
Re: use module question. by kwaping (Priest) on Aug 04, 2005 at 15:34 UTC
I think the discussion in How to do a conditional load of a module ? applies here. Enjoy!	[reply]
Re: use module question. by gellyfish (Monsignor) on Aug 04, 2005 at 15:38 UTC
You probably want something like: `if ($species eq "H") { require human; human->import(); } elsif ($species eq "M") { use mouse; mouse->import(); }` [download] The reason that what you had didn't work is that the `use` happens are compile time before the code is executed. You might want to consider using proper cased names for your modules as all lowercase names are conventionally reserved for pragmatic modules such as `strict` /J\	[reply] [d/l] [select]
Re: use module question. by Zaxo (Archbishop) on Aug 04, 2005 at 15:41 UTC
The if.pm module is useful for that, `use if $species eq 'H', 'human'; use if $species eq 'M', 'mouse';` [download] Make sure that $species is set at compile time (that's probably why it's being ignored). This looks like a natural place for inheritance, though. Let Human.pm and Mouse.pm inherit from Mammal.pm, which carries the common subroutines. After Compline, Zaxo	[reply] [d/l]
Re: use module question. by friedo (Prior) on Aug 04, 2005 at 16:00 UTC
The Monks above are all correct that using `require` or `use if` will allow you to load the module at runtime based on your conditional. But I think this is a perfect place for taking advantage of one of the most powerful OO features, polymorphism. `use Human; use Mouse; my %classes = ( H => 'Human', M => 'Mouse' ); my $species = get_species(); # returns 'H' or 'M' my $organism = $classes{$species}->new;` [download] This will allow you to instantiate a different object depending on whatever is returned by the `get_species` function. For more, take a look at perltoot.	[reply] [d/l]
Re: use module question. by blazar (Canon) on Aug 04, 2005 at 15:35 UTC
No, no, no! It may be confusing, but indeed `use` is executed at `BEGIN` time. Check perldoc -f require.	[reply] [d/l] [select]
Re: use module question. by jeteve (Pilgrim) on Aug 04, 2005 at 15:45 UTC
Hi ! Most dynamic way to do it: `my $spec = .... ; # H, M , etc... my $mod = "mod".$spec ; # Modules named modH , modM , etc.. eval "require $mod;"; if( $@ ){ die "Cannot load module $mod for specie $spec : $@"; } #Then, use your module function f1 , f2.. by: $mod->f1() or $mod::f2() etc...` [download] This way, anytime you have a new specie to handle you just have to write a new module, without modifying your using code. In general, avoid to construct case like structure. It makes your code hard to maintain and is a bit outdated. Dynamicity of perl is easy to write and very rewarding to use. Use it !! J. Nice photos of naked perl sources here !	[reply] [d/l]