Defining a subroutine one of two ways depending on whether a package exists

bikeNomad has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (subroutines)

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Re: Defining a subroutine one of two ways depending on whether a package exists⭐ by ariels (Curate) on Jun 08, 2001 at 11:01 UTC
If you'd like to separate the `use Package;` command from the test, you can still do it. Use the `%INC` hash (see perlvar). You need to look for a filename, though, not the package name: `eval { use Compress::Zlib; } # ... much other code, or in some other module... BEGIN { *myCRC = (exists $INC{'Compress/Zlib.pm'}) ? sub { crc32(@_) } : sub { 0 }; }` [download]	[reply] [d/l]
Re: Defining a subroutine one of two ways depending on whether a package exists by bikeNomad (Priest) on Jun 07, 2001 at 22:26 UTC
You can do something like this: `BEGIN { eval { require Compress::Zlib; import Compress::Zlib 'crc32'; }; myCRC = $@ ? sub { 0 } # don't have it : sub { crc32(@_) }; }` [download] This calls Compress::Zlib::crc32 or just returns 0. The way this works is: The BEGIN* block forces this to happen at the very beginning of the program. The eval tries to load the package (in this case Compress::Zlib) and import the symbol 'crc32' into the current package. If this process succeeds, the eval will leave a null string in $@. The *myCRC = $@ ? sub { } : sub { } part sets up the name 'myCRC' in the symbol table of the current package to refer to one of two subroutines that are defined anonymously here.	[reply] [d/l]
Re: Defining a subroutine one of two ways depending on whether a package exists by bikeNomad (Priest) on Jun 07, 2001 at 22:56 UTC
The way this works is: The BEGIN block forces this to happen at the very beginning of the program. The eval tries to load the package (in this case Compress::Zlib) and import the symbol 'crc32' into the current package. If this process succeeds, the eval will leave a null string in $@. The myCRC = $@ ? sub { } : sub { } part sets up the name 'myCRC' in the symbol table of the current package to refer to one of two subroutines that are defined anonymously here. Originally posted as a Categorized Answer.*	[reply]
Re: Defining a subroutine one of two ways depending on whether a package exists by bimleshsharma (Beadle) on Nov 16, 2011 at 06:45 UTC
You can do something like this: BEGIN { eval { require Compress::Zlib; import Compress::Zlib 'crc32'; }; myCRC = $@ ? sub { 0 }: sub { crc32(@_) }; } } Actually "eval" works like error handling and even after wrong in statment inside eval it does not die and thier returned result can be track using $@. Originally posted as a Categorized Answer.*	[reply]