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Think about Loose Coupling


by tedv (Pilgrim)
on Dec 13, 2000 at 02:24 UTC ( #46338=note: print w/replies, xml ) Need Help??

in reply to Re: Module Submission for CPAN
in thread Module Submission for CPAN

To be honest, I don't understand a lot of how module exporting and importing works-- I just know use and require. Is there a brief tutorial on this exporting stuff, or is it explainable in a paragraph or so? I've read through the Camel Book's section on exporting, but I just got a syntatical feel, not a feel for the purpose behind it or how it's best used.


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Re: Exporting
by chipmunk (Parson) on Dec 13, 2000 at 02:57 UTC
    Here's a quick, off-the-cuff explanation of exporting/importing:

    First, exporting and importing are the same thing; like emigrating/immigrating, it just depends which side you're looking from. :)

    Exporting/importing is simply the process of making a link from a symbol (or part of a symbol) in one package to a symbol in another package, so that the symbol can be accessed from either package. For example, if you import CPAN's install method, then CPAN exports it (by using Exporter) with this assignment: (*{"${callpkg}::$sym"} = \&{"${pkg}::$sym"}. That actually makes an assignment to the symbol table for the calling package. Assuming $callpkg is main, now main::install and CPAN::install refer to the same subroutine. The \& on the RHS of the assignment means that only the subroutine part of the CPAN::install symbol was duplicated; the scalar, array, hash, and other parts were not. Exporter does separate assignments for each data type, so that only what was asked for is imported.

    The reason for exporting/importing is simply to make accessing symbols in other packages easier, by bringing them into your package. Someone using the functional interface to CGI, for example, would have to write out CGI::header(), CGI::start_html(), CGI::param(), etc., for every function, if they couldn't import the methods.

Re: Exporting
by cwest (Friar) on Dec 13, 2000 at 02:37 UTC
    Read the Exporter manpage... here's a simple example:
    package Ctweten::Export; use Exporter; our @ISA = qw(Exporter); # we now inherit from Exporter, namely the im +port() method our @EXPORT_OK = qw(this that); # you must ask for these: use Ctweten: +:Export 'this'; our @EXPORT = qw(other); # exporter by default sub this { print "Hello\n" } *that = *other = *this;
    In a program, if you want to use this() and other(), you do the following: <code> #!/usr/local/bin/perl -w use strict; $|++; use Ctweten::Export qw(this); other; this; that; # this would die under these circumstances.
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