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RFC 233 (v1) Replace Exporter by a better scaling mechanism

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From:
Perl6 RFC Librarian
Date:
September 15, 2000 12:25
Subject:
RFC 233 (v1) Replace Exporter by a better scaling mechanism
Message ID:
20000915192500.20873.qmail@tmtowtdi.perl.org
This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

Replace Exporter by a better scaling mechanism

=head1 VERSION

  Maintainer: Ilya Zakharevich <ilya@math.ohio-state.edu>
  Date: 15 September 2000
  Mailing List: perl6-stdlib@perl.org
  Number: 233
  Version: 1
  Status: Developing

=head1 ABSTRACT

This RFC proposes a minimal efficient well-scaling mechanism for exporting.
Only export of subroutines and tags are supported by this mechanism.

=head1 DESCRIPTION

C<Exporter> gives an enormous efficiency hit for modules with a large
list of exportable symbols (but which are very small nevertheless, say,
because everything is autoloaded).  It is supposed to be optimized for
speed, with a lot of caching and keeping things close at hand.  However,
all these actions are in fact only slowing things down, since they create
a lot of code to compile, and a lot of data structures to maintain.

In many cases the Exporter-support data creates up to 90% of memory overhead
of the module (so may be supposed to provide a similar fraction of the load
time overhead).

Lessons learned: 

=over

=item *

instead of keeping a lot of arrays and a hash storing a lot of redundant data,
store the information in a string, and use RExen to extract it from
this string.

This provides a crucial advantage in the memory footprint, and may provide
speed advantages (maybe even if the module is imported from many times).

This string may contain items of two kinds: symbols, and tag lists of the
form C<":tag[ :subtag1 :subtag2 symbol1 symbol2 ]">.  If symbols appears
inside a tag, it need not be repeated again.  The default exporting list
should be provided the same as the list for the C<:DEFAULT> tag.

=item *

The symbols may be followed by the prototypes.  Example:

     ' :DEFAULT[:micro :mini additional_f(\%)] :micro[ func1($$) ]
       :mini[ func2($) ] :extra[ func3() ] func4($) '

Having the prototypes in this list may be useful, since the same list
may be used by the compile-time action of autoloading, reducing the
overhead yet more.  (See RFC on flexible autoloading.)

=item *

The export string should be registered as the argument to the pragma.
Example:

     use export q( func1($$) func2($) );	# No default

=item * 

The export list may be substituted by a subroutine reference which returns
such a string (cached to avoid such a call for future exporting).  Example:

     use export sub { local $/; my $l = <DATA>; eval 'use export $l'; $l };
     ...
     __DATA__
     func1($$)
     func2($)

This is desirable since the subroutine may be autoloaded (either explicitly,
or as above), thus reducing yet more the overhead of import-less

     use Module ();

=item *

export.pm should support only this barebone API.  Anything hairer
should be implicitly requested via a separate module (Export::Methods),
so that the overhead of export.pm is as small as possible.

=back

=head1 MIGRATION ISSUES

None.

=head1 IMPLEMENTATION

Straightforward.

=head1 REFERENCES

None.


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