NAME

Exporter::Proxy - Simplified symbol export & proxy dispatch.

SYNOPSIS

    package My::Module;

    use Exporter::Proxy qw( foo Bar );

    # at this point users of My::Module will get
    # *My::Module::foo and *My::Module::Bar 
    # installed.
    #
    # My::Module also gets an 'exports' method
    # that lists the exported items; array refs
    # are exported as copies by value.

    my @exported    = My::Module->exports;

    my $object      = My::Module->construct;

    my $exported    = $object->exports;

    package Some::Other;

    use My::Module qw( foo );   # only exports foo
    use My::Module qw( Bar );   # only exports Bar

    use My::Module qw( bar );   # croaks, 'bar' is not exported.

    # caller can specify the items to export by 
    # name -- not type. foo might be used as a
    # subroutine, Bar as an array, or foo may 
    # be overloaded with &foo, %foo, @foo, $foo.

    $value eq $Bar[0]
    or croak "Invalid '$value'";

    delete $foo{ somekey }
    or croak "Oops: foo is missing 'somekey'";

    my $bletch  = $foo || 'oops, no $foo';

    # if the caller does not want to import
    # anything from the module by default:

    use My::Module qw( :noexport );

    # there are times when it is easier to use
    # a dispatcher for things like service classes
    # than to pollute the caller's namespace with 
    # all of the available methods.

    use Exporter::Proxy qw( dispatch=do_something );

    # at this point 'do_something' is installed in 
    # My::Module. it splices out the second
    # argument, uses My::Module->can( $name ) to
    # check if the module has the service availble
    # and then dispatches to it via goto.
    #
    # My::Module->exports will include the dispatcher,
    # in the last example it will have only the
    # dispatcher since no other names were included.
    #
    # now modules use-ing this one look like:

    use My::Module;

    my $object  = My::Module->construct;

    $object->do_something( foo => @foo_args );

    my @test_these  = $object->exports;

    my $test_ref    = $objeect->exports;

    # @test_these will be qw( do_something )
    # $test_ref will be an arrayref of a 
    # copy of the exported values (i.e., 
    # modifying $test_ref does not affect
    # the exported items.

    # some modules may want to use their own import, pulling
    # in only exports and the dispatcher.

    package Foo::Bar;

    use Exporter::Proxy qw( import=0 );

    sub import
    {
        # left as-is, package has to implement its own
        # export utility for @exports.

        ...
    }

DESCRIPTION

This installs 'import' and 'exports' subroutines into the callers namespace. The 'import' does the usual deed: exporting symbols by name; 'exports' simplifies introspection by listing the exported symbols (useful for testing).

The optional "dispather=name" argument is used to install a dispatcher. This allows the module to offer a variety of services without polluting the caller's namespace with too many of them. All it does is check for $module->can( $name ) and goto &$handler if the module can handle the request.

Public Interface

import

The arguments to this are the symbol names to export with an optional "dispatch=<name>" or "import=<name>" for installing the dispatcher and extra import handler.

Installed methods

import

Import handles two optional directives: "dispatch" and "filter". The former installs a dispatch and adds its name to the exports, the latter allows pre- processing the arguments to import without having to write an entire import sub to override this one (see examples below).

With no arguments the import uses the original exports list, pushing all of the symbols into the caller's space.

The optional argument ':noexport' avoids exporting any symbols to the caller's space.

Other than ':noexport' any arguments with leading colons are silently ignored by import.

Anything without a leading colon is assumed to be a name, and is checked againsed the exports list. If it is on the list then the caller's $name symbol is aliased to the source module's.

Note that this is not a copy-by-value into the caller's space, it is aliaing via the symbol table.

i.e.,

    my $dest    = qualify_to_ref $name, $caller;
    my $src     = qualify_to_ref $name, $source;

    *$dest  = *$src;

Callers modifying their copy of the item will be modifiying a global copy.

Aside: Once read-only references are avaialble then they will be an option.

exports

Mainly for testing, calling:

    $module->exports;

or

    $object->exports

returns an array[ref] copy of the exported names.

dispatch=... (optional)

When exporting a large number of symbols is problematic, a dispatcher can be installed instead. This splices off the second argument, checks that the module can perform the name, and does a goto.

Calls to the dispatcher look like:

    $module->$dispatch( $name => @name_argz );

The dispatcher splices $name off the stack, checks that $module->can( $name ) (or $object can), croaks if it cannot or does a goto &$handler.

Note that the dispatcher can only be exported once: the last dispatch=name will be the only one installed.

For example:

    package Query::Services;

    use Exporter::Proxy qw( dispatch=query );

    sub lookup
    {
        ...
    }

    sub modify
    {
        ...
    }

    sub insert_returning
    {
        ...
    }

allows the caller to:

    use Query::Services;

    # caller now can 'query', which can dispatch
    # calls to lookup, modify, and insert_returning.

    __PACKAGE__->query( modify => $sql, @argz );

    $object->query( lookup => @lookup_argz );
prefilter

Where present this gets first crack at the stack via &$subref. This allows the caller to deal with any extra ":foo" arguments without having to write a complete import sub. Since Exporter::Proxy's import silently ignores anything with a leading colon (other than ":noexport") directives can be handled without haviing to munge the stack.

Normal use of this will be the usuall sorts of side-effects handled in import subs like reading config files or installing variables. This can also deal with any specialized directives by pushing standard lists of exports onto the stack.

    use Exporter::Proxy qw( prefilter=add_groups bim bam foo bar );

    sub bim{}
    sub bam{}

    sub foo{}
    sub bar{}

    sub add_groups
    {
        state $groupz =
        {
            default => [ qw( bim bam ) ],
            others  => [ qw( foo bar ) ],
        };

        for my $i ( 0 .. $#_ )
        {
            my ( $name ) = $_[$i] =~ m{^ : (\w+) }x
            or next;

            my $altz    = $groupz->{ $name }
            or die "Bogus group: '$name' unknown";

            splice @_, $i, 1, @$altz;

            redo
        }
    }

The prefilter can consume or munge the stack as necessary. Whatever is left will be treated as names for export.

    use Exporter::Proxy qw( prefilter=read_config bim bam foo bar );

    sub read_config
    {
        for my $i ( 0 .. $#_ )
        {
            my ( $path ) = $_[$i] =~ m{^ : (\w+) }x
            or next;

            splice @_, $i, 1;

            -e $path    or die "Missing config: '$path'";

            do_something_with $path;

            redo
        }
    }

Aside: In later versions of perl

    while( my ( $i, $val ) = each @_ ){ ... }

makes it easier to get $i and the value.

The postfilter is called as a final step in the import sub via goto &$post_handler. It cannot affect the stack used by import but will have access to caller() for determining the calling class. THi can be usefuil for cases where some information is managed by class such as default files or per-package counts:

    use Exporter::Proxy qw( postfilter=store_exports ... );

    sub store_exports
    {
        # this returns the package calling import

        my $caller  = caller;

        $info{ $caller } = ...
    }

A more general use of this is combining a number of service classes with a single 'dispatcher' class that users others. In this case various separate My::Query::* modules help break up what would otherwise be a monstrosity into manageable chunks. They can use fairly short names that are obvious in context becuase the names only propagate up to My::Query.

My::Query can even use "if" to limit the number of services available (e.g., only packages that already have an 'IsSafe' method have the modify calls available).

        package My::Query::Handle;
    
        use Exporter::Proxy 
        qw
        (
            connect
            prepare
            disconnect
            fetch
            non_fetch
            insert_returning
        );
    
        # implementations...
    
    
        package My::Query::Lookup
    
        use Exporter::Proxy
        qw
        (
            lookup
            single_vale
        );
    
        ...
    
        package My::Query::Modify
    
        use Exporter::Proxy
        qw
        (
            insert
            insert_returning
            update
        );
    
        ...
    
        # all this needs is to install a dispatcher
        # and pull in the modules that implement the
        # methods it dispatches into.
    
        package My::Query;
    
        use Exporter::Proxy qw( dispatch=query );
    
        use My::Query::Handle;
        use My::Query::Lookup;
    
        use if $::can_modify, 'My::Query::Modify';
    
        __END__
    
    
        # the object class use-ing My::Query gets a
        # "query" method without having its namespace
        # polluted with "insert", "modify", etc.
    
        use My::Query;
    
        ...
    
        $object->query( lookup => $sql, @valz );

Simple Test

The exports method provides a simple technique for baseline testing of modules: check that they can be used and actually can do what they've claimed to export.

Say your tests are standardized as '00-Module-Name-Here.t'.

    use Test::More;
    use File::Basename;

    # whatever your naming convention is, 
    # munge it into a package name.

    my $madness = basename $0, '.t';

    $madness    =~ s/^ \d+ - //;
    $madness    =~ s/-/::/g;

    use_ok $madness;

    my @methodz = 
    (
        qw
        (
            import
            exports
        ),
        $madness->exports
    );

    ok $madness->can( $_ ), "$madness can '$_'"
    for @methodz;

    done_testing;

    __END__

Symlink this to whatever modules you need testing and "prove t/00*.t" will give a quick, standard first pass as to whether they compile and are minimally usable.

SEE ALSO

Symbol

Used to export symbols w/o turning off strict.

AUTHOR

Steven Lembark <lembark@wrkhors.com>

LICENSE

Copyright (C) 2009-2016 Workhorse Computing. This code is released under the same terms as Perl 5.22, or any later version of Perl, itself.