=begin pod

=TITLE Modules

=SUBTITLE How to create, use and distribute Perl 6 modules

=head1 Creating and Using Modules

A module is usually a source file or set of source filesN<Technically
a module is a set of I<compunits> which are usually files but could
come from anywhere as long as there is a I<compunit repository> that
can provide it. See L<S11|https://design.perl6.org/S11.html>.> that
expose Perl 6 constructs. These are typically packages
(L<classes|/language/objects#Classes>,
L<roles|/language/objects#Roles>, L<grammars|Grammar>),
L<subroutines|/language/functions>, and sometimes
L<variables|/language/variables>. In Perl 6 I<module> can also refer
to a type of package declared with the C<module> keyword (see example
below) but here we mostly mean "module" as a set of source
files in a namespace.

=head2 Basic Structure

Module distributions (in the I<set of related source files> sense) in Perl 6
have the same structure as any distribution in the Perl family of languages:
there is a main project directory containing a C<README> and a C<LICENSE> file,
a C<lib> directory for the source filesE<mdash>which may be individually
referred to as modules and/or may themselves define modules with the C<module>
keywordN<As L<synopsis S11|https://design.perl6.org/S11.html#Units> says:
Confusing? Yes it is.>, a C<t> directory for tests, and possibly a C<bin>
directory for executable programs and scripts.

Source files generally use the standard C<.pm> extension, and scripts or
executables use C<.pl>.  However, if you wish to highlight that the file is
written in Perl 6 you can use the C<.pm6> extension for modules, and the
C<.p6> extension for scripts.  Test files still use the normal C<.t>
extension.

=head2 Loading and Basic Importing

Loading a module makes the packages in the same namespace declared
within available in the file scope of the loader. Importing from a
module makes the symbols exported available in the lexical scope of
the importing statement.

=head3 X<need|compunit>

C<need> loads a C<compunit> at compile time.

    need MyModule;

Any packages in the namespace defined within will also be available.

    # MyModule.pm
    unit module MyModule;

    class MyModule::Class { ... }

C<MyModule::Class> will be defined when C<MyModule> is loaded.

=comment class Class { ... } won't get exported automatically on loading...not sure if bug or..

=head3 X<use|compunit>

C<use> loads and then imports from a compunit at compile time.

    use MyModule;

It is equivalent to:

    =begin code :allow<L>
    L<need|/language/modules#need> MyModule;
    import MyModule;
    =end code

=head3 X<require|compunit>

C<require> loads a compunit and imports definite symbols at runtime.

    sub load-mymodule {
       say "loading MyModule";
       require MyModule;
    }

    load-mymodule();

The compunit name can be in a runtime variable if you put it inside an
indirect lookup.

    sub load-a-module($name){
       require ::($name);
    }

    load-a-module('MyModule');

To import symbols you must define them at compile time.

    sub do-something {
       require MyModule <&something>;
       something() # &something will be defined here
    }

    do-something();
    # &something will not be defined here


If C<MyModule> doesn't export C<&something> then it will fail.

=head2 Exporting and Selective Importing

=head3 is export

Packages, subroutines, variables, constants and enums are exported by marking
them with the L<is export> trait.

    =begin code
    unit module MyModule;
    our $var is export = 3;
    sub foo is export { ... };
    constant FOO is export = "foobar";
    enum FooBar is export <one two three>;

    # Packages like classes can be exported too
    class MyClass is export {};

    # If a subpackage is in the namespace of the current package
    # it doesn't need to be explicitly exported
    class MyModule::MyClass {};
    =end code

As with all traits, if applied to a routine, "is export" should appear after
any argument list.

    =begin code
    sub foo(Str $string) is export { ... }
    =end code

You can pass named parameters to C<is export> to group symbols for exporting
so that the importer can pick and choose. There are three predefined
tags: C<ALL>, C<DEFAULT> and C<MANDATORY>.

    =begin code
    # lib/MyModule.pm
    unit module MyModule;
    sub bag        is export             { ... }
    sub pants      is export(:MANDATORY) { ... }
    sub sunglasses is export(:day)       { ... }
    sub torch      is export(:night)     { ... }
    sub underpants is export(:ALL)       { ... }
    =end code

    =begin code
    # main.pl
    use lib 'lib';
    use MyModule;          # bag, pants
    use MyModule :DEFAULT; # the same
    use MyModule :day;     # pants, sunglasses
    use MyModule :night;   # pants, torch
    use MyModule :ALL;     # bag, pants, sunglasses, torch, underpants
    =end code

Notes:

1. The C<:MANDATORY> tag on an exported sub ensures it will be exported
no matter whether the using program adds any tag or not.

2. All exported subs without an explicit tag are implicitly C<:DEFAULT>.

3. The space after the module name and before the tag is mandatory.

4. Multiple tags may be used (separated by commas).  For example:

    =begin code
    # main.pl
    use lib 'lib';
    use MyModule :day, :night; # pants, sunglasses, torch
    =end code

=head3 UNIT::EXPORT::*

Beneath the surface, C<is export> is adding the symbols to a C<UNIT>
scoped package in the C<EXPORT> namespace. For example, C<is
export(:FOO)> will add the target to the C<UNIT::EXPORT::FOO>
package. This is what Perl 6 is really using to decide what to import.

    =begin code
    unit module MyModule;

    sub foo is export { ... }
    sub bar is export(:other) { ... }
    =end code

Is the same as:

    =begin code
    unit module MyModule;

    my package EXPORT::DEFAULT {
        our sub foo { ... }
    }

    my package EXPORT::other {
        our sub bar { ... }
    }
    =end code

For most purposes C<is export> is sufficient but the C<EXPORT>
packages are useful when you want to produce the exported symbols
dynamically. For example:

    =begin code
    # lib/MyModule.pm
    unit module MyModule;

    my package EXPORT::DEFAULT {
       for <zero one two three four>.kv -> $number, $name {
          for <sqrt log> -> $func {
             OUR::{'&' ~ $func ~ '-of-' ~ $name } := sub { $number."$func"() };
          }
       }
    }

    =end code

    =begin code
    # main.pl
    use MyModule;
    say sqrt-of-four; #-> 2
    say log-of-zero;  #-> -Inf
    =end code

=head3 EXPORT
X<|sub EXPORT>

You can export arbitrary symbols with an C<EXPORT> sub. C<EXPORT>
must return a L<Map>, where the keys are the symbol names and
the values are the desired values. The names should include the sigil
(if any) for the associated type.

    =begin code
    # lib/MyModule.pm

    class MyModule::Class { ... }

    sub EXPORT {
        {
         '$var'   => 'one',
         '@array' => <one two three>,
         '%hash'  => { one => 'two', three => 'four' },
         '&doit'   => sub { ... },
         'ShortName' => MyModule::Class
        }
    }
    =end code

    =begin code
    # main.pl
    use lib 'lib';
    use MyModule;
    say $var;
    say @array;
    say %hash;
    doit();
    say ShortName.new; #-> MyModule::Class.new
    =end code

Note, C<EXPORT> can't be declared inside a package because
presently Rakudo (2015.09) seems to treat C<EXPORT> as part of the
compunit rather than the package.

Whereas C<UNIT::EXPORT> packages deal with the named parameters passed
to C<use>, the C<EXPORT> sub handles positional parameters. If you
pass positional parameters to C<use> they will be passed to
C<EXPORT>. If a positional is passed the module no longer exports
default symbols. You may still import them explicitly by
passing C<:DEFAULT> to C<use> along with your positional parameters.

    =begin code
    # lib/MyModule

    class MyModule::Class {}

    sub EXPORT($short_name?) {
        {
          do $short_name => MyModule::Class if $short_name
        }
    }

    sub always is export(:MANDATORY) { say "works" }

    #import with :ALL or :DEFAULT to get
    sub shy is export { say "you found me!" }
    =end code

    =begin code
    # main.pl
    use lib 'lib';
    use MyModule 'foo';
    say foo.new(); #MyModule::Class.new
    always();      #OK   - is imported
    shy();         #FAIL - won't be imported
    =end code

You can combine `EXPORT` with type captures for interesting
effect. This example creates a `?` postfix which will only work on
L<Cool>s.

    =begin code
    # lib/MakeQuestionable.pm
    sub EXPORT(::Questionable) {
        my multi postfix:<?>(Questionable $_) { .so };
        {
          '&postfix:<?>' => &postfix:<?>,
        }
    }
    =end code

    =begin code
    use MakeQuestionable Cool;
    say 0?, 1?, {}?, { a => "b" }?; # False True False True
    =end code

=head2 Introspection

To list exported symbols of a module first query the export tags supported by
the module.

    use URI::Escape;$
    dd URI::Escape::EXPORT::.keys;$
    # OUTPUT«("DEFAULT", "ALL").Seq»

Then use the tag you like and pick the symbol by it's name.

    dd URI::Escape::EXPORT::DEFAULT::.keys;
    # OUTPUT«("\&uri-escape", "\&uri_escape", "\&uri-unescape", "\&uri_unescape").Seq»
    my &escape-uri = URI::Escape::EXPORT::DEFAULT::<&uri_escape>;

=head2 Finding Modules

A user may have a collection of modules not found in the normal
ecosystem but needed regularly.  Instead of using the C<lib> pragma
one can use the C<PERL6LIB> environment variable to point to module
locations.  For example:

    export PERL6LIB=/path/to/my-modules,/path/to/more/modules

Note that the comma (',') is used as the directory separator (instead
of the colon (':') as with Perl 5 for C<PERL5LIB> or C<PERLLIB>).

=head1 Distributing Modules

If you've written a Perl 6 module and would like to share it with the
community, we'd be delighted to have it listed in the L<Perl 6 modules
directory|https://modules.perl6.org>. C<:)>

For now, the process requires that you use git for your module's version
control.

The instructions herein assume that you have a
L<GitHub|https://github.com> account so that your module can be
shared from its GitHub repository, however another provider such as
L<GitLab|https://about.gitlab.com/> should work as long as it works in
a similar way.

To share your module, do the following:

    =item Create a project directory named after your module. For
    example, if your module is C<Vortex::TotalPerspective>, then create a
    project directory named C<Vortex-TotalPerspective>. This project
    directory name will also be used as the GitHub repo name.

    =begin item
    Make your project directory look like this:

        Vortex-TotalPerspective/
        |-- lib
        |   `-- Vortex
        |       `-- TotalPerspective.pm
        |-- doc
        |   `-- Vortex
        |       `-- TotalPerspective.pod6
        |-- LICENSE
        |-- META6.json
        |-- README.md
        `-- t
            `-- basic.t

    If your project contains other modules that help the main module do
    its job, they should go in your lib directory like so:

        lib
        `-- Vortex
            |-- TotalPerspective.pm
            `-- TotalPerspective
                |-- FairyCake.pm
                `-- Gargravarr.pm
    =end item

    =item If you have any additional files (such as templates or a dynamic
    library) that you wish to have installed so you can access them at
    runtime, they should be placed in a C<resources> sub-directory of your project.

    =item The C<README.md> file is a L<markdown-formatted|https://help.github.com/articles/markdown-basics/>
    text file, which will later be automatically rendered as HTML by GitHub.

    =item Regarding the C<LICENSE> file, if you have no other preference,
    you might just use the same one that Rakudo Perl 6 uses. Just
    copy/paste the raw form of L<its license|https://github.com/rakudo/rakudo/blob/nom/LICENSE>
    into your own C<LICENSE> file.

    =item If you don't yet have any tests, you can leave out the C<t>
    directory and C<basic.t> file for now. For more info on how to write
    tests (for now), you might have a look at how other modules use
    C<Test>. It's quite similar to Perl 5's C<Test::More>.

    =item To document your modules, use L<Perl 6 Pod |
    https://design.perl6.org/S26.html> markup inside your modules. Module
    documentation is most appreciated and will be especially important once
    the Perl 6 module directory (or some other site) begins rendering Pod docs
    as HTML for easy browsing.
    N«
        Note, described above is a minimal project directory. If your project
        contains scripts that you'd like distributed along with your module(s),
        put them in a C<bin> directory. If you have extra docs (in addition to the
        Pod docs in your module(s)), create a C<doc> directory for them. If you'd
        like a graphical logo to appear next to your module at the module
        directory, create a C<logotype> directory and put into it a C<logo_32x32.png>
        file. At some point, you might also consider adding C<CONTRIBUTORS>, C<NEWS>,
        C<TODO>, or other files.
    »

    =begin item
    Make your C<META6.json> file look something like this:

    =for code :allow<R>
    {
        "perl" : "6.c",
        "name" : "Vortex::TotalPerspective",
        "version" : "0.1.0",
        "description" : "Wonderful simulation to get some perspective.",
        "authors" : [ "R<Your Name>" ],
        "provides" : {
            "Vortex::TotalPerspective" : "lib/Vortex/TotalPerspective.pm"
        },
        "depends" : [ ],
        "resources" : [ ],
        "source-url" : "git://github.com/R<you>/Vortex-TotalPerspective.git"
    }

    For choosing a version numbering scheme, perhaps use
    "major.minor.patch" (see L<the spec on versioning |
    https://design.perl6.org/S11.html#Versioning> for further
    details). If the version number doesn't matter to you or your users
    right now, you can just put in an asterisk (*) for the version.

    The C<authors> section includes a list of all the module authors. In
    the case there is only one author, a single element list must be
    supplied.

    In the C<provides> section, include all the namespaces provided by
    your distribution and that you wish to be installed, only module
    files that are explicitly included here will be installed and
    available with C<use> or C<require> in other programs.

    Set C<perl> version to the minimum perl version your module works with.

    The C<resources> section is optional, but, if present, should contain a
    list of the files in your C<resources> directory that you wish to be
    installed, these will be installed with hashed names alongside your
    library files and their installed location can be determined through the
    C<%?RESOURCES> Hash indexed on the name provided.

    The L<Test::META module | https://github.com/jonathanstowe/Test-META/>
    can help you check the correctness of the META6.json file.

    =end item

    =item Put your project under git version control if you haven't done so
    already.

    =item Once you're happy with your project, create a repo for it at GitHub.
    See L<GitHub's help docs|https://help.github.com/> if necessary. Your
    GitHub repo should be named the same as your project directory. Immediately
    after creating the GitHub repo, GitHub shows you how to configure your
    local repo to know about your GitHub repo.

    =item Push your project to GitHub.

    =item Consider setting up automated testing (see L<https://docs.travis-ci.com/user/languages/perl6>).

    =item Create a PR on L<ecosystem|https://github.com/perl6/ecosystem> adding
    your module to META.list, or ping someone on IRC (#perl6 at freenode) to
    get help having it added.

    =item After the pull request has been accepted, wait for an hour. If
    your module doesn't show up on L<https://modules.perl6.org/>, please
    view the log file at L<https://modules.perl6.org/update.log> to see
    if it identifies an error with your module or meta file.

B<That's it! Thanks for contributing to the Perl 6 community!>

If you'd like to try out installing your module, use the X<panda> module
installer tool which is included with Rakudo Star Perl 6:

=code panda install Vortex::TotalPerspective

This will download your module to its own working directory (C<~/.panda>),
build it there, and install the module into C<~/.perl6>.

To use C<Vortex::TotalPerspective> from your scripts, just write
C<use Vortex::TotalPerspective>, and your Perl 6 implementation will
know where to look for the module file(s).

=head1 Modules and Tools related to module authoring

You can find a list of modules and tools that aim to improve the experience of
writing/test modules at L<Modules Extra|/language/modules-extra>

=head1 The Future of Ecosystem

L<https://modules.perl6.org> and github-based infrastructure is temporary. The
plan is to establish something similar to Perl 5's PAUSE/CPAN/MetaCPAN
infrastructure. B<Volunteers needed!>

The rough plan is:

        1. fix EVAL precomp bug (nine)
        2. get Repository API straight
        3. get zef up to speed
        4. continue with the metacpan fork for perl6 (jdv79)

The repository with jdv's fork can be found at L<https://github.com/jdv/metacpan-web>

You can also already upload your Perl 6 modules to
L<Perl 5's PAUSE|https://pause.perl.org/>, selecting `Perl6` directory during the
upload. That will ensure your module is indexed in Perl 6's space and not Perl 5's.

=head2 Contact Information

To help out or learn more, join the dedicated channel
L<#perl6-toolchain on irc.freenode.net|irc://irc.freenode.net/#perl6-toolchain>
L<(channel logs)|https://irclog.perlgeek.de/perl6-toolchain/today>. A repository
to discuss tooling issues is also available at
L<https://github.com/perl6/toolchain-bikeshed>

=end pod

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