NAME
    Module::Features - Define features for modules

SPECIFICATION VERSION
    0.1

VERSION
    This document describes version 0.1.3 of Module::Features (from Perl
    distribution Module-Features), released on 2021-02-23.

DESCRIPTION
    This document specifies a very easy and lightweight way to define and
    declare features for modules. A definer module defines some features in
    a feature set, other modules declare these features that they have or
    don't have, and user can easily check and select modules based on
    features he/she wants.

SPECIFICATION STATUS
    The series 0.1.x version is still unstable.

GLOSSARY
  feature definer module
    A module in ""Module::Features::"*FeatureSetName*" namespace that
    contains "feature set specification". This module describes what each
    feature in the feature set means, what values are valid for the feature,
    and so on. A "feature declarer module" follows this specification and
    declares features.

  feature declarer module
    A regular Perl module that wants to declare some features defined by
    "feature definer module". Module name must not end with
    "::_ModuleFeatures", in which case it is a "feature declarer proxy
    module".

  feature declarer proxy module
    A module that declares features for another module. Module name must end
    with "::_ModuleFeatures" and the name of the module it delares features
    for is its own name sans the "::_ModuleFeatures" suffix. For example,
    the module Text::Table::Tiny::_ModuleFeatures contains "features
    declaration" for Text::Table::Tiny.

  feature name
    A string, preferably an identifier matching regex pattern /\A\w+\z/.

  feature value
    The value of a feature.

  feature specification
    A DefHash, containing the feature's summary, description, schema for
    value, and other things.

  feature set name
    A string following regular Perl namespace name, e.g. "JSON::Encoder" or
    "TextTable".

  feature set specification
    A collection of "feature name"s along with each feature's specification.

  features declaration
    A DefHash containing a list of feature set names and feature values for
    features of those feature sets.

SPECIFICATION
  Defining feature set
    A "feature definer module" specifies feature set by putting the "feature
    set specification" in %FEATURES_DEF package variable. Specifying feature
    set should not require any module dependency.

    For example, in Module::Features::TextTable:

     # a DefHash
     our %FEATURES_DEF = (
         summary => 'Features of a text table generator',
         description => <<'_',
     This feature set defines features of a text table generator. By declaring these
     features, the module author makes it easier for module users to choose an
     appropriate module.
     _
         features => {
             # each key is a feature name. each value is the feature's specification.

             align_cell_containing_color_codes => {
                 # a regular DefHash with common properties like 'summary',
                 # 'description', 'tags', etc. can also contain these properties:
                 # 'schema', 'req' (whether the feature must be declared by user
                 # module).

                 summary => 'Whether the module can align cells that contain ANSI color codes',
                 # schema => 'bool*', # Sah schema. if not specified, the default is 'bool*'
             },
             align_cell_containing_multiple_lines => {
                 summary => 'Whether the module can align cells that contain multiple lines of text',
             },
             align_cell_containing_wide_characters => {
                 summary => 'Whether the module can align cells that contain wide Unicode characters',
             },
             speed => {
                 summary => 'The speed of the module, according to the author',
                 schema => ['int', in=>['slow', 'medium', 'fast']],
             },
         },
     );

  Declaring features
    A "feature declarer module" declares features that it supports (or does
    not support) via putting the "features declaration" in %FEATURES package
    variable. Declaring features should not require any module dependency,
    but a helper module can be written to help check that declared feature
    sets and features are known and the feature values conform to defined
    schemas.

    Not all features from a feature set need to be declared by the feature
    declarer module. The undeclared features will have "undef" as their
    values for the declarer module. However, features defined as required
    ("req => 1" in the specification) MUST be declared.

    For example, in Text::Table::More:

     # a DefHash
     our %FEATURES = (
         # optional, specifies which module version this declaration pertains to
         #module_v => "0.002",

         # optional, a numeric value to be compared against other declarations for
         # the same module. recommended form is YYYYMMDD. for multiple serials in a
         # single day, you can use YYYYMMDD.1, YYYYMMDD.2, YYYYMMDD.91, and so on.
         #serial => 20210223,

         features => {
             # each key is a feature set name.
             TextTable => {
                 # each key is a feature name defined in the feature set. each value
                 # is either a feature value, or a DefHash that contains the feature
                 # value in the 'value' property, and notes in 'summary', and other
                 # things.
                 align_cell_containing_color_codes     => 1,
                 align_cell_containing_wide_characters => 1,
                 align_cell_containing_multiple_lines  => 1,
                 speed => {
                     value => 'slow', # if unspecified, value will become undef
                     summary => "It's certainly slower than Text::Table::Tiny, etc; and it can still be made faster after some optimization",
                 },
             },
         },
     );

    While in Text::Table::Sprintf:

     our %FEATURES = (
         features => {
             TextTable => {
                 align_cell_containing_color_codes     => 0,
                 align_cell_containing_wide_characters => 0,
                 align_cell_containing_multiple_lines  => 0,
                 speed                                 => 'fast',
             },
         },
     );

    and in Text::Table::Any:

     our %FEATURES = (
         features => {
             TextTable => {
                 align_cell_containing_color_codes     => {value => undef, summary => 'Depends on the backend used'},
                 align_cell_containing_wide_characters => {value => undef, summary => 'Depends on the backend used'},
                 align_cell_containing_multiple_lines  => {value => undef, summary => 'Depends on the backend used'},
                 speed                                 => {value => undef, summary => 'Depends on the backend used'},
             },
         },
     );

    Features declaration can also be put in other places:

    *   %FEATURES package variable in the "feature declarer proxy module"

    *   database

    *   others

    The %FEATURES package variable in the feature declarer module itself is
    considered to be authoritative, but other places can be checked first to
    avoid having to load the feature declarer module. When multiple features
    declaration exist, the "module_v" and/or "serial" can be used to find
    out which declaration is the most recent or suitable.

  Checking whether a module has a certain feature
    The user of a "feature declarer module" can check whether the module has
    a certain feature simply by checking the module's "features declaration"
    (%FEATURES). Checking features of a module should not require any module
    dependency.

    For example, to check whether Text::Table::Sprintf supports aligning
    cells that contain multiple lines:

     if (do { my $val = $Text::Table::Sprintf::FEATURES{features}{TextTable}{align_cell_containing_multiple_lines}; ref $val eq 'HASH' ? $val->{value} : $val }) {
         ...
     }

    A utility module can be written to help make this more convenient.

FAQ
  Why not roles?
    Role frameworks like Role::Tiny allow you to require a module to have
    certain subroutines, i.e. to follow some kind of interface. This can be
    used to achieve the same goal of defining and declaring features, by
    representing features as required subroutines and feature sets as roles.
    However, Module::Features wants declaring features to have negligible
    overhead, including no extra runtime dependency.

HOMEPAGE
    Please visit the project's homepage at
    <https://metacpan.org/release/Module-Features>.

SOURCE
    Source repository is at
    <https://github.com/perlancar/perl-Module-Features>.

BUGS
    Please report any bugs or feature requests on the bugtracker website
    <https://github.com/perlancar/perl-Module-Features/issues>

    When submitting a bug or request, please include a test-file or a patch
    to an existing test-file that illustrates the bug or desired feature.

SEE ALSO
    DefHash

    Sah

AUTHOR
    perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2021 by perlancar@cpan.org.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.

