Module::Features - Define features for modules




This document describes version 0.1.7 of Module::Features (from Perl distribution Module-Features), released on 2021-08-27.


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.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.


The 0.1.x version series is still unstable.


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 (the target module) is its own name sans the ::_ModuleFeatures suffix. For example, the module Text::Table::Tiny::_ModuleFeatures contains "features declaration" for Text::Table::Tiny.

The point of proxy module is to allow a different author declare features for a target module.

feature name

A non-empty 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.

See "Recommendation for feature name".

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.


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 = (

     # version number of the feature set. positive integer, begins at 1.
     # optional, default is 1 if unspecified. should be increased whenever
     # there's a backward-incompatible change in the feature set, i.e. when one
     # or more features are renamed, deleted, change meaning, or change the
     # schema in a backward-incompatible way (e.g. become more restricted or
     # change type). when a feature set changes in a backward-compatible wa
     # (e.g. a new feature is added, just the summary is revised, etc) then the
     # version number need not be increased.
     v => 1,

     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. see recommendation on feature name in this
         # specification.

         can_align_cell_containing_color_code => {
             # 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'

             tags => ['category:alignment'],
         can_align_cell_containing_newline => {
             summary => 'Whether the module can align cells that contain multiple lines of text',
             tags => ['category:alignment'],
         can_align_cell_containing_wide_character => {
             summary => 'Whether the module can align cells that contain wide Unicode characters',
             tags => ['category:alignment', 'category:unicode'],
         speed => {
             summary => 'The speed of the module, according to the author',
             schema => ['str', in=>['slow', 'medium', 'fast']],

Recommendations for feature name

Features should be written in lower case and words are separated by underscores, e.g. can_color, max_colors. The name should be self-explanatory when possible and should use English.

Singular noun is preferred (e.g can_align_cell_containing_wide_character instead of can_align_cells_containing_wide_characters) unless when is is grammatically required to be plurals, e.g. max_colors.

Abbreviations should be avoided unless when an abbrevation is common, e.g. require_filesystem is preferred over req_fs, but max_colors is okay.

Infinitive form of verb is preferred, e.g. require_filesystem_access instead of requires_filesystem_access.

Features that refer to whether a module has a specific ability should be named with can_ prefix. Examples: can_align_cell_containing_wide_character, can_color. These features have a bool value ("yes" or "no"). have_ or able_to_ prefix is not preferred.

Features that refer to whether a module needs (requires) a specific feature/resource to function should be named with require_ prefix. need_ prefix is not preferred. Examples: require_filesystem_access.

Features that refer to whether a module can optionally use or prefers something should be named with can_use_ prefix. prefer_ or want_ prefix is not preferred.

Features that specify an upper or lower limit of something should be named with max_ or min_ prefix. They typically have int/float/num schemas.

Recommendations for feature definer module

The distribution that ships a feature definer module should add a dependency (phase=develop, rel=x_spec) to Module::Features, to express that it follows the Module::Features specification.

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. specify the versions of the feature sets this declaration uses.
     # the version of feature set must match. versions defaults to 1 if
     # unspecified.
     #set_v => {TextTable => 1},

     # 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.
             can_align_cell_containing_color_code     => 1,
             can_align_cell_containing_wide_character => 1,
             can_align_cell_containing_newline        => 1,
             speed => {
                 value => 'slow', # if unspecified, value will become undef (which means N/A [not available])
                 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 => {
             can_align_cell_containing_color_code     => 0,
             can_align_cell_containing_wide_character => 0,
             can_align_cell_containing_newline        => 0,
             speed                                 => 'fast',

and in Text::Table::Any:

 our %FEATURES = (
     features => {
         TextTable => {
             can_align_cell_containing_color_code     => {value => undef, summary => 'Depends on the backend used'},
             can_align_cell_containing_wide_character => {value => undef, summary => 'Depends on the backend used'},
             can_align_cell_containing_newline        => {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:

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.

Recommendations for feature declarer module

The distribution that ships a feature declarer module should add a dependency (phase=develop, rel=x_spec) to Module::Features, to express that it follows the Module::Features specification. It should also add a dependency (phase=develop, x=features_from> to associated feature definer module(s), to express that it declares features defined in the associated feature definer module(s).

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.


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.


Please visit the project's homepage at


Source repository is at





perlancar <>


To contribute, you can send patches by email/via RT, or send pull requests on GitHub.

Most of the time, you don't need to build the distribution yourself. You can simply modify the code, then test via:

 % prove -l

If you want to build the distribution (e.g. to try to install it locally on your system), you can install Dist::Zilla, Dist::Zilla::PluginBundle::Author::PERLANCAR, and sometimes one or two other Dist::Zilla plugin and/or Pod::Weaver::Plugin. Any additional steps required beyond that are considered a bug and can be reported to me.


This software is copyright (c) 2021 by perlancar <>.

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


Please report any bugs or feature requests on the bugtracker website

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.