NAME
    CXC::Exporter::Util - Tagged Based Exporting

VERSION
    version 0.13

SYNOPSIS
    In the exporting code:

      package My::Exporter;
      use CXC::Exporter::Util ':constants';

      use parent 'Exporter' # or Exporter::Tiny

      # install sets of constants, with automatically generated
      # enumerating functions
      install_CONSTANTS( {
            DETECTORS => {
                ACIS => 'ACIS',
                HRC  => 'HRC',
            },

            AGGREGATES => {
                ALL  => 'all',
                NONE => 'none',
                ANY  => 'any',
            },
        } );

      # install some functions
      install_EXPORTS(
                { fruit => [ 'tomato', 'apple' ],
                  nut   => [ 'almond', 'walnut' ],
                } );

    In importing code:

      use My::Exporter   # import ...
       ':fruit',         # the 'fruit' functions
       ':detector',      # the 'detector' functions
       'DETECTORS'       # the function enumerating the DETECTORS constants  values
       'DETECTORS_NAMES' # the function enumerating the DETECTORS constants' names
       ;

      # print the DETECTORS constants' values;
      say $_ for DETECTORS;

      # print the DETECTORS constants' names;
      say $_ for DETECTORS_NAMES;

DESCRIPTION
    "CXC::Exporter::Util" provides *tag-centric* utilities for modules which
    export symbols. It doesn't provide exporting services; its sole purpose
    is to manipulate the data structures used by exporting modules which
    follow the API provided by Perl's core Exporter module (e.g.
    Exporter::Tiny).

    In particular, it treats %EXPORT_TAGS as the definitive source for
    information about exportable symbols and uses it to generate @EXPORT_OK
    and @EXPORT. Consolidation of symbol information in one place avoids
    errors of omission.

  Exporting Symbols
    At it simplest, the exporting module calls "install_EXPORTS" with a hash
    specifying tags and their symbols sets, e.g.,

      package My::Exporter;
      use CXC::Exporter::Util;

      use parent 'Exporter'; # or your favorite compatible exporter

      install_EXPORTS(
                { fruit => [ 'tomato', 'apple' ],
                  nut   => [ 'almond', 'walnut' ],
                } );

      sub tomato {...}
      sub apple  {...}
      sub almond {...}
      sub walnut {...}

    An importing module could use this via

      use My::ExportingModule ':fruit'; # import tomato, apple
      use My::ExportingModule ':nut';   # import almond, walnut
      use My::ExportingModule ':all';   # import tomato, apple,
                                        #        almond, walnut,

    For more complicated setups, %EXPORT_TAGS may be specified first:

      package My::ExportingModule;
      use CXC::Exporter::Util;

      use parent 'Exporter';
      our %EXPORT_TAGS = ( tag => [ 'Symbol1', 'Symbol2' ] );
      install_EXPORTS;

    "install_EXPORTS" may be called multiple times

  Exporting Constants
    "CXC::Exporter::Util" provides additional support for creating,
    organizing and installing constants via "install_CONSTANTS". Constants
    are created via Perl's constant pragma.

    "install_CONSTANTS" is passed sets of constants grouped by tags, e.g.:

      use CXC::Export::Util ':constants';

      install_CONSTANTS( {
            DETECTORS => {
                ACIS => 'ACIS',
                HRC  => 'HRC',
            },

            AGGREGATES => {
                ALL  => 'all',
                NONE => 'none',
                ANY  => 'any',
            },
       });

       # A call to install_EXPORTS (with or without arguments) must follow
       # install_CONSTANTS;
       install_EXPORTS;

    This results in the definition of

    *   the constant functions, i.e.,

          ACIS HRC ALL NONE ANY

        returning their specified values,

    *   functions enumerating the constants' values, i.e.

          DETECTORS -> ( 'ACIS', 'HRC' )
          AGGGREGATES -> ( 'all', 'none', 'any' )

    *   functions enumerating the constants' names, i.e.

          DETECTORS_NAMES -> ( 'ACIS', 'HRC' )
          AGGGREGATES_NAMES -> ( 'ALL', 'NONE', 'ANY' )

    The enumerating functions are useful for generating enumerated types via
    e.g. Type::Tiny:

      Enum[ DETECTORS ]

    or iterating:

      say $_ for DETECTORS;

    "install_CONSTANTS" may be called multiple times. If the constants are
    used later in the module for other purposes, constant definition should
    be done in a BEGIN block:

      BEGIN {
          install_CONSTANTS( {
              CCD => {nCCDColumns  => 1024, minCCDColumn => 0,},
          } );
      }

      install_CONSTANTS( {
          CCD => {
              maxCCDColumn => minCCDColumn + nCCDColumns - 1,
          } }
      );

      install_EXPORTS;

    For more complex situations, the lower level "install_constant_tag" and
    "install_constant_func" routines may be useful.

  UI Helpers
    *Note*: These subroutines are available only if Exporter::Tiny is
    installed.

    The UI Helper routines make it easier to expose constants' names to
    users and convert them to constant values. For example, say the
    constants are for colors,

       install_CONSTANTS( { COLORS =>
                              { RED => 0xFF0000, YELLOW_GREEN => 0x00FFFF } } );

    and the application provides a "--color" command line option. The
    application can accept lower-case, user-friendly representations of the
    constant names and convert them back to constant values.

    Each helper takes the constant tag name (as used by "install_CONSTANTS"
    or "install_constant_tag") and an optional $prefix, which strips a
    common literal constant-name prefix before aliases are generated. For
    example, with a prefix of "COLOR", "COLOR_YELLOW_GREEN" is exposed as
    "yellow_green" and "yellow-green".

    "ui_list_constants" returns a list of accepted human-friendly aliases
    for the constants' names.

    The UI can use these to provide the user with values for "--color", e.g.

       if ( $opt{color} eq '-list' ) {
         say join( "\n", ui_list_constants( 'colors' ) );
         exit (0);
       }

    or as parameters for Type::Tiny::Enum.

    "ui_coerce_constant" and "ui_assert_coerce_constant" match the passed
    constant name alias to the constant value. They differ in that
    "ui_coerce_constant" returns the original passed name if there is no
    match, while "ui_assert_coerce_constant" throws an exception.

    To make these helpers available to users of a constants module, import
    the "ui_helpers" tag from "CXC::Exporter::Util" into that module:

      package My::Constants;
      use parent 'Exporter::Tiny';
      use CXC::Exporter::Util ':constants', ':ui_helpers';

      install_CONSTANTS( { COLORS =>
                             { RED => 0xFF0000, YELLOW_GREEN => 0x00FFFF } } );
      install_EXPORTS;

    This adds a "ui_helpers" export tag to "My::Constants". Users of
    "My::Constants" may then import all of the UI helpers with

      use My::Constants ':ui_helpers';

    or import them individually.

SUBROUTINES
  install_EXPORTS
      install_EXPORTS( [\%export_tags], [$package], [\%options]  );

    Populate $package's @EXPORT and @EXPORT_OK arrays based upon
    %EXPORT_TAGS and %export_tags.

    If not specified, $package defaults to the caller's package.

    Available Options:

    overwrite => [Boolean]
        If the "overwrite" option is true, the contents of %export_tags will
        overwrite %EXPORT_TAGS in $package, otherwise %export_tags is merged
        into %EXPORT_TAGS.

        Note that overwriting will remove the tags and symbols installed
        into %EXPORT_TAGS by previous calls to "install_CONSTANTS".

        This defaults to false.

    package => [Package Name]
        This provides another means of indicating which package to install
        into. Setting this overrides the optional $package argument.

    all => [Boolean | 'auto' ]
        This determines whether "install_EXPORTS" creates an "all" tag based
        on the contents of %EXPORT_TAGS in $package. Some exporters, such as
        Exporter::Tiny and Sub::Exporter automatically handle the "all" tag,
        but Perl's default Exporter does not.

        If set to "auto" (the default), it will install the "all" tag if
        $package is *not* a subclass of Exporter::Tiny.

        (At present I don't know how to determine if Sub::Exporter is used).

    This routine does the following in $package based upon the contents of
    $package's %EXPORT_TAGS hash:

    *   Copy the symbol names in $EXPORT_TAGS{default} into @EXPORT.

    *   Copy the symbol names in all of %EXPORT_TAGS into @EXPORT_OK.

    *   Create an "all" tag which includes all of the symbols in
        %EXPORT_TAGS.

    *   Create a "ui_helper" tag if requested (See "UI Helpers").

  install_CONSTANTS
      install_CONSTANTS( @specs, ?$package  );

    Create sets of constants and make them available for export in $package.

    If not specified, $package defaults to the caller's package.

    The passed @specs arguments are either hashrefs or arrayrefs and contain
    one or more set specifications. A set specification consists of a unique
    identifier and a list of name-value pairs, specified either as a hash or
    an array. For example,

      @spec = ( { $id1 => \%set1, $id2 => \@set2 },
                [ $id3 => \%set3, $id4 => \@set4 ],
              );

    The identifier is used to create an export tag for the set, as well as
    to name enumerating functions returning constant's names and values. The
    individual $id, $set pairs are passed to "install_constant_tag"; see
    that function for more information on how the identifiers are used.

    A call to "install_EXPORTS" *must* be made after the last call to
    "install_CONSTANTS" or

    *   The constants won't be added to the exports.

    *   The enumerating functions won't be created.

    "install_CONSTANTS" may be called more than once to add symbols to a
    tag, but don't split those calls across a call to "install_EXPORTS".

    In other words,

      # DON'T DO THIS, IT'LL THROW
      install_CONSTANTS( { Foo => { bar => 1 } } );
      install_EXPORTS;
      install_CONSTANTS( { Foo => { baz => 1 } } );
      install_EXPORTS;

      # DO THIS
      install_CONSTANTS( { Foo => { bar => 1 } } );
      install_CONSTANTS( { Foo => { baz => 1 } } );
      install_EXPORTS;

    Each call to "install_EXPORTS" installs the enumerating functions for
    sets modified since the last call to it, and each enumerating function
    can only be added once.

  install_constant_tag
    Create and install constant functions for a set of constants. Called
    either as

    install_constant_tag( \@names, $constants, [$package] )
        where @names contains

          $tag,
          $fn_values, # optional name of function returning constants' values
          $fn_names,  # optional name of function returning constants' names

        and, if not specified,

          $fn_values //= uc($tag);
          $fn_names //= $fn_values . '_NAMES';

    install_constant_tag( $fn_values, $constants, [$package] )
        where

          $tag = lc($fn_values);
          $fn_names = $fn_values . '_NAMES';

    where

    $tag
        is the name of the tag representing the set of constants

    $fn_values
        is the name of the function which will return a list of the
        constants' values

    $fn_names
        is the name of the function which will return a list of the
        constants' names

    $constants
        specifies the constants' names and values, as either a hashref or an
        arrayref containing *name* - *value* pairs.

    $package
        is the name of the package (the eventual exporter) into which the
        constants will be installed. It defaults to the package of the
        caller.

    "install_constant_tag" will

    1   use Perl's constant pragma to create a function named *name*
        returning *value* for each *name*-*value* pair in $constants.

        The functions are installed in $package and their names appended to
        the symbols in %EXPORT_TAGS with export tag $tag. If $constants is
        an arrayref they are appended in the ordered specified in the array,
        otherwise they are appended in random order.

    2   Add a hook so that the next time "install_EXPORTS" is called, Perl's
        constant pragma will be used to create

        *   an enumerating function named $fn_values which returns a list of
            the *values* of the constants associated with $tag, in the order
            they were added to $EXPORT_TAGS{$tag}.

        *   an enumerating function named $fn_names which returns a list of
            the *names* of the constants associated with $tag, in the order
            they were added to $EXPORT_TAGS{$tag}.

        These enumerating functions are added to the symbols in %EXPORT_TAGS
        tagged with "contant_funcs".

        Just as you shouldn't interleave calls to "install_CONSTANTS" for a
        single tag with calls to "install_EXPORTS", don't interleave calls
        to "install_constant_tag" with calls to "install_EXPORTS".

    For example, after

      $id = 'AGGREGATES';
      $constants = { ALL => 'all', NONE => 'none', ANY => 'any' };
      install_constant_tag( $id, $constants );
      install_EXPORTS:

    1   The constant functions, "ALL", "NONE", "ANY" will be created and
        installed in the calling package.

        A new element will be added to %EXPORT_TAGS with an export tag of
        "aggregates".

          $EXPORT_TAGS{aggregates} = [ 'ALL', 'NONE', 'ANY ];

    2   A function named "AGGREGATES" will be created and installed in the
        calling package. "AGGREGATES" will return the values

          'all', 'none', 'any'

        (in a random order, as $constants is a hashref).

        "AGGREGATES" will be added to the symbols tagged by "constant_funcs"
        in %EXPORT_TAGS

    3   A function named "AGGREGATES_NAMES" will be created and installed in
        the calling package. "AGGREGATES_NAMES" will return the values

          'ALL', 'NONE', 'ANY'

        (in a random order, as $constants is a hashref).

        "AGGREGATES_NAMES" will be added to the symbols tagged by
        "constant_name_funcs" in %EXPORT_TAGS

    After this, a package importing from $package can

    *   import the constant functions "ALL", "NONE", "ANY" via the
        "aggregate" tag:

           use Package ':aggregate';

    *   import the enumerating function "AGGREGATES" directly, via

          use Package 'AGGREGATES';

    *   import "AGGREGATES" via the "constant_funcs" tag:

          use Package ':constant_funcs';

    As mentioned above, if the first argument to "install_constant_tag" is
    an arrayref, $tag, $fn_values, and $fn_names may be specified directly.
    For example,

      $id = [ 'Critters', 'Animals', 'Animal_names' ];
      $constants = { HORSE => 'horse', GOAT   => 'goat' };
      install_constant_tag( $id, $constants );

    will create the export tag "Critters" for the "GOAT" and "HORSE"
    constant functions, an enumerating function called "Animals" returning

      ( 'horse', 'goat' )

    and a function called "Animal_names" returning

      ( 'HORSE', 'GOAT')

    "install_constant_tag" uses "install_constant_func" to create and
    install the constant functions which return the constant values.

    Because of when enumerating functions are created, all enumerating
    functions associated with a set will return all of the set's values,
    regardless of when the function was specified. For example,

      install_constant_tag( 'TAG', { HIGH => 'high' }  );
      install_constant_tag( [ 'TAG', 'ATAG' ], { LOW => 'low' } );

    will create functions "TAG" and "ATAG" which both return "high", "low".

  install_constant_func( $name, \@values, $caller )
    This routine does the following in $package, which defaults to the
    caller's package.

    1   Create a constant subroutine named $name which returns @values;

    2   Adds $name to the "constant_funcs" tag in %EXPORT_TAGS.

    For example, after calling

      install_constant_func( 'AGGREGATES', [ 'all', 'none', 'any' ]  );

    1   The function "AGGREGATES" will return "all", "none", "any".

    2   A package importing from $package can import the "AGGREGATE"
        constant function via the "constant_funcs" tag:

          use Package ':constant_funcs';

        or directly

          use Package 'AGGREGATES';

  ui_list_constants
      @names = ui_list_constants( $tag, ?$prefix );

    Returns a list of accepted names for the constants with the given $tag.
    These names are lower-case alternatives with underscores preserved and
    with underscores translated to hyphens. The optional $prefix strips a
    common literal constant-name prefix before aliases are generated.

  ui_coerce_constant
      $value = ui_coerce_constant( $name, $tag, ?$prefix );

    Maps an accepted constant name alias (see "ui_list_constants") to the
    constant's value, returning unknown values unchanged.

  ui_assert_coerce_constant
      $value = ui_assert_coerce_constant( $name, $tag, ?$prefix );

    Performs the same coercion as "ui_coerce_constant", but throws an
    exception for unknown values.

EXPORTS
    CXC::Exporter::Util supports the following export tags

    all *DEPRECATED*
          install_EXPORTS
          install_CONSTANTS
          install_constant_tag
          install_constant_func

        Use of the "all" tag is deprecated. Due to backwards compatibility
        issues, it does not actually import all of CXC::Exporter::Util's
        symbols. Use the other tags instead.

    default, exports
          install_EXPORTS

        These are synonymous:

          use CXC::Exporter::Util;
          use CXC::Exporter::Util ':default';
          use CXC::Exporter::Util ':exports';

    constants
          install_EXPORTS
          install_CONSTANTS

    utils
          install_constant_tag
          install_constant_func

    ui_helpers
          ui_list_constants
          ui_coerce_constant
          ui_assert_coerce_constant

BUGS AND INCOMPATIBILITIES
    *   No attempt is made to complain if enumerating functions' names clash
        with constant function names.

    *   namespace::clean

        The "UI Helpers" routines are exported into your module, and are
        made available for import by users of your module when they request
        them directly or via the "ui_helpers" tag.

        When your module uses namespace::clean, it's important not to scrub
        the UI helpers from your module's namespace. For example,

          package Broken {
            use parent 'Exporter::Tiny';
            use CXC::Exporter::Util ':constants', ':ui_helpers';
            use namespace::clean;
            ...
          }

          package Use::Broken {
            use Foo ':ui_helpers';
          }

        will fail because the UI helpers have been removed from "Broken"'s
        namespace, while this works:

          package Working {
            use parent 'Exporter::Tiny';
            use CXC::Exporter::Util ':constants';
            use namespace::clean;
            use CXC::Exporter::Util ':ui_helpers';

            ...
          }

          package Use::Working {
            use Foo ':ui_helpers';
          }

EXAMPLES
    Alternate constant generation modules.
        To use an alternate constant generation function bypass
        "install_CONSTANTS" and load things manually.

        For example, using enum:

          package My::Exporter;

          use CXC::Exporter::Util ':all';

          our @DaysOfWeek;
          BEGIN{ @DaysOfWeek = qw( Sun Mon Tue Wed Thu Fri Sat ) }
          use enum @DaysOfWeek;
          use constant DaysOfWeek => map { &$_ } @DaysOfWeek;
          install_EXPORTS( { days_of_week => \@DaysOfWeek,
                             constant_funcs => [ 'DaysOfWeek' ],
                            });

        and then

          use My::Exporter -days_of_week;

          say Sun | Mon;

    Using a constant in the exporting module
        When a constant is used in an exporting module (to create another
        constant, for example), it's tempting to do something like this:

          # DON'T DO THIS
          %CCD = ( minCCDColumn => 0, nCCDColumns = 1024 );
          $CCD{maxCCDColumn} = $CCD{minCCDColumn} + $CCD{nCCDColumns} - 1;
          install_CONSTANTS( { CCD => \%CCD } );
          install_EXPORTS;

        Not only is this noisy code, if the hash keys are mistyped, there's
        an error, which is exactly what constants are supposed to avoid.

        Instead, create an initial set of constants in a BEGIN block, which
        will make them available for the rest of the code:

          BEGIN {
              install_CONSTANTS( {
                  CCD => {nCCDColumns  => 1024, minCCDColumn => 0,},
              } );
          }

          install_CONSTANTS( {
              CCD => {
                  maxCCDColumn => minCCDColumn + nCCDColumns - 1,
              } }
          );

          install_EXPORTS;

        A bit more verbose, but it uses the generated constant functions and
        avoids errors.

SUPPORT
  Bugs
    Please report any bugs or feature requests to
    bug-cxc-exporter-util@rt.cpan.org or through the web interface at:
    <https://rt.cpan.org/Public/Dist/Display.html?Name=CXC-Exporter-Util>

  Source
    Source is available at

      https://codeberg.org/CXC-Optics/p5-CXC-Exporter-Util

    and may be cloned from

      https://codeberg.org/CXC-Optics/p5-CXC-Exporter-Util.git

SEE ALSO
    Please see those modules/websites for more information related to this
    module.

    *   Exporter

    *   Exporter::Tiny

    *   Exporter::Almighty

AUTHOR
    Diab Jerius <djerius@cpan.org>

COPYRIGHT AND LICENSE
    This software is Copyright (c) 2022 by Smithsonian Astrophysical
    Observatory.

    This is free software, licensed under:

      The GNU General Public License, Version 3, June 2007

