NAME
    Hash::Subset - Produce subset of a hash

VERSION
    This document describes version 0.004 of Hash::Subset (from Perl
    distribution Hash-Subset), released on 2019-09-20.

SYNOPSIS
     use Hash::Subset qw(hash_subset hashref_subset);

     # using keys specified in an array
     my %subset = hash_subset   ({a=>1, b=>2, c=>3}, ['b','c','d']); # => (b=>2, c=>3)
     my $subset = hashref_subset({a=>1, b=>2, c=>3}, ['b','c','d']); # => {b=>2, c=>3}

     # using keys specified in another hash
     my %subset = hash_subset   ({a=>1, b=>2, c=>3}, {b=>20, c=>30, d=>40}); # => (b=>2, c=>3)
     my $subset = hashref_subset({a=>1, b=>2, c=>3}, {b=>20, c=>30, d=>40}); # => {b=>2, c=>3}

     # filtering keys using a coderef
     my %subset = hash_subset   ({a=>1, b=>2, c=>3}, sub {$_[0] =~ /[bc]/}); # => (b=>2, c=>3)
     my $subset = hashref_subset({a=>1, b=>2, c=>3}, sub {$_[0] =~ /[bc]/}); # => {b=>2, c=>3}

    A use case is when you use hash arguments:

     sub func1 {
         my %args = @_; # known arguments: foo, bar, baz
         ...
     }

     sub func2 {
         my %args = @_; # known arguments: all func1 arguments as well as qux, quux

         # call func1 with all arguments passed to us
         my $res = func1(hash_subset(\%args, [qw/foo bar baz/]));

         # postprocess result
         ...
     }

    If you use Rinci metadata in your code, this will come in handy, for
    example:

     my %common_args = (
         foo => {...},
         bar => {...},
         baz => {...},
     );

     $SPEC{func1} = {
        v => 1.1,
        args => {
            %common_args,
        },
     };
     sub func1 {
         my %args = @_;
         ...
     }

     $SPEC{func2} = {
        v => 1.1,
        args => {
            %common_args,
            # func2 supports all func1 arguments plus a couple of others
            qux  => { ... },
            quux => { ... },
        },
     };
     sub func2 {
         my %args = @_;

         # call func1 with all arguments passed to us
         my $res = func1(hash_subset(\%args, $SPEC{func1}{args}));

         # postprocess result
         ...
     }

DESCRIPTION
    Keywords: hash arguments, hash picking, hash grep, hash filtering

FUNCTIONS
    None exported by default.

  hash_subset
    Usage:

     my %subset  = hash_subset   (\%hash, \@keys);
     my $subset  = hashref_subset(\%hash, \@keys);

     my %subset  = hash_subset   (\%hash, \%another_hash);
     my $subset  = hashref_subset(\%hash, \%another_hash);

     # filter_sub is called with args ($key, $value) and return true when key should be included
     my %subset  = hash_subset   (\%hash, \&filter_sub);
     my $subset  = hashref_subset(\%hash, \&filter_sub);

    Produce subset of %hash, returning the subset hash (or hashref, in the
    case of "hashref_subset" function).

    Perl lets you produce a hash subset using the hash slice notation:

     my %subset = %hash{"b","c","d"};

    The difference with "hash_subset" is: 1) hash slice is only available
    since perl 5.20 (in previous versions, only array slice is available);
    2) when the key does not exist in the array, perl will create it for you
    with "undef" as the value:

     my %hash   = (a=>1, b=>2, c=>3);
     my %subset = %hash{"b","c","d"}; # => (b=>2, c=>3, d=>undef)

    So basically "hash_subset" is equivalent to:

     my %subset = %hash{grep {exists $hash{$_}} "b","c","d"}; # => (b=>2, c=>3)

    and available for perl earlier than 5.20.

  hashref_subset
    See "hash_subset".

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

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

BUGS
    Please report any bugs or feature requests on the bugtracker website
    <https://rt.cpan.org/Public/Dist/Display.html?Name=Hash-Subset>

    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
    Hash::Util::Pick also allows you to create a hash subset by specifying
    the wanted keys in a list or via filtering using a coderef. This XS
    module should perhaps be preferred over Hash::Subset for its
    performance, but there are some cases where you cannot use XS modules.

    See some benchmarks in Bencher::Scenarios::HashPicking.

AUTHOR
    perlancar <perlancar@cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2019 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.