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Author image Johan Lindström
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NAME

autobox::Transform - Autobox methods to transform Arrays and Hashes

CONTEXT

autobox provides the ability to call methods on native types, e.g. strings, arrays, and hashes as if they were objects.

autobox::Core provides the basic methods for Perl core functions like uc, map, and grep.

This module, autobox::Transform, provides higher level and more specific methods to transform and manipulate arrays and hashes, in particular when the values are hashrefs or objects.

SYNOPSIS

    use autobox::Core;  # map, uniq, sort, join, sum, etc.
    use autobox::Transform;

Arrays

    # use autobox::Core for ->map etc.

    # filter (like a more versatile grep)
    $book_locations->filter(); # true values
    $books->filter(sub { $_->is_in_library($library) });
    $book_names->filter( qr/lord/i );
    $book_types->filter("scifi");
    $book_types->filter({ fantasy => 1, scifi => 1 }); # hash key exists

    $book_types->order;
    $book_types->order("desc");
    $book_prices->order([ "num", "desc" ]);
    $books->order([ sub { $_->{price} }, "desc", "num" ]);
    $log_lines->order([ num => qr/pid: "(\d+)"/ ]);
    $books->order(
        [ sub { $_->{price} }, "desc", "num" ] # first price
        sub { $_->{name} },                    # then name
    );

    # Flatten arrayrefs-of-arrayrefs
    $authors->map_by("books") # ->books returns an arrayref
    # [ [ $book1, $book2 ], [ $book3 ] ]
    $authors->map_by("books")->flat;
    # [ $book1, $book2, $book3 ]

    # Return reference, even in list context, e.g. in a parameter list
    $book_locations->filter()->to_ref;

    # Return array, even in scalar context
    @books->to_array;

Arrays with hashrefs/objects

    # $books and $authors below are arrayrefs with either objects or
    # hashrefs (the call syntax is the same)

    $books->map_by("genre");
    $books->map_by([ price_with_tax => $tax_pct ]);

    $books->filter_by("is_sold_out");
    $books->filter_by([ is_in_library => $library ]);
    $books->filter_by([ price_with_tax => $rate ], sub { $_ > 56.00 });
    $books->filter_by("price", sub { $_ > 56.00 });
    $books->filter_by("author", "James A. Corey");
    $books->filter_by("author", qr/corey/i);

    # grep_by is an alias for filter_by
    $books->grep_by("is_sold_out");

    $books->uniq_by("id");

    $books->order_by("name");
    $books->order_by(name => "desc");
    $books->order_by(price => "num");
    $books->order_by(price => [ "num", "desc" ]);
    $books->order_by(name => [ sub { uc($_) }, "desc" ]);
    $books->order_by([ price_with_tax => $rate ] => "num");
    $books->order_by(author => "str", price => [ "num", "desc" ]);
    $books->order_by(
        author                      => [ "desc", sub { uc($_) } ],
        [ price_with_tax => $rate ] => [ "num", "desc" ],
        "name",
    );


    $books->group_by("title"),
    # {
    #     "Leviathan Wakes"       => $books->[0],
    #     "Caliban's War"         => $books->[1],
    #     "The Tree-Body Problem" => $books->[2],
    #     "The Name of the Wind"  => $books->[3],
    # },

    $authors->group_by([ publisher_affiliation => "with" ]),
    # {
    #     'James A. Corey with Orbit'     => $authors->[0],
    #     'Cixin Liu with Head of Zeus'   => $authors->[1],
    #     'Patrick Rothfuss with Gollanz' => $authors->[2],
    # },

    $books->group_by_count("genre"),
    # {
    #     "Sci-fi"  => 3,
    #     "Fantasy" => 1,
    # },

    my $genre_books = $books->group_by_array("genre");
    # {
    #     "Sci-fi"  => [ $sf_book_1, $sf_book_2, $sf_book_3 ],
    #     "Fantasy" => [ $fantasy_book_1 ],
    # },

Hashes

    # Upper-case the genre name, and make the count say "n books"
    #     (return a key => value pair)
    $genre_count->map_each(sub { uc( $_[0] ) => "$_ books" });
    # {
    #     "FANTASY" => "1 books",
    #     "SCI-FI"  => "3 books",
    # },

    # Make the count say "n books"
    #     (return the new value)
    $genre_count->map_each_value(sub { "$_ books" });
    # {
    #     "Fantasy" => "1 books",
    #     "Sci-fi"  => "3 books",
    # },

    # Transform each pair to the string "n: genre"
    #     (return list of items)
    $genre_count->map_each_to_array(sub { "$_: $_[0]" });
    # [ "1: Fantasy", "3: Sci-fi" ]

    # Genres with more than five books
    $genre_count->filter_each(sub { $_ > 5 });

    # Return reference, even in list context, e.g. in a parameter list
    %genre_count->to_ref );

    # Return hash, even in scalar context
    $author->book_count->to_hash;

Combined examples

    my $order_authors = $order->books
        ->uniq_by("isbn")
        ->map_by("author")
        ->map_by("name")->uniq->sort->join(", ");

    my $total_order_amount = $order->books
        ->filter_by([ not_covered_by_vouchers => $vouchers ])
        ->map_by([ price_with_tax => $tax_pct ])
        ->sum;

DESCRIPTION

High level autobox methods you can call on arrays, arrayrefs, hashes and hashrefs.

  • $array->filter()

  • $array->flat()

  • $array->as_ref()

  • $array->as_array()

  • $array->map_by()

  • $array->filter_by()

  • $array->uniq_by()

  • $array->group_by()

  • $array->group_by_count()

  • $array->group_by_array()

  • $hash->map_each

  • $hash->map_each_value

  • $hash->map_each_to_array

  • $hash->filter_each

  • $array->as_ref()

  • $array->as_hash()

Transforming lists of objects vs list of hashrefs

map_by, filter_by etc. (all methods named *_by) work with sets of hashrefs or objects.

These methods are called the same way regardless of whether the array contains objects or hashrefs. The items in the list must be either all objects or all hashrefs.

If the array contains objects, a method is called on each object (possibly with the arguments provided).

If the array contains hashrefs, the hash key is looked up on each item.

Calling accessor methods with arguments

For method calls, it's possible to provide arguments to the method.

Consider filter_by:

    $array->filter_by($accessor, $predicate)

If the $accessor is a string, it's a simple method call.

    # method call without args
    $books->filter_by("price", sub { $_ < 15.0 })
    # becomes $_->price() or $_->{price}

If the $accessor is an arrayref, the first item is the method name, and the rest of the items are the arguments to the method.

    # method call with args
    $books->filter_by([ price_with_discount => 5.0 ], sub { $_ < 15.0 })
    # becomes $_->price_with_discount(5.0)

Deprecated syntax

There is an older syntax for calling methods with arguments. It was abandoned to open up more powerful ways to use grep/filter type methods. Here it is for reference, in case you run into existing code.

    $array->filter_by($accessor, $args, $subref)
    $books->filter_by("price_with_discount", [ 5.0 ], sub { $_ < 15.0 })

Call the method $accessor on each object using the arguments in the $args arrayref like so:

    $object->$accessor(@$args)

This style is deprecated, and planned for removal in version 2.000, so if you have code with the old call style, please:

  • Replace your existing code with the new style as soon as possible. The change is trivial and the code easily found by grep/ack.

  • If need be, pin your version to < 2.000 in your cpanfile, dist.ini or whatever you use to avoid upgrading modules to incompatible versions.

Filter predicates

There are several methods that filter items, e.g. @array-filter> (duh), @array-filter_by>, and %hash-filter_each>. These methods take a $predicate argument to determine which items to retain or filter out.

If $predicate is an unblessed scalar, it is compared to each value with string eq.

    $books->filter_by("author", "James A. Corey");

If $predicate is a regex, it is compared to each value with =~.

    $books->filter_by("author", qr/Corey/);

If $predicate is a hashref, values in @array are retained if the $predicate hash key exists (the hash values are irrelevant).

    $books->filter_by(
        "author", {
            "James A. Corey"   => undef,
            "Cixin Liu"        => 0,
            "Patrick Rothfuss" => 1,
        },
    );

If $predicate is a subref, the subref is called for each value to check whether this item should remain in the list.

The $predicate subref should return a true value to remain. $_ is set to the current $value.

    $authors->filter_by(publisher => sub { $_->name =~ /Orbit/ });

List and Scalar Context

Almost all of the methods are context sensitive, i.e. they return a list in list context and an arrayref in scalar context, just like autobox::Core.

Beware: you might be in list context when you need an arrayref.

When in doubt, assume they work like map and grep, and convert the return value to references where you might have an unobvious list context. E.g.

    $self->my_method(
        # Wrong, this is list context and wouldn't return an arrayref
        books => $books->filter_by("is_published"),
    );

    $self->my_method(
        # Correct, convert the returned list to an arrayref
        books => [ $books->filter_by("is_published") ],
    );
    $self->my_method(
        # Correct, ensure scalar context to get an array ref
        books => scalar $books->filter_by("is_published"),
    );

    # Probably the nicest, since it goes at the end
    $self->my_method(
        # Correct, use ->to_ref to ensure an array reference is returned
        books => $books->filter_by("is_published")->to_ref,
    );

Sorting using order and order_by

Let's first compare how sorting is done with Perl's sort and autobox::Transform's order/order_by.

Sorting with sort

provide a sub that returns the comparison outcome of two values: $a and $b
in case of a tie, provide another comparison of $a and $b
    # If the name is the same, compare age (oldest first)
    sort {
        uc( $a->{name} ) cmp uc( $b->{name} )
        ||
        int( $b->{age} / 10 ) <=> int( $a->{age} / 10 )
    } @users

(note the opposite order of $a and $b for the age comparison, something that's often difficult to discern at a glance)

Sorting with order, order_by

provide order options for how one value should be compared with the others
how to compare (cmp or <=>)
which direction to sort (ascending or descending)
the value can be transformed using an optional subref, e.g. by uc($_)
in case of a tie, provide another comparison
    # If the name is the same, compare age (oldest first)

    @users->order(
        sub { $_->{name} },                               # first comparison
        [ "num", sub { int( $_->{age} / 10 ) }, "desc" ], # second comparison
    )

    @users->order_by(
        name => "str",                                     # first comparison
        age  => [ num => desc => sub { int( $_ / 10 ) } ], # second comparison
    )

Comparison Options

If there's only one option for a comparison (e.g. num), provide a single option (string/regex/subref) value. If there are many options, provide them in an arrayref in any order.

There are comparison options for how to compare values (string/numeric), sort order, and how to get at the value to compare.

Comparison operator

str (cmp) - default
num (<=>)

Sort order

asc (ascending) - default
desc (descending)

The value to compare

A subref - default is: sub { $_ }
The return value is used in the comparison
A regex
The value of join("", @captured_groups) are used in the comparison (@captured_groups are $1, $2, $3 etc.)

Examples

    ## A single comparison

    # order: the first arg is the comparison options (one or an
    # arrayref with many)
    ->order()  # Defaults to str, asc, $_, just like sort
    ->order("num")
    ->order(sub { uc($_) })
    ->order( qr/first_name: (\w+), last_name: (\w+)/ )
    ->order([ num => qr/id: (\d+)/ ])
    ->order([ sub { int($_) }, "num" ])

    # order_by: the first arg is the accessor, second arg is the
    # comparison options (one or an arrayref with many)
    ->order_by("id")
    ->order_by("id", "num")
    ->order_by("id", [ "num", "desc" ])
    ->order_by("name", sub { uc($_) })
    ->order_by(log_line => qr/first_name: (\w+), last_name: (\w+)/ )
    ->order_by("log_line", [ num => qr/id: (\d+)/ ])
    ->order_by(age => [ sub { int($_) }, "num" ])
    ->order_by([ age_by_interval => 10 ] => [ sub { int($_) }, "num" ])
    ->order_by([ name_with_title => $title ], sub { uc($_) })


    ## Multiple comparisons
    # order: subsequent comparison options are added as needed (one or
    # an arrayref with many, per comparison)
    ->order(
        [ sub { uc($_) }, "desc" ],
        "str",
    )
    ->order(
        [ sub { $_->{price} }, "num" ], # First a numeric comparison of price
        [ sub { $_->{name} }, "desc" ], # or if same, a reverse comparison of the name
    )
    ->order(
        qr/type: (\w+)/,
        [ num => desc => qr/duration: (\d+)/ ]
        [ num => sub { /id: (\d+)/ } ],
        "str",
    )

    # order_by:
    ->order(
        price => "num", # First a numeric comparison of price
        name => "desc", # or if same, a reverse comparison of the name
    )
    ->order(
        price => [ "num", "desc" ],
        name  => "str",
    )
    ->order(
        [ price_with_discount => $discount ] => [ "num", "desc" ],
        name                                 => [ str => sub { uc($_) } ],
        "id",
    )

Order methods

See "order" and "order_by"

METHODS ON ARRAYS

@array->filter($predicate = *is_true_subref*) : @array | @$array

Similar to Perl's grep, return an @array with values for which $predicate yields a true value.

$predicate can be a subref, string, undef, regex, or hashref. See "Filter predicates".

The default (no $predicate) is a subref which retains true values in the @array.

Examples:

    my @apples     = $fruit->filter("apple");
    my @any_apple  = $fruit->filter( qr/apple/i );
    my @publishers = $authors->filter(
        sub { $_->publisher->name =~ /Orbit/ },
    );

filter and grep

autobox::Core's grep method takes a subref, just like this method. filter also supports the other predicate types, like string, regex, etc.

@array->order(@comparisons = ("str")) : @array | @$array

Return @array ordered according to the @comparisons. The default comparison is the same as the default sort, e.g. a normal string comparison of the @array values.

If the first item in @comparison ends in a tie, the next one is used, etc.

Each comparison consists of a single option or an arrayref of options, e.g. str/num, asc/desc, or a subref/regex. See "Sorting using order and order_by" for details about how these work.

Examples:

    @book_types->order;
    @book_types->order("desc");
    @book_prices->order([ "num", "desc" ]);
    @books->order([ sub { $_->{price} }, "desc", "num" ]);
    @log_lines->order([ num => qr/pid: "(\d+)"/ ]);
    @books->order(
        [ sub { $_->{price} }, "desc", "num" ] # first price
        sub { $_->{name} },                    # then name
    );

@array->order_by(@accessor_comparison_pairs) : @array | @$array

Return @array ordered according to the @accessor_comparison_pairs.

The comparison value comes from an initial map_by($accessor) on each @array item. It then works just like with ->order_by>.

    $books->order_by(price => "num");
    $books->order_by(price => [ "num", "desc" ]);

It is important that the map_by call returns exactly a single scalar that can be compared with the other values.

Just like with order, the value to actually compare can be transformed using a sub, or be matched against a regex.

    $books->order_by(price => [ num => sub { int($_) } ]);

    # Ignore leading "The" in book titles by optionally matching it
    # with a non-capturing group and the rest with a capturing paren
    $books->order_by( title => qr/^ (?: The \s+ )? (.+) /x );

If a comparison is missing for the last pair, the default is a normal str comparison.

    $books->order_by("name"); # default "str"

If the first comparison ends in a tie, the next pair is used, etc. Note that in order to provide accessor-comparison pairs, it's often necessary to provide a default "str" comparison just to make it a pair.

    $books->order_by(
        author => "str",
        price  => [ "num", "desc" ],
    );

@array->flat() : @array | @$array

Return a (one level) flattened array, assuming the array items themselves are array refs. I.e.

    [
        [ 1, 2, 3 ],
        [ "a", "b" ],
        [ [ 1, 2 ], { 3 => 4 } ]
    ]->flat

returns

    [ 1, 2, 3, "a", "b ", [ 1, 2 ], { 3 => 4 } ]

This is useful if e.g. a ->map_by("some_method") returns arrayrefs of objects which you want to do further method calls on. Example:

    # ->books returns an arrayref of Book objects with a ->title
    $authors->map_by("books")->flat->map_by("title")

Note: This is different from autobox::Core's ->flatten, which reurns a list rather than an array and therefore can't be used in this way.

@array->to_ref() : $arrayref

Return the reference to the @array, regardless of context.

Useful for ensuring the last array method return a reference while in scalar context. Typically:

    do_stuff(
        books => $author->map_by("books")->to_ref,
    );

map_by is called in list context, so without ->to_ref it would have return an array, not an arrayref.

@array->to_array() : @array

Return the @array, regardless of context. This is mostly useful if called on a ArrayRef at the end of a chain of method calls.

METHODS ON ARRAYS CONTAINING OBJECTS/HASHES

@array->map_by($accessor) : @array | @$array

$accessor is either a string, or an arrayref where the first item is a string.

Call the $accessor on each object in @array, or get the hash key value on each hashref in @array. Like:

    map { $_->$accessor() }
    # or
    map { $_->{$accessor} }

Examples:

    my @ahthor_names = $authors->map_by("name");
    my $author_names = @publishers->map_by("authors")->map_by("name");

Or get the hash key value. Example:

    my @review_scores = $reviews->map_by("score");

Alternatively the $accessor is an arrayref. The first item is the accessor name, and the rest of the items are passed as args the method call. This only works when working with objects, not with hashrefs.

Examples:

    my @prices_including_tax = $books->map_by([ "price_with_tax", $tax_pct ]);
    my $prices_including_tax = $books->map_by([ price_with_tax => $tax_pct ]);

@array->filter_by($accessor, $predicate = *is_true_subref*) : @array | @$array

$accessor is either a string, or an arrayref where the first item is a string.

Call the $accessor on each object in the list, or get the hash key value on each hashref in the list.

Example:

    my @prolific_authors = $authors->filter_by("is_prolific");

Alternatively the $accessor is an arrayref. The first item is the accessor name, and the rest of the items are passed as args the method call. This only works when working with objects, not with hashrefs.

Example:

    my @books_to_charge_for = $books->filter_by([ price_with_tax => $tax_pct ]);

Use the $predicate to determine whether the value should remain. $predicate can be a subref, string, undef, regex, or hashref. See "Filter predicates".

The default (no $predicate) is a subref which retains true values in the result @array.

Examples:

    # Custom predicate subref
    my @authors = $authors->filter_by(
        "publisher",
        sub { $_->name =~ /Orbit/ },
    );

    # Call method with args and match a regex
    my @authors = $authors->filter_by(
        [ publisher_affiliation => "with" ],
        qr/Orbit/ },
    );

Note: if you do something complicated with a $predicate subref, it might be easier and more readable to simply use $array-$<gtfilter()>.

Alias

grep_by is an alias for filter_by. Unlike grep vs filter, this one works exaclty the same way.

@array->uniq_by($accessor) : @array | @$array

$accessor is either a string, or an arrayref where the first item is a string.

Call the $accessor on each object in the list, or get the hash key value on each hashref in the list. Return list of items wich have a unique set of return values. The order is preserved. On duplicates, keep the first occurrence.

Examples:

    # You have gathered multiple Author objects with duplicate ids
    my @authors = $authors->uniq_by("author_id");

Alternatively the $accessor is an arrayref. The first item is the accessor name, and the rest of the items are passed as args the method call. This only works when working with objects, not with hashrefs.

Examples:

    my @example_book_at_price_point = $books->uniq_by(
        [ price_with_tax => $tax_pct ],
    );

@array->group_by($accessor, $value_subref = object) : %key_value | %$key_value

$accessor is either a string, or an arrayref where the first item is a string.

Call ->$accessor on each object in the array, or get the hash key for each hashref in the array (just like ->map_by) and group the values as keys in a hashref.

The default $value_subref puts each object in the list as the hash value. If the key is repeated, the value is overwritten with the last object.

Example:

    my $title_book = $books->group_by("title");
    # {
    #     "Leviathan Wakes"       => $books->[0],
    #     "Caliban's War"         => $books->[1],
    #     "The Tree-Body Problem" => $books->[2],
    #     "The Name of the Wind"  => $books->[3],
    # },

The $value_subref

This is a bit tricky to use, so the most common thing would probably be to use one of the more specific group_by-methods (see below). It should be capable enough to achieve what you need though, so here's how it works:

The hash key is whatever is returned from $object->$accessor.

The hash value is whatever is returned from

    my $new_value = $value_sub->($current_value, $object, $key);

where:

  • $current value is the current hash value for this key (or undef if the first one).

  • $object is the current item in the list. The current $_ is also set to this.

  • $key is the key returned by $object->$accessor(@$args)

@array->group_by_count($accessor) : %key_count | %$key_count

$accessor is either a string, or an arrayref where the first item is a string.

Just like group_by, but the hash values are the the number of instances each $accessor value occurs in the list.

Example:

    $books->group_by_count("genre"),
    # {
    #     "Sci-fi"  => 3,
    #     "Fantasy" => 1,
    # },

$book->genre() returns the genre string. There are three books counted for the "Sci-fi" key.

@array->group_by_array($accessor) : %key_objects | %$key_objects

$accessor is either a string, or an arrayref where the first item is a string.

Just like group_by, but the hash values are arrayrefs containing the objects which has each $accessor value.

Example:

    my $genre_books = $books->group_by_array("genre");
    # {
    #     "Sci-fi"  => [ $sf_book_1, $sf_book_2, $sf_book_3 ],
    #     "Fantasy" => [ $fantasy_book_1 ],
    # },

$book->genre() returns the genre string. The three Sci-fi book objects are collected under the Sci-fi key.

METHODS ON HASHES

%hash->map_each($key_value_subref) : %new_hash | %$new_hash

Map each key-value pair in the hash using the $key_value_subref. Similar to how to how map transforms a list into another list, map_each transforms a hash into another hash.

$key_value_subref->($key, $value) is called for each pair (with $_ set to the value).

The subref should return an even-numbered list with zero or more key-value pairs which will make up the %new_hash. Typically two items are returned in the list (the key and the value).

Example

    { a => 1, b => 2 }->map_each(sub { "$_[0]$_[0]" => $_ * 2 });
    # Returns { aa => 2, bb => 4 }

%hash->map_each_value($value_subref) : %new_hash | %$new_hash

Map each value in the hash using the $value_subref, but keep the keys the same.

$value_subref->($key, $value) is called for each pair (with $_ set to the value).

The subref should return a single value for each key which will make up the %new_hash (with the same keys but with new mapped values).

Example

    { a => 1, b => 2 }->map_each_value(sub { $_ * 2 });
    # Returns { a => 2, b => 4 }

%hash->map_each_to_array($item_subref) : @new_array | @$new_array

Map each key-value pair in the hash into a list using the $item_subref.

$item_subref->($key, $value) is called for each pair (with $_ set to the value) in key order.

The subref should return zero or more list items which will make up the @new_array. Typically one item is returned.

Example

    { a => 1, b => 2 }->map_each_to_array(sub { "$_[0]-$_" });
    # Returns [ "a-1", "b-2" ]

%hash->filter_each($predicate = *is_true_subref*) : @hash | @$hash

Return a %hash with values for which $predicate yields a true value.

$predicate can be a subref, string, undef, regex, or hashref. See "Filter predicates".

The default (no $predicate) is a subref which retains true values in the @array.

Examples:

    my @apples     = $fruit->filter("apple");
    my @any_apple  = $fruit->filter( qr/apple/i );
    my @publishers = $authors->filter(
        sub { $_->publisher->name =~ /Orbit/ },
    );

If the $predicate is a subref, $predicate->($key, $value) is called for each pair (with $_ set to the value).

The subref should return a true value to retain the key-value pair in the result %hash.

Example

    $book_author->filter_each(sub { $_->name =~ /Corey/ });

%hash->to_ref() : $hashref

Return the reference to the %hash, regardless of context.

Useful for ensuring the last hash method return a reference while in scalar context. Typically:

    do_stuff(
        genre_count => $books->group_by_count("genre")->to_ref,
    );

%hash->to_hash() : %hash

Return the %hash, regardless of context. This is mostly useful if called on a HashRef at the end of a chain of method calls.

AUTOBOX AND VANILLA PERL

Raison d'etre

autobox::Core is awesome, for a variety of reasons.

  • It cuts down on dereferencing punctuation clutter, both by using methods on references and by using ->elements to deref arrayrefs.

  • It makes map and grep transforms read in the same direction it's executed.

  • It makes it easier to write those things in a natural order. No need to move the cursor around a lot just to fix dereferencing, order of operations etc.

On top of this, autobox::Transform provides a few higher level methods for mapping, filtering and sorting common cases which are easier to read and write.

Since they are at a slightly higher semantic level, once you know them they also provide a more specific meaning than just "map" or "grep".

(Compare the difference between seeing a "map" and seeing a "foreach" loop. Just seeing the word "map" hints at what type of thing is going on here: transforming a list into another list).

The methods of autobox::Transform are not suitable for all cases, but when used appropriately they will lead to much more clear, succinct and direct code, especially in conjunction with autobox::Core.

Code Comparison

These examples are only for when there's a straightforward and simple Perl equivalent.

    ### map_by - method call: $books are Book objects
    my @genres = map { $_->genre() } @$books;
    my @genres = $books->map_by("genre");

    my $genres = [ map { $_->genre() } @$books ];
    my $genres = $books->map_by("genre");

    # With sum from autobox::Core / List::AllUtils
    my $book_order_total = sum(
        map { $_->price_with_tax($tax_pct) } @{$order->books}
    );
    my $book_order_total = $order->books
        ->map_by([ price_with_tax => $tax_pct ])->sum;

    ### map_by - hash key: $books are book hashrefs
    my @genres = map { $_->{genre} } @$books;
    my @genres = $books->map_by("genre");



    ### filter_by - method call: $books are Book objects
    my $sold_out_books = [ grep { $_->is_sold_out } @$books ];
    my $sold_out_books = $books->filter_by("is_sold_out");
    my $sold_out_books = $books->grep_by("is_sold_out");

    my $books_in_library = [ grep { $_->is_in_library($library) } @$books ];
    my $books_in_library = $books->filter_by([ is_in_library => $library ]);

    ### filter_by - hash key: $books are book hashrefs
    my $sold_out_books = [ grep { $_->{is_sold_out} } @$books ];
    my $sold_out_books = $books->filter_by("is_sold_out");



    ### uniq_by - method call: $books are Book objects
    my %seen; my $distinct_books = [ grep { ! %seen{ $_->id // "" }++ } @$books ];
    my $distinct_books = $books->uniq_by("id");

    ### uniq_by - hash key: $books are book hashrefs
    my %seen; my $distinct_books = [ grep { ! %seen{ $_->{id} // "" }++ } @$books ];
    my $distinct_books = $books->uniq_by("id");


    #### flat - $author->books returns an arrayref of Books
    my $author_books = [ map { @{$_->books} } @$authors ]
    my $author_books = $authors->map_by("books")->flat

DEVELOPMENT

Author

Johan Lindstrom, <johanl [AT] cpan.org>

Source code

https://github.com/jplindstrom/p5-autobox-Transform

Bug reports

Please report any bugs or feature requests on GitHub:

https://github.com/jplindstrom/p5-autobox-Transform/issues.

COPYRIGHT & LICENSE

Copyright 2016- Johan Lindstrom, All Rights Reserved.

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