class List

Sequence of values

my class List is Iterable does Positional { .. }

List stores items sequentially and potentially lazily.

Indexes into lists and arrays start at 0 by default.

You can assign to list elements if they are containers. Use Arrays to have every value of the list stored in a container.

Items, Flattening and Sigils

In Perl 6, assigning a List to a scalar variable does not lose information. The difference is that iteration generally treats a list (or any other list-like object, like a Parcel or an Array) inside a scalar as a single element.

my @a = 1, 2, 3;
for @a { }      # three iterations
my $s = @a;
for $s { }      # one iteration
for @a.item { } # one iteration
for $s.list { } # three iterations

Lists generally interpolate (flatten) unless they are accessed via an item (scalar) container.

my @a = 1, 2, 3;
my @flat   = @a, @a;           # six elements
my @nested = @a.item, @a.item; # two elements

.item can often be written as $( ... ), and on an array variable even as $@a.

Methods

routine elems

multi sub    elems($list)  returns Int:D
multi method elems(List:D:) returns Int:D

Returns the number of elements in the list.

routine end

multi sub    end($list)  returns Int:D
multi method end(List:D:) returns Int:D

Returns the index of the last element.

routine keys

multi sub    keys($list)  returns List:D
multi method keys(List:D:) returns List:D

Returns a list of indexes into the list (e.g., 0..(@list.elems-1)).

routine values

multi sub    values($list)  returns List:D
multi method values(List:D:) returns List:D

Returns a copy of the list.

routine kv

multi sub    kv($list)  returns List:D
multi method kv(List:D:) returns List:D

Returns an interleaved list of indexes and values. For example

<a b c>.kv

Returns

0, 'a', 1, 'b', 2, 'c'

routine pairs

multi sub    pairs($list)   returns List:D
multi method pairs(List:D:) returns List:D

Returns a list of pairs, with the indexes as keys and the list values as values.

<a b c>.pairs   # 0 => 'a', 1 => 'b', 2 => 'c'

routine join

multi sub    join($separator, *@list) returns Str:D
multi method join(List:D: $separator) returns Str:D

Treats the elements of the list as strings, interleaves them with $separator and concatenates everything into a single string.

Example:

join ', ', <a b c>;     # 'a, b, c'

routine map

multi sub    map(&code, *@elems) returns List:D
multi method map(List:D: &code) returns List:D

Invokes &code for each element and gathers the return values in another list and returns it. This happens lazily, i.e. &code is only invoked when the return values are accessed.

Examples:

> ('hello', 1, 22/7, 42, 'world').map: { .WHAT.perl }
Str Int Rat Int Str
> map *.Str.chars, 'hello', 1, 22/7, 42, 'world'
5 1 8 2 5

routine grep

multi sub    grep(Mu $matcher, *@elems) returns List:D
multi method grep(List:D:  Mu $matcher) returns List:D

Returns a lazy list of elements against which $matcher smart-matches. The elements are returned in the order in which they appear in the original list.

Examples:

> ('hello', 1, 22/7, 42, 'world').grep: Int
1 42
> grep { .Str.chars > 3 }, 'hello', 1, 22/7, 42, 'world'
hello 3.142857 world

routine grep-index

multi method grep-index(List:D: Mu $matcher) returns List:D

Returns a lazy list of indices against which the associated elements smart-match. The indicies are returned in order.

routine first

multi sub    first(Mu $matcher, *@elems)
multi method first(List:D:  Mu $matcher)

Returns the first item of the list which smart-matches against $matcher, fails when no values match.

Examples:

say (1, 22/7, 42).first: * > 5;     # 42
say $f = ('hello', 1, 22/7, 42, 'world').first: Complex;
say $f.perl;                        #  Failure.new(exception => X::AdHoc.new(payload => "No values matched"))

routine first-index

multi method first-index(List:D: Mu $matcher)

Returns the first index against which $matcher smart-matches, or Nil if no match was found.

routine last-index

multi method last-index(List:D: Mu $matcher)

Returns the last index against which $matcher smart-matches, or Nil if no match was found.

routine classify

multi sub    classify(&mapper, *@values) returns Hash:D
multi method classify(List:D: &mapper)   returns Hash:D

Transforms a list of values into a hash representing the classification of those values according to a mapper; each hash key represents the classification for one or more of the incoming list values, and the corresponding hash value contains an array of those list values classified by the mapper into the category of the associated key.

Example:

say classify { $_ %% 2 ?? 'even' !! 'odd' }, (1, 7, 6, 3, 2);
            # ("odd" => [1, 7, 3], "even" => [6, 2]).hash;;
say ('hello', 1, 22/7, 42, 'world').classify: { .Str.chars }
            # ("5" => ["hello", "world"], "1" => [1], "8" => [22/7], "2" => [42]).hash

method Bool

multi method Bool(List:D:) returns Bool:D

Returns True if the list has at least one element, and False for the empty list.

method Str

multi method Str(List:D:) returns Str:D

Stringifies the elements of the list and joins them with spaces (same as .join(' ')).

method Int

multi method Int(List:D:) return Int:D

Returns the number of elements in the list (same as .elems).

routine pick

multi sub    pick($count, *@list) returns List:D
multi method pick(List:D: $count = 1)

Returns $count elements chosen at random and without repetition from the invocant. If * is passed as $count, or $count is greater than or equal to the size of the list, then all elements from the invocant list are returned in a random sequence.

Examples:

say <a b c d e>.pick;           # b
b
say <a b c d e>.pick: 3;        # c a e
say  <a b c d e>.pick: *;       # e d a b c

routine roll

multi sub    roll($count, *@list) returns List:D
multi method roll(List:D: $count = 1)

Returns a lazy list of $count elements, each randomly selected from the list. Each random choice is made independently, like a separate die roll where each die face is a list element.

If * is passed to $count, returns a lazy, infinite list of randomly chosen elements from the original list.

Examples:

say <a b c d e>.roll;       # b
b
say <a b c d e>.roll: 3;    # c c e
say roll 8, <a b c d e>;    # b a e d a e b c
my $random_digits := (^10).roll(*);
say $random_digits[^15];    # 3 8 7 6 0 1 3 2 0 8 8 5 8 0 5

method eager

multi method eager(List:D:) returns List:D

Evaluates all elements in the list eagerly, and returns the invocant. If a List signals that it is "known infinite", eager evaluation may stop at the point where the infinity is detected.

routine reverse

multi sub    reverse(*@list ) returns List:D
multi method reverse(List:D:) returns List:D

Returns a list with the same elements in reverse order.

Note that reverse always refers to reversing elements of a list; to reverse the characters in a string, use flip.

Examples:

say <hello world!>.reverse      #  world! hello
say reverse ^10                 # 9 8 7 6 5 4 3 2 1 0

routine rotate

multi sub    rotate(@list,  Int:D $n = 1) returns List:D
multi method rotate(List:D: Int:D $n = 1) returns List:D

Returns the list rotated by $n elements.

Examples:

<a b c d e>.rotate(2);   # <c d e a b>
<a b c d e>.rotate(-1);  # <e a b c d>

routine sort

multi sub    sort(*@elems)      returns List:D
multi sub    sort(&by, *@elems) returns List:D
multi method sort(List:D:)      returns List:D
multi method sort(List:D:, &by) returns List:D

Sorts the list, smallest element first. By default infix:<cmp> is used for comparing list elements.

If &by is provided, and it accepts two arguments, it is invoked for pairs of list elements, and should return Order::Increase, Order::Same or Order::Decrease.

If &by accepts only one argument, the list elements are sorted according to by($a) cmp by($b) . The return values of &by are cached, so that &by is only called once per list element.

Examples:

say (3, -4, 7, -1, 2, 0).sort;                  # -4 -1 0 2 3 7
say (3, -4, 7, -1, 2, 0).sort: *.abs;           # 0 -1 2 3 -4 7
say (3, -4, 7, -1, 2, 0).sort: { $^b leg $^a }; # 7 3 2 0 -4 -1

routine reduce

multi sub    reduce(&with, *@elems)
multi method reduce(List:D: &with)

Applies &with to the first and the second value of the list, then to the result of that calculation and the third value and so on. Returns a single item generated that way.

Note that reduce is an implicit loop, and thus responds to next, last and redo statements.

Example:

say (1, 2, 3).reduce: * - *;    # -4

routine splice

multi sub    splice(@list,  $start, $elems?, *@replacement) returns List:D
multi method splice(List:D: $start, $elems?, *@replacement) returns List:D

Deletes $elems elements starting from index $start from the list, returns them and replaces them by @replacement. If $elems is omitted, all the elements starting from index $start are deleted.

Example:

my @foo = <a b c d e f g>;
say @foo.splice(2, 3, <M N O P>);       # c d e
say @foo;                               # a b M N O P f g

routine pop

multi sub    pop(List:D )
multi method pop(List:D:)

Removes and returns the last item from the list, fails for an empty list.

Example:

> my @foo = <a b>;
a b
> @foo.pop;
b
> pop @foo
a
> pop @foo
Element popped from empty list

routine push

multi sub    push(List:D, *@values) returns List:D
multi method push(List:D: *@values) returns List:D

Adds the @values to the end of the list, and returns the modified list. Fails for infinite lists.

Example:

my @foo = <a b c>;
@foo.push: 1, 3 ... 11;
say @foo;                   # a b c 1 3 5 7 9 11

routine shift

multi sub    shift(List:D )
multi method shift(List:D:)

Removes and returns the first item from the list. Fails for an empty list.

Example:

my @foo = <a b>;
say @foo.shift;     # a
say @foo.shift;     # b
say @foo.shift;     # Element shifted from empty list

routine unshift

multi sub    unshift(List:D, *@values) returns List:D
multi method unshift(List:D: *@values) returns List:D

Adds the @values to the start of the list, and returns the modified list. Fails if @values is infinite.

Example:

my @foo = <a b c>;
@foo.unshift: 1, 3 ... 11;
say @foo;                   # 1 3 5 7 9 11 a b c

routine combinations

multi method combinations (List:D: Int:D $of)          returns List:D
multi method combinations (List:D: Range:D $of = 0..*) returns List:D
multi sub    combinations ($n, $k)                     returns List:D

The Int variant returns all $of-combinations of the invocant list. For example

say .join('|') for <a b c>.combinations(2);

prints

a|b
a|c
b|c

because all the 2-combinations of 'a', 'b', 'c' are ['a', 'b'], ['a', 'c'], ['b', 'c'].

The Range variant combines all the individual combinations into a single list, so

say .join('|') for <a b c>.combinations(2..3);

prints

a|b
a|c
b|c
a|b|c

because that's the list of all 2- and 3-combinations.

The subroutine form combinations($n, $k) is equivalent to (^$n).combinations($k), so

.say for combinations(4, 2)

prints

0 1
0 2
0 3
1 2
1 3
2 3

routine permutations

multi method permutations(List:D:) returns List:D
multi sub    permutations($n)      returns List:D

Returns all possible permutations of a list as a list of arrays. So

say .join('|') for <a b c>.permutations

prints

a|b|c
a|c|b
b|a|c
b|c|a
c|a|b
c|b|a

permutations treats all list elements as distinguishable, so (1, 1, 2).permutations still returns a list of 6 elements, even though there are only three distinct permutations.

The subroutine form permutations($n) is equivalent to (^$n).permutations, so

.say for permutations 3;

prints

1 2 3
1 3 2
2 1 3
2 3 1
3 1 2
3 2 1

Type graph

Below you should see an image showing the type relations for List. If not, try the PNG version.

perl6-type-graph List List Iterable Iterable List->Iterable Cool Cool List->Cool Positional Positional List->Positional Mu Mu Any Any Any->Mu Iterable->Any Cool->Any Array Array Array->List LoL LoL LoL->List Backtrace Backtrace Backtrace->List

Methods supplied by role Positional

List does role Positional, which provides the following methods:

method of

method of()

Returns the type constraint for elements of the positional container. Defaults to Mu.

Methods supplied by class Any

List inherits from class Any, which provides the following methods:

method ACCEPTS

multi method ACCEPTS(Any:D: Mu $other)

Returns True if $other === self (i.e. it checks object identity).

method any

Interprets the invocant as a list and creates an any-Junction from it.

method all

Interprets the invocant as a list and creates an all-Junction from it.

method one

Interprets the invocant as a list and creates an one-Junction from it.

method none

Interprets the invocant as a list and creates an none-Junction from it.

Methods supplied by class Mu

List inherits from class Mu, which provides the following methods:

method Str

multi method Str()   returns Str

Returns a string representation of the invocant, intended to be machine readable.

method clone

method clone(*%twiddles)

Creates a shallow clone of the invocant. If named arguments are passed to it, their values are used in every place where an attribute name matches the name of a named argument.

method new

multi method new(*%attrinit)

Default method for constructing (create + initialize) new objects of a class. This method expects only named arguments which are then used to initialize attributes with accessors of the same name.

Classes may provide their own new method to override this default.

method bless

method bless(*%attrinit) returns Mu:D

Lower-level object construction method than new.

Creates a new object of the same type as the invocant, uses the named arguments to initialize attributes, and returns the created object.

You can use this method when writing custom constructors:

class Point {
    has $.x;
    has $.y;
    multi method new($x, $y) {
        self.bless(:$x, :$y);
    }
}
my $p = Point.new(-1, 1);

(Though each time you write a custom constructor, remember that it makes subclassing harder).

method CREATE

method CREATE() returns Mu:D

Allocates a new object of the same type as the invocant, without initializing any attributes.

method print

multi method print() returns Bool:D

Prints value to $*OUT after stringification using .Str method without newline at end.

method say

multi method say() returns Bool:D

Prints value to $*OUT after stringification using .gist method with newline at end.

method ACCEPTS

multi method ACCEPTS(Mu:U: $other)

Performs a type check. Returns True if $other conforms to the invocant (which is always a type object or failure).

This is the method that is triggered on smart-matching against type objects, for example in if $var ~~ Int { ... }.

method WHICH

multi method WHICH() returns ObjAt:D

Returns an object of type ObjAt which uniquely identifies the object. Value types override this method which makes sure that two equivalent objects return the same return value from WHICH.