class Pair

Key/value pair

class Pair does Associative { ... }

Consists of two parts, a key and a value. Pairs can be seen as the atomic units in Hashes, and they are also used in conjunction with named arguments and parameters.

: => :! There are three syntaxes for Pairs:

'key' => 'value'    # this...
:key<value>         # ...means the same as this
:$foo               # short for foo => $foo

Variants of this are

:key                # same as   key => True
:!key               # same as   key => False

Methods

method antipair

Defined as:

method antipair() returns Pair:D

Usage:

PAIR.antipair

Returns a new Pair object with key and value exchanged.

my $p = (6 => 'Perl').antipair;
say $p.key;         # Perl
say $p.value;       # 6

method value

Defined as:

multi method value(Pair:D:) is rw

Usage:

PAIR.value

Returns the value part of the Pair.

infix cmp

Defined as:

multi sub infix:<cmp>(Pair:D, Pair:D)

Usage:

PAIR1 cmp PAIR2

The type-agnostic comparator; compares two Pairs. Compares first their key parts, and then compares the value parts if the keys are equal.

method fmt

Defined as:

multi method fmt(Pair:D:) returns Str:D

Usage:

PAIR.fmt(FORMATSTRING)

Takes a format string, and returns a string the key and value parts of the Pair formatted. Here's an example:

my $pair = :Earth(1);
say $pair.fmt("%s is %.3f AU away from the sun")
# Prints "Earth is 1.000 AU away from the sun"

For more about format strings, see sprintf.

method kv

Defined as:

multi method kv(Pair:D:) returns List:D

Usage:

PAIR.kv

Returns a two-element List with the key and value parts of Pair, in that order. This method is a special case of the same-named method on Hash, which returns all its entries as a list of keys and values.

method pairs

Defined as:

multi method pairs(Pair:D:)

Usage:

PAIR.pairs

Returns a list of one Pair, namely this one.

Type graph

Below you should see a clickable image showing the type relations for Pair that links to the documentation pages for the related types. If not, try the PNG version instead.

perl6-type-graph Pair Pair Any Any Pair->Any Associative Associative Pair->Associative Mu Mu Any->Mu

Routines supplied by class Any

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

method ACCEPTS

Defined as:

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

Usage:

EXPR.ACCEPTS(EXPR);

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

Many built-in types override this for more specific comparisons

method any

Defined as:

method any() returns Junction:D

Usage:

LIST.any

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

say so 2 == <1 2 3>.any;        # True
say so 5 == <1 2 3>.any;        # False

method all

Defined as:

method all() returns Junction:D

Usage:

LIST.any

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

say so 1 < <2 3 4>.all;         # True
say so 3 < <2 3 4>.all;         # False

method one

Defined as:

method one() returns Junction:D

Usage:

LIST.one

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

say so 1 == (1, 2, 3).one;      # True
say so 1 == (1, 2, 1).one;      # False

method none

Defined as:

method none() returns Junction:D

Usage:

LIST.none

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

say so 1 == (1, 2, 3).none;     # False
say so 4 == (1, 2, 3).none;     # True

method list

Interprets the invocant as a list, and returns that List.

say 42.list.^name;           # List
say 42.list.elems;           # 1

method push

The method push is defined for undefined invocants and allowes to autovivify undefined to an empty Array, unless the undefined value implements Positional already. The argument provided will then be pushed into the newly created Array.

my %h;
dd %h<a>; # Any (and therefor undefined)
%h<a>.push(1); # .push on Any
dd %h; # «Hash %h = {:a($[1])}␤» # please not the Array

routine reverse

Defined as:

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

Usage:

reverse(LIST)
LIST.reverse

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

method sort

Sorts iterables with infix:<cmp> or given code object.

Examples:

say <b c a>.sort;                           # a b c
say 'bca'.comb.sort.join;                   # abc
say 'bca'.comb.sort({$^b cmp $^a}).join;    # cba
say '231'.comb.sort(&infix:«<=>»).join;     # 123

method flat

Interprets the invocant as a list, flattens it, and returns that list. Please note that .flat will not solve the halting problem for you. If you flat an infinite list .flat may return that infinite list, eating all your RAM in the process. Further, say is not a good way to test lists because it will truncate after 100 elements.

say ((1, 2), (3)).elems;        # 2
say ((1, 2), (3)).flat.elems;   # 3

method eager

Interprets the invocant as a list, evaluates it eagerly, and returns that list.

say (1..10).eager;              # 1 2 3 4 5 6 7 8 9 10

method elems

Interprets the invocant as a list, and returns the number of elements in the list.

say 42.elems;                   # 1
say <a b c>.elems;              # 3

method end

Interprets the invocant as a list, and returns the last index of that list.

say 6.end;                      # 0
say <a b c>.end;                # 2

method pairup

method pairup() returns List

Interprets the invocant as a list, and constructs a list of pairs from it, in the same way that assignment to a Hash does. That is, it takes two consecutive elements and constructs a pair from them, unless the item in the key position already is a pair (in which case the pair is passed is passed through, and the next list item, if any, is considered to be a key again).

say (a => 1, 'b', 'c').pairup.perl;     # ("a" => 1, "b" => "c").list

sub exit

sub exit(Int() $status = 0)

Exits the current process with return code $status.

Routines supplied by class Mu

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

routine defined

multi sub    defined(Mu) returns Bool:D
multi method defined()   returns Bool:D

Returns False on the type object, and True otherwise.

say Int.defined;                # False
say 42.defined;                 # True

Very few types (like Failure) override defined to return False even for instances:

sub fails() { fail 'oh noe' };
say fails().defined;            # False

routine Bool

multi sub    Bool(Mu) returns Bool:D
multi method Bool()   returns Bool:D

Returns False on the type object, and True otherwise.

Many built-in types override this to be False for empty collections, the empty string or numerical zeros

say Mu.Bool;                    # False
say Mu.new.Bool;                # True
say [1, 2, 3].Bool;             # True
say [].Bool;                    # False
say { 'hash' => 'full'}.Bool;   # True
say {}.Bool;                    # False

method Str

multi method Str()   returns Str

Returns a string representation of the invocant, intended to be machine readable. Method Str warns on type objects, and produces the empty string.

say Mu.Str;                     #!> use of uninitialized value of type Mu in string context

routine gist

multi sub    gist(Mu) returns Str
multi method gist()   returns Str

Returns a string representation of the invocant, optimized for fast recognition by humans.

The default gist method in Mu re-dispatches to the perl method for defined invocants, and returns the type name in parenthesis for type object invocants. Many built-in classes override the case of instances to something more specific that may truncate output.

gist is the method that say calls implicitly, so say $something and say $something.gist generally produce the same output.

say Mu.gist;        # (Mu)
say Mu.new.gist;    # Mu.new()

routine perl

multi sub    perl(Mu) returns Str
multi method perl()   returns Str

Returns a Perlish representation of the object (i.e., can usually be re-evaluated with EVAL to regenerate the object). The exact output of perl is implementation specific, since there are generally many ways to write a Perl expression that produces a particular value

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.

class Point2D {
    has ($.x, $.y);
    multi method gist(Point2D:D:) {
        "Point($.x, $.y)";
    }
}

my $p = Point2D.new(x => 2, y => 3);

say $p;                     # Point(2, 3)
say $p.clone(y => -5);      # Point(2, -5)

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.

new triggers an object construction mechanism that calls submethods named BUILD in each class of an inheritance hierarchy, if they exist. See the documentation on object construction for more information.

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.

say Mu.CREATE.defined;  # True

method print

multi method print() returns Bool:D

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

"abc\n".print;          # abc␤

method put

multi method put() returns Bool:D

Prints value to $*OUT after stringification using .Str method adding a newline at end.

"abc".put;              # abc␤

method say

multi method say() returns Bool:D

Prints value to $*OUT after stringification using .gist method with newline at end. To produce machine readable output use .put.

say 42;                 # 42␤

method ACCEPTS

multi method ACCEPTS(Mu:U: $other)

ACCEPTS is the method that smart matching with the infix ~~ operator and given/when invokes on the right-hand side (the matcher).

The Mu:U multi performs a type check. Returns True if $other conforms to the invocant (which is always a type object or failure).

say 42 ~~ Mu;           # True
say 42 ~~ Int;          # True
say 42 ~~ Str;          # False

Note that there is no multi for defined invocants; this is to allow autothreading of junctions, which happens as a fallback mechanism when no direct candidate is available to dispatch to.

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.

say 42.WHICH eq 42.WHICH;       # True

method WHERE

method WHERE() returns Int

Returns an Int representing the memory address of the object.

method WHY

multi method WHY()

Returns the attached Pod value. For instance,

    sub cast(Spell $s)
    #= Initiate a specified spell normally
    #= (do not use for class 7 spells)
    {
	do-raw-magic($s);
    }
    say &cast.WHY;

prints

Initiate a specified spell normally (do not use for class 7 spells)

See the documentation specification for details about attaching Pod to variables, classes, functions, methods, etc.

trait is export

multi sub trait_mod:<is>(Mu:U \type, :$export!)

Marks a type as being exported, that is, available to external users.

my class SomeClass is export { }

A user of a module or class automatically gets all the symbols imported that are marked as is export.

See /language/modules#Exporting_and_Selective_Importing for more details.

method take

method take()

Takes the given item and passes it to the enclosing gather block.

#| randomly select numbers for lotto
my $num-selected-numbers = 6;
my $max-lotto-numbers = 49;
gather for ^$num-selected-numbers {
    take (1 .. $max-lotto-numbers).pick(1);
}.say;    #-> 32 22 1 17 32 9  (for example)

method so

method so()

Returns a Bool value representing the logical non-negation of an expression. One can use this method similarly to the English sentence: "If that is so, then do this thing". For instance,

my @args = <-a -e -b -v>;
my $verbose-selected = any(@args) eq '-v' | '-V';
if $verbose-selected.so {
    say "Verbose option detected in arguments";
} #-> Verbose option detected in arguments

method not

method not()

Returns a Bool value representing the logical negation of an expression. Thus it is the opposite of so.

my @args = <-a -e -b>;
my $verbose-selected = any(@args) eq '-v' | '-V';
if $verbose-selected.not {
    say "Verbose option not present in arguments";
} #-> Verbose option not present in arguments

Since there is also a prefix version of not, the above code reads better like so:

my @args = <-a -e -b>;
my $verbose-selected = any(@args) eq '-v' | '-V';
if not $verbose-selected {
    say "Verbose option not present in arguments";
} #-> Verbose option not present in arguments