Documentation for routine map

Documentation for routine map, assembled from the following types:

class Any

From Any

(Any) method map

Defined as:

multi method map(\SELF: █; :$label:$item)
multi method map(HyperIterable:D: █; :$label)

map will iterate over the invocant and apply the number of positional parameters of the code object from the invocant per call. The returned values of the code object will become elements of the returned Seq.

The :$label and :$item are useful only internally, since for loops get converted to maps. The :$label takes an existing Label to label the .map's loop with and :$item controls whether the iteration will occur over (SELF,) (if :$item is set) or SELF.

class List

From List

(List) routine map

Defined as:

sub    map(&code*@elems --> Seq:D)
method map(List:D: &code --> Seq:D)

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

say ('hello'122/742'world').map: { .WHAT.perl } # OUTPUT: «(Str Int Rat Int Str)␤» 
say map *.Str.chars'hello'122/742'world';     # OUTPUT: «(5 1 8 2 5)␤» 

map inspects the arity of the code object, and tries to pass as many arguments to it as expected:

sub b($a$b{ "$a before $b" };
say <a b x y>.map(&b).join('');   # OUTPUT: «a before b, x before y␤» 

iterates the list two items at a time.

Note that map does not flatten embedded lists and arrays, so

((12), <a b>).map({ .join(',')})

passes (1, 2) and <a b> in turn to the block, leading to a total of two iterations and the result sequence "1,2", "a,b". See method flatmap for an alternative that flattens.

If &code is a Block loop phasers will be executed and loop control statements will be treated as in loop control flow. Please note that return is executed in the context of its definition. It is not the return statement of the block but the surrounding Routine. Using a Routine will also handle loop control statements and loop phasers. Any Routine specific control statement or phaser will be handled in the context of that Routine.

sub s {
    my &loop-block = {
        return # return from sub s 
    say 'hi';
    (1..3).map: &loop-block;
    say 'oi‽' # dead code 
# RESULT: «hi» 

class Supply

From Supply

(Supply) method map

method map(Supply:D: &mapper --> Supply:D)

Returns a new supply that maps each value of the given supply through &mapper and emits it to the new supply.

my $supplier =;
my $all      = $supplier.Supply;
my $double   = $> $value { $value * 2 });
$supplier.emit(4);           # RESULT: «8»