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Perl 6 and Parrot Essentials, Second Edition by Leopold Tötsch, Dan Sugalski, Allison Randal

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Arguments

The standard way of passing arguments is by position. The first argument passed in goes to the first parameter, the second to the second, and so on:

sub matchparams ($first, $second) { . . . }

matchparams($one, $two);  # $one is bound to $first
                          # $two is bound to $second

Named Argument Passing

You can also pass arguments in by name, using a list of anonymous pairs. The key of each pair gives the parameter’s name and the value of the pair gives the value to be bound to the parameter. When passed by name, the arguments can come in any order. Optional parameters can be left out, even if they come in the middle of the parameter list. This is particularly useful for subroutines with a large number of optional parameters:

sub namedparams ($first, ?$second, ?$third is rw) { . . . }

namedparams(third => 'Trillian', first => $name);

Sometimes the option syntax for pairs is clearer than the pair constructor syntax:

namedparams :third('Trillian'), :first($name);

Flattening Arguments

To get the Perl 5-style behavior where the elements of an array (or the pairs of a hash) flatten out into the parameter list, use the flattening operator in the call to the subroutine. Here, $first binds to @array[0] and $second binds to @array[1]:

sub flat ($first, $second) { . . . }

flat(*@array);

A flattened hash argument acts as a list of pairs, which are bound to the parameters just like ordinary named arguments. So, $first is bound to %hash{'first'}, and $second is bound to %hash{'second'}:

sub flat_hash ...

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