Perl 6 FAQ - Multiple Dispatch

This FAQ is part of the Programmer's Heaven Perl 6 FAQ. It discusses multiple dispatch, including multi-subs and multi-methods.

What is multiple dispatch?

Multiple dispatch, new in Perl 6, is somewhat similar to the idea of method overloading in C# and Java. You can write multiple subroutines or methods with the same name, but with different signatures. Then, when the method is called, the number of variables being passed and their types are used to choose the correct subroutine or method to call, provided an appropriate method exists.

Note that all subroutines (or methods) with the given name must be marked with the "multi" keyword; it is not possible to have an existing subroutine or method and then add another with a different signature and just mark the one that was added as "multi". Single dispatch is the default.

Unlike static overloading in Java and C#, the decision of which method to call is made at run time. With static overloading, it is determined at compile time. Making the decision at run time introduces some overhead to the call, but clever caching techniques can help to mitigate this. It is not possible to make the decision at compile time, since new methods may be added to classes at runtime, inheritance hierarchies may change and other such evil fun.

How do I write multi-subs?

To specify that a call to a sub should do multiple dispatch, add the keyword "multi" before "sub". The following example demonstrates defining two multi-subs, both named "what_is_it" but taking parameters of different types. They are then called with parameters of different types, and the results recorded in the comment next to them.

multi sub what_is_it(Int $x) {
    say "This is the integer $x";
}
multi sub what_is_it(Str $s) {
    say "This is the string $s";
}
what_is_it(42);       # This is the integer 42
what_is_it("weasel"); # This is the string weasel

As can be seen, when an integer is passed the multi-sub taking an integer is called; when a string is passed, the multi-sub taking a string is called.

How do I write multi-methods?

Multi-methods are written by placing the "multi" keyword before "method". the main difference between multi-methods and multi-subs is that multi-methods always dispatch first like a normal method call (that is, based upon the type of the invocant). Then the possible multi-methods that could be called are searched and the most appropriate one is called.

This example is very similar to the subs one, but uses methods instead.

class Test {
    multi method what_is_it(Int $x) {
        say "This is the integer $x";
    }
    multi method what_is_it(Str $s) {
        say "This is the string $s";
    }
}
my Test $t .= new();
$t.what_is_it(42);       # This is the integer 42
$t.what_is_it("weasel"); # This is the string weasel

How do I state that only some parameters should be used for multiple dispatch?

It is possible to denote that only some of the parameters of a multi sub or multi method should be considered when doing multiple dispatch. This is indicated by placing a double semicolon after the last parameter that should be considered during dispatch.

multi sub write_to_file(str $filename ;; int mode, str $text) {
}
multi sub write_to_file(str $filename ;; str $text) {
}

Here only the first parameter will be considered when building a candidate list of subroutines that may be called. This means that the above two subroutines would always be in conflict (so in reality you could never want to write this example); without the ;; they would not be as the conflict could be resolved by considering number and types of other parameters.

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riya From India (Report as abusive)	"Very Useful" this faqs made to know about many things unknown and it helped me a lot
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