Perl 6 and Parrot Essentials, Second Edition

Multiple Dispatch

In the previous chapter, we mentioned multi subroutines. The multi keyword actually applies to any code object: subroutines, methods, or submethods. As we said before, multi allows you to define multiple, different routines all with the same name but different signatures. This example dispatches to a variant of the lunch method depending on the types of the arguments:

multi method lunch (Lunching::Friar $who, Megadodo::Office $location) {
    print "Jolly nice restaurant.";
}

multi method lunch (Hitchhiker $who, Cargo::Hold $location) {
    print "Towel again.";
}

A member of the Lunching Friars of Voondon must always eat at a nice restaurant when he visits the offices of Megadodo Publications. A hitchhiker in a cargo hold, however, will just have to settle for the nutrient solution soaked into the corner of his towel.

A call to a multimethod has the same syntax as a call to a subroutine—the name of the routine followed by a list of arguments:

lunch($zaphod, $where);

This call searches outward through its lexical, package, and global scopes for a matching name. If it finds a nonmulti sub it makes an ordinary subroutine call. Otherwise, it generates a list of multi subs, methods, or submethods with that name and dispatches to the closest matching signature.(For more complete details on the dispatch process, see Apocalypse 12.)

You can also call a multimethod with an ordinary single-dispatch method call:

$zaphod.lunch($where);

In this case, the call will only failover to ...