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 ...
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