Traits — Meta Data With Character

Traits are a nice, extensible way to attach meta data to all sorts of objects in Perl 6.

An example is the is cached trait that automatically caches the functions return value, based on the argument(s) passed to it.

Here is a simple implementation of that trait:

 # this gets called when 'is cached' is added
 # to a routine
 multi sub trait_mod:<is>(Routine $r, :$cached!) {
     my %cache;
     #wrap the routine in a block that..
     $r.wrap(-> $arg {
         # looks up the argument in the cache
         %cache{$arg}:exists
             ?? %cache{$arg}
             # ... and calls the original, if it
             # is not found in the cache
             !! (%cache{$arg} = callwith($arg))
         }
     );
 }
 
 # example aplication:
 sub fib($x) is cached {
     say("fib($x)");
     $x <= 1 ?? 1 !! fib($x - 1) + fib($x - 2);
 }

 # only one call for each value from 0 to 10
 say fib(10);

A trait is applied with a verb, here is. That verb appears in the routine name that handles the trait, here trait_mod:<is>. The arguments to that handler are the object on which the trait is applied, and the name of the trait (here cached) as a named argument.

Note that a production grade is cached would need to handle multiple arguments, and maybe things like limiting the cache size.

In this example, the .wrap method is called on the routine, but of course you can do whatever you want. Common applications are mixing roles into the routine or adding them to a dispatch table.

Traits can not only be applied to routines, but also to parameters, attributes and variables. For example writable accessors are realized with the is rw trait:

 class Book {
     has @.pages is rw;
     ...
 }

Traits are also used to attach documentation to classes and attributes (stay tuned for an addvent calendar post on Pod6), marking routine parameters as writable and declaring class inheritance and role application.

This flexibility makes them ideal for writing libraries that make the user code look like a domain-specific language, and supplying meta data in a safe way.