[Pdx-pm] Poll: traits

chromatic chromatic at wgz.org
Sat Nov 19 11:22:41 PST 2005

On Sat, 2005-11-19 at 10:54 -0800, Eric Wilhelm wrote:

> "A trait is essentially a group of pure methods that serves as a 
> building block for classes and is a primitive unit of code reuse."
> So, can you spell that in Perl?  It sounds suspiciously like it's 
> nothing more than a module that allows you to import some methods into 
> your namespace.  Or is that a mixin?

It's a lot more than a mixin.  It's an abstraction that gives a
semantically useful name to a group of methods.

Occasionally people show up in Perl QA to ask "I have these functions
that print their output and I want to collect that output instead!"  The
answer is somewhere between select(), tie(), and IO::Handle or another
module.  You really have to know what you're doing to make it work.

You also could define a trait for an appendable collection of lines.
That might represent a file (you can print to it) or an array (same) or
a log (if it expires the oldest content) or a cache (the same).

If every place you want to append a line to something you check that
what you're going to use does that trait -- instead of being a
filehandle or an array or however it implements that trait -- you get a
lot more flexibility.  In particular, you're checking for the
appropriate *intended behavior* of an object, not its implementation.

Likewise you could have a KeyedLookup trait for a hash or a cache or a
remote object storage system or whatever.

The important thing is the name of the collection of methods.  You can't
just check that the object you have has a method of the right name --
there are too many homonyms possible for that to work reliably.  It's
the collection of methods under a trait name that makes this useful.

-- c

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