From: David Alan Black <dblack@...>
Date: 2002-01-31T10:55:23+09:00
Subject: Re: Newbie question about mixins

Hello --

Welcome to Ruby :-)

On Thu, 31 Jan 2002, Mark Wilson wrote:

> I have a module that I want to use as a mixin.  I have defined a method
> within the module that attempts to assign a value to a hash 'slot':
>
>     module myMod
>         def mixinMethod(param1, param2)
>             @hashVar[param1] = param2
>         end
>     end

My Ruby balks at being asked to call a module "myMod"....

  dblack@laptop:~$ ruby -ve 'module myMod; end'
  ruby 1.6.5 (2001-09-19) [i686-linux]
  -e:1: class/module name must be CONSTANT

(You need a constant identifier, which begins with an uppercase
letter.)

> I include this module into my class, and call 'mixinMethod'.  I get a
> NameError because, apparently, I have not told the program that 'hashVar' is
> a hash table.  Where should I do a Hash.new() or other initialization?

Here's one pretty common idiom for telling Ruby that you only want to
assign something to a variable if that variable is uninitialized
(well, technically, if that variable evaluates to "false" in a Boolean
sense, which covers the case of uninitialized variables):

  h ||= Hash.new

which is shorthand for:

  h = h || Hash.new

Once h is initialized, this essentially functions as "h = h".


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav