From: Robert Klemme <shortcutter@...>
Date: 2007-07-11T22:19:14+09:00
Subject: Re: Syntax sugar: method[](...) => method(...)

On 29.06.2007 22:17, Erwin Abbott wrote:
> Hi,
> 
> The [] method is really handy when you have an object that should
> behave like a hash or array. One thing I find myself wishing I could
> also do is something like
> 
> def logins[](date)
>  @events.select{|e| e.action == :login and e.date == date}
> end
> 
> There may be a better way to deal with this example, but this is just
> to demonstrate the circumstances where you don't have an array or hash
> called logins, but you'd like to be able to use that [] syntax. One
> way I though about doing it is
> 
> def logins
>  EventList.new @events.select{|e| e.action == :login}
> end
> 
> class EventList
>  def initialize data
>    @data = data
>  end
>  def [] date
>    @data.select{|e| e.date == date}
>  end
> end
> 
> logins[Date.today] # => all logins today from @events
> 
> I guess some people might prefer to write a method like
> get_logins(date), but I like this syntax.  It seems reasonable to also
> define []= in EventList to update the original @events. What I'd like
> to know is if there's another way to solve this problem, or if there
> are any simple optimizations (maybe only keep one instance of
> EventList and update its contents with each call to #events).

It's pretty easy to achieve what you want:

irb(main):001:0> class Foo
irb(main):002:1> def logins
irb(main):003:2> x = %w{a b c}
irb(main):004:2> def x.[](a)
irb(main):005:3> select {|e| e == a}
irb(main):006:3> end
irb(main):007:2> x
irb(main):008:2> end
irb(main):009:1> end
=> nil
irb(main):010:0> f=Foo.new
=> #<Foo:0x7ff7abc8>
irb(main):014:0> f.logins["w"]
=> []
irb(main):015:0> f.logins["a"]
=> ["a"]

Here's another variant:

irb(main):016:0> class Bar
irb(main):017:1> def logins
irb(main):018:2> x = %w{a b c}
irb(main):019:2> lambda {|a| x.select {|e| e == a}}
irb(main):020:2> end
irb(main):021:1> end
=> nil
irb(main):022:0> b=Bar.new
=> #<Bar:0x7ff5bfe8>
irb(main):023:0> b.logins["a"]
=> ["a"]
irb(main):024:0> b.logins["w"]
=> []

I.e. you just create a proxy object that will handle the [] call. 
However I am not sure it's a good idea - it's probably better to return 
the original array or a copy of it.

Kind regards

	robert