Asp Forum - Syntax sugar: method[](...) => method(...

Erwin Abbott

6/29/2007 8:17:00 PM

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).

Lastly, has there been any mention of allowing function names like so
in ruby 1.9:
def logins[] date
# ...
end

Thanks,
- Erwin

3 Answers

Trans

6/30/2007 8:49:00 AM

On Jun 29, 4:17 pm, "Erwin Abbott" <erwin.abb...@gmail.com> 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).

def logins
@_logins ||= Proc.new do |date|
@events.select{|e| e.action == :login and e.date == date}
end
end

However, I feel you are limiting yourself. Try something like this
instead:

def logins
@_logins ||= (
l = @events.select{|e| e.action == :login}
def l.by_date(date)
select{|e| e.date == date}
end
l
)
end

Then you can use:

logins.by_date(date)

If needed, you could change #logins to lazy evaluate too.

T.

Robert Klemme

7/9/2007 8:55:00 PM

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

Erwin Abbott

7/10/2007 4:53:00 AM

Hi Logan,

Really cool stuff going on there, that was worth a thousand words. I
never knew about this UnboundMethod#bind thing, curious if Proc has a
similar mechanism. Also happy to learn Method#[] is an alias to
Method#call

Let me summarize what's going on here to make sure I understand. First
we're creating an UnboundMethod from the given block. Next we create
the named method, which returns a blank object with a [] method that
calls the block. Okay, that was simpler than I thought!

One thing I'll be trying to work out is calling the method "logins"
will return a plain Object instead of a list of @events that are of
the type :login. I think for my purposes defining a few more singleton
methods like obj.each and obj.size would be good enough.

On 7/9/07, Logan Capaldo <logancapaldo@gmail.com> wrote:
> class Module
> def bracket(name, &code)
> define_method("#{name}_bracketed", &code)
> m = instance_method("#{name}_bracketed")
> remove_method "#{name}_bracketed"
>
> define_method(name) do ||
> obj = Object.new
> m1 = m.bind(self)
> (class << obj; self; end).module_eval { define_method(:[]) { |x|
> m1[x] } }
> obj
> end
> end
> end
>
>
> class Foo
> bracket :logins do |date|
> @data.select { |e| e.date == date }
> end
> end
>
> Who needs sugar?
>
> (I so didn't test this at all)

Thanks,
- Erwin

comp.lang.ruby

Syntax sugar: method[](...) => method(...

Erwin Abbott

Trans

Robert Klemme

Erwin Abbott

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