From: ptkwt@... (Phil Tomson)
Date: 2005-03-29T16:49:49+09:00
Subject: Re: User defined loops (applicable to DSLs)

In article <d29fv40210q@enews4.newsguy.com>,
Phil Tomson <ptkwt@aracnet.com> wrote:
>In article <Pine.LNX.4.21.0503280111230.10981-100000@frieza>,
>Relm  <relm@3tlk.net> wrote:
>>On Mon, 28 Mar 2005, Phil Tomson wrote:
>>
>>> Actually, I figured out a way to make it much more natural:
>>> 
>>> Change Whiler to accept an optional condition in it's constructor like 
>>> so:
>>> 
>>>   class Whiler
>>>     def initialize(cond=nil)
>>>       @ret = nil
>>>       @cond = cond
>>>     end
>>>     def while(cond=@cond)
>>>       break @ret unless cond
>>>       @ret = yield
>>>       retry
>>>     end
>>>   end
>>> 
>>> 
>>> Define the my_while top-level method like so:
>>> 
>>>   def my_while(cond,&block)
>>>     Whiler.new(cond).method(:while).call(&block)
>>>   end
>>
>>This new my_while doesn't seem to return the last block.  The following
>>snippet outputs nil values instead of "outer" and "inner":
>>
>>  i = 0
>>  x = nil
>>  y = nil
>>
>>  x = my_while(i<10) do
>>    j=0
>>    y = my_while(j<10) do
>>      j+=1
>>      "inner"
>>    end
>>    i+=1
>>    "outer"
>>  end
>>
>>  puts x
>>  puts y
>>
>
>Hmmm...  I'm seeing the  same thing you are, but I'm not sure why.  
>Anyone got any ideas?

Ah, I see the problem now.  my_while calls Whiler.new every time it is 
called of course, thus creating a new Whiler object each time.  The last 
time through each loop, the condition is false when Whiler.new is 
called and thus @ret is nil because my_while returns before getting to 
the '@ret = yield'.  Definately problematic.  You can see the problem by 
adding a 'puts' to the Whiler constructor like so:

  class Whiler
    def initialize(cond=nil)
      puts "Whiler::new(#{cond})"
      @ret = nil
      @cond = cond
    end
  end


>>This makes your original my_while thread-safe.  It was already safe for
>>nesting.
>>
>>  def my_while(cond)
>>    @ret ||= {}
>>    key = Thread.current
>>    return @ret.delete(key) unless cond
>>    @ret[key] = yield
>>    retry
>>  end
>>
>

This is probably the best bet for being both threadsafe and nestable.

Phil