From: ptkwt@... (Phil Tomson)
Date: 2005-03-29T03:09:56+09:00
Subject: Re: User defined loops (applicable to DSLs)

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?

>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
>

That'll work.

Phil