From: ptkwt@... (Phil Tomson)
Date: 2005-03-28T03:24:48+09:00
Subject: Re: Evaluator for a mini-Ruby in Haskell

In article <3anjpeF6d91alU1@individual.net>,
Robert Klemme <bob.news@gmx.net> wrote:
>
>"Phil Tomson" <ptkwt@aracnet.com> schrieb im Newsbeitrag 
>news:d24b8d02fq1@enews4.newsguy.com...
>> In article <3alo9bF6d2vo8U1@individual.net>,
>> Robert Klemme <bob.news@gmx.net> wrote:
>>>
>>>
>>>I'm surprised about "retry", too.  His analysis sounds all very 
>>>resonable -
>>>only that "break" does not work for me but "return" does:
>>>
>>>>> def my_while(cond)
>>>>>   break unless cond
>>>>>   yield
>>>>>   retry
>>>>> end
>>>=> nil
>>>>> i = 0
>>>=> 0
>>>>> my_while i < 10 do
>>>?>   puts i
>>>>>   i += 1
>>>>> end
>>>0
>>>1
>>>2
>>>3
>>>4
>>>5
>>>6
>>>7
>>>8
>>>9
>>>LocalJumpError: unexpected break
>>>        from (irb):2:in `my_while'
>>>        from (irb):7
>>
>>
>> Hmmm...  break works find for me in 1.8.2.  Time to upgrade?
>
>Yeah, maybe.
>
>>>Btw, can anybody think of a way to make my_while return the result of the
>>>last block evaluation?  It seems impossible because the return occurs 
>>>before
>>>the yield...
>>>
>>
>>
>> How about:
>>
>>  def my_while(cond)
>>    return @ret unless cond
>>    @ret = yield
>>    retry
>>  end
>
>Not thread safe.  And I think also not nesting safe.
>

Yes, I know.  Did you see my later post?  I don't think it made it from 
the newsgroup to the mailing list.

Here's the code:

  class Whiler
    def initialize
      @ret = nil
    end

    def while(cond)
      break @ret unless cond
      @ret = yield
      retry
    end
  end

  outer = Whiler.new
  inner = Whiler.new

  i=0
  outer.while i<5 do
    puts "i: #{i}"
    j=0
    inner.while j<5 do
      puts "  j: #{j}"
      j+=1
    end
    i+=1
  end

A bit cumbersome, perhaps, but it is definately nestable and should be 
threadsafe as well.

Phil