From: Robert Klemme <bob.news@...>
Date: 2005-03-27T03:34:48+09:00
Subject: Re: Evaluator for a mini-Ruby in Haskell


"Phil Tomson" <ptkwt@aracnet.com> schrieb im Newsbeitrag 
news:d22ob001uag@enews1.newsguy.com...
> In article <1111806229.622292.217680@l41g2000cwc.googlegroups.com>,
> Daniel Berger <djberg96@hotmail.com> wrote:
>>Maybe I need to rethink my view of Haskell after all:
>>
>>http://community.moertel.com/ss/space/start/2005-03-25/1#Writing_a_simple_Ruby_evaluator
>>
>
> Interesting.
>
> Anyone care to comment on the analysis of 'retry' there?  He presents the
> following code for a do-it-yourself while loop:
>
>  def my_while(cond)
>    break unless cond
>    yield
>    retry
>  end
>
>  i = 0 my_while i < 10 do print i i += 1 end
>
>  # output: 0123456789
>
>
> And offers this analysis:
>
> "This is where the Pickaxe II let me down. It said, "retry will
> reevaluate
> any arguments to the iterator before restarting it." Yes, clearly, that
> is what is happening. But how is it happening and what exactly does that
> simple English statement really mean?
>
> So, after thinking about it, I concluded that what is going on is that a
> function call in Ruby works like this. Given a function f, a block b, and
> arguments xs, the call f(xs){b} means this:
>
>   1. let k be the current continuation (i.e., just before the call)
>   2. bind xs to f's formal arguments
>   3. bind b internally to the current block
>   4. evaluate the body of f
>
> Now, if inside of f's body we encounter a retry, the evaluator basically
> calls k (with a nil argument, I expect). This jumps back to step 2, from
> which evaluation continues. Any side effects up to this point are
> retained (so we could have previously incremented i, for example), which
> is what allows the code within the function body eventually to choose an
> execution path which does not contain a retry expression, and thus avoid
> looping forever."

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
>> def my_while(cond)
>>   return 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
=> nil

ruby 1.8.1

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...

Kind regards

    robert