From: Alex Young <alex@...>
Date: 2007-11-14T19:47:17+09:00
Subject: Re: recursion with blocks

Douglas A. Seifert wrote:
> Pat Maddox wrote:
>> On Nov 13, 2007 4:15 PM, James Edward Gray II 
>> <james@grayproductions.net> wrote:
>>  
>>> On Nov 13, 2007, at 6:01 PM, Mike Perham wrote:
>>>
>>>    
>>>> You aren't actually recursing inside the block,
>>>> just using the block as a conditional.  I was trying to figure out how
>>>> to recurse inside the block.
>>>>       
>>> Yeah, I was just using the block inside a recursive method.  You are
>>> right.
>>>
>>> Can you elaborate on what you mean by "recurse inside a block?"  I'm
>>> having trouble thinking of an example.
>>>
>>> James Edward Gray II
>>>
>>>
>>>     
>>
>> I think he means something like this?
>>
>> fact = lambda {|val| return 1 if val <= 0; val * fact[val - 1] }
>> fact[4]  => 24
>>
>> but I'm not sure...
>>
>> Pat
>>
>>   
> This works:
> 
> irb(main):001:0> fact = lambda {|val| val <= 0 ? 1 : val * fact[val-1] }
<snip>

Another example I coincidentally came up with for another thread, 
looking at how the stack is printed on 1.8 and 1.9:

$ ruby -e "a = Proc.new{|d| raise 'foo' if d==0; a.call(d-1)}; a.call(20)"
-e:1: foo (RuntimeError)
         from -e:1:in `call'
         from -e:1
         from -e:1:in `call'
         from -e:1
         from -e:1:in `call'
         from -e:1
         from -e:1:in `call'
         from -e:1
          ... 30 levels...
         from -e:1:in `call'
         from -e:1
         from -e:1:in `call'
         from -e:1

-- 
Alex