From: Joel VanderWerf <vjoel@...>
Date: 2009-07-06T07:45:50+09:00
Subject: Re: to_proc and Proc/block conversion with &

Russ McBride wrote:
> Thanks.  Interesting discussion.
> 
>> given &expr, if expr is a Proc object, use it as
>> the block; if not, call to_proc on it.
> ... and then use the resulting proc as a block.
> 
> Maybe the best way to think of & is a a proc/block converter, which,
> if called on something that is neither looks first for a to_proc method 
> before
> performing its conversion.

That still makes & sound like an operator method that can be called on 
an object to produce a different object, though. Blocks are syntactic 
entities, Procs are semantic entities (a.k.a. values, objects), and & is 
a way of relating them. The conversion from one class to another is kind 
of distracting here, because it really only needs to happen in the 
special cases like &:sym .

The & is really more like variable assignment ( = ) in ruby rather than 
an operator, in that '=' affects bindings (another syntax-semantics 
relation) but does not operate on objects. This is something that often 
trips up people coming to ruby from C et al.

>> The only use I can think of for this feature[1] is if you want to 
>> conditionally instantiate the Proc, as in:
>>
>>  def defer time
>>    if time < Time.now
>>      yield
>>    else
>>      pr = Proc.new
>>      # store pr somewhere and schedule it for execution
>>    end
>>  end
> 
> But, you could just do something like this, right?
> 
>  def defer(time, &some_block)
>    if time < Time.now
>      yield   # or some_block.call
>    else
>      pr = some_block
>      # store pr somewhere and schedule it for execution
>    end
>  end

Sure, that works, but it always instantiates a Proc (the &some_block 
does that), even if the yield branch is taken. It's not a big 
difference, since instantiation and GC are not usually bottlenecks, but 
it's still something to be aware of.

> Cheers,
> Russ
> 
> P.S.  Joel, I seen now why you were inclined toward Rhodes.  :-)

Ruby everywhere :-)

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407