From: Joel VanderWerf <vjoel@...>
Date: 2007-09-16T08:42:50+09:00
Subject: Re: one line to print the statement AS WELL AS the evaluated value like in C

Summercool wrote:
> 
> On Sep 15, 10:49 am, Joel VanderWerf <vj...@path.berkeley.edu> wrote:
>> kendear wrote:
>>> i wonder in Ruby, is there a line method to do something like in C
>> def dbg(&bl)
>>    s = yield
>>    puts "#{s} = #{eval(s, bl).inspect}"
>> end
>>
>> array=[0,1,2,3]
>> dbg{"array[3]+1"}  # ==> array[3]+1 = 4
>>
>> It seems a little awkward to use both {...} and "..." around the
>> expression, but the {} allows the #dbg method to capture the binding of
>> the caller.
> 
> It works!  both for globals and for instance variables...
> but what is this call   dbg{"n"}  using  {  } instead of  (  )
> does it have a name?  and does any book talk about it?

The {..} is just ruby's block notation, just as in "each {...}"

> i guess there is no way to do
> 
> dbg(array[3] + 1)  or   dbg{array[3] + 1}  without quoting it as
> string huh?   C was able to do it due to the preprocessor.

Not easy.

Another advantage of using the block notation is that you can disable 
evaluation with a global flag:

def dbg
   if $debug
     s = yield
     puts "#{s} = #{eval(s, Proc.new).inspect}"
   end
end

If the flag is set, the dbg call is just a method call and a variable 
test. It doesn't even instantiate a Proc (note that I removed the &bl 
and added Proc.new).

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