From: Joel VanderWerf <vjoel@...>
Date: 2004-06-02T08:01:00+09:00
Subject: Re: Blocks and Closures

Gavin Sinclair wrote:
> On Wednesday, June 2, 2004, 6:38:19 AM, Joel wrote:
> 
> 
>>You're right, there's no reason to use the &bl notation. You can use the
>>Kernel#block_given? method:
> 
> 
> 
> Just a note for the OP.  The following example shows the advantage of
> the &block notation in certain circumstances.
> 
>   def bar
>     return 5 + yield
>   end
> 
>   def foo(&block)
>     bar(block)
>   end
> 
> 'foo' is now a proxy for 'bar'.

Slight nit: 'bar(block)' should be 'bar(&block)', if you want to 
propagate the block.

Actually, though, there is a more efficient way to propagate a block:

   def bar
     return 5 + yield
   end

   def foo
     bar {yield}
   end

   p foo {3}    # ==> 8

I guess it's more efficient because no Proc object is created. This 
technique doesn't work if you need to store the block somewhere for 
later access. Then you really do need to use & or Proc.new.

Here's some benchmarking (ruby-1.9.0):

require 'benchmark'

def outer11(&bl)
   inner1(&bl)
end

def outer12(&bl)
   inner2(&bl)
end

def outer21
   inner1 {yield}
end

def outer22
   inner2 {yield}
end

def inner1(&bl)
   bl.call
end

def inner2
   yield
end

n = 100000

Benchmark.bmbm(10) do |rpt|
   rpt.report("outer11") do
     n.times {outer11{}}
   end

   rpt.report("outer12") do
     n.times {outer12{}}
   end

   rpt.report("outer21") do
     n.times {outer21{}}
   end

   rpt.report("outer22") do
     n.times {outer22{}}
   end
end

__END__

Output:

Rehearsal ---------------------------------------------
outer11     1.850000   0.010000   1.860000 (  1.856130)
outer12     1.600000   0.000000   1.600000 (  1.600134)
outer21     2.210000   0.000000   2.210000 (  2.214836)
outer22     0.520000   0.000000   0.520000 (  0.526088)
------------------------------------ total: 6.190000sec

                 user     system      total        real
outer11     1.870000   0.000000   1.870000 (  1.871797)
outer12     1.620000   0.000000   1.620000 (  1.623072)
outer21     2.230000   0.000000   2.230000 (  2.236854)
outer22     0.500000   0.000000   0.500000 (  0.501914)