From: Brian Candler <b.candler@...>
Date: 2009-04-15T21:19:48+09:00
Subject: Re: Proc.call with custom bindings

Robert Klemme wrote:
>> The reason I am asking, is that I am thinking about improving
>> performance of template engines that generate Ruby code upon parsing
>> template, and then call eval(@precompiledsrc) upon each render of
>> template.
> 
> Why would anyone want to do that?

I think he's saying that he had found a template engine which stored the 
string and eval'd it each time it needed to be rendered; he wanted to 
improve on that situation.

I think vanilla ERB falls into that category.

irb(main):001:0> require 'erb'
=> true
irb(main):002:0> e = ERB.new("<%= foo %>")
=> #<ERB:0xb7c90498 @src="_erbout = ''; _erbout.concat(( foo ).to_s); 
_erbout", @filename=nil, @safe_level=nil>
irb(main):003:0> lambda { foo = 123; e.result(binding) }.call
=> "123"
irb(main):004:0> lambda { foo = 456; e.result(binding) }.call
=> "456"

Source of ERB#result:

  def result(b=TOPLEVEL_BINDING)
    if @safe_level
      th = Thread.start {
        $SAFE = @safe_level
        eval(@src, b, (@filename || '(erb)'), 1)
      }
      return th.value
    else
      return eval(@src, b, (@filename || '(erb)'), 1)
    end
  end
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