From: Stefano Crocco <stefano.crocco@...>
Date: 2010-07-18T01:09:07+09:00
Subject: Re: Instance eval in 1.8 and 1.9

On Saturday 17 July 2010, Gavin Sinclair wrote:
> |Hi,
> |
> |Porting a program from 1.8 to 1.9, I hit a stumbling block in the use
> |of instance_eval.
> |
> |The following code works in 1.8 but not 1.9.  instance_eval in 1.9
> |does not allow you to use a lambda as a block.  The solution is to use
> |a proc instead.  I understand that lambdas are like methods and procs
> |are like blocks, but what gets me is the incredibly unhelpful error
> |message.
> |
> |Anyway, the code (can also see it at http://gist.github.com/479572 )
> |
> |  class InstanceEval
> |    def initialize(x)
> |      @block = x
> |      @context = Object.new
> |    end
> |
> |    def run
> |      code = @block
> |      result = @object.instance_eval(&code)
> |      "* run --> #{result.inspect}"
> |    end
> |  end
> |
> |  p = proc { :p }
> |  l = lambda { :l }               # (line 16 -- for the error message
> |below)
> |
> |  puts InstanceEval.new(p).run
> |  puts InstanceEval.new(l).run
> |
> |
> |The output in 1.8:
> |
> |  * run --> :p
> |  * run --> :l
> |
> |The output in 1.9:
> |
> |  * run --> :p
> |  code.rb:16:in `block in <main>': wrong number of arguments (1 for 0)
> |(ArgumentError)
> |          from code.rb:10:in `instance_eval'
> |          from code.rb:10:in `run'
> |          from code.rb:19:in `<main>'
> |
> |To what method have I passed one argument where zero arguments were
> |expected?
> |
> |Is the average Ruby programmer supposed to know that instance_eval
> |can't take a lambda in Ruby 1.9?  Perhaps it should say so in the
> |documentation.  (http://bit.ly/96HBeb)
> |
> |Is there a good reason ( instance_eval &lambda_object ) can't work?
> |
> |Thanks for any light you can shed on it.
> |
> |Gavin

Your code runs without errors for me with ruby 1.9.1-p376.

Stefano