From: Gavin Sinclair <gsinclair@...>
Date: 2010-07-18T00:25:18+09:00
Subject: Instance eval in 1.8 and 1.9

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