From: Thomas Gantner <thomas.gantner@...>
Date: 2007-08-24T08:40:08+09:00
Subject: accessor_writer methods and instance_eval

Hi

I stumbled upon a problem with 'instance_eval' (using ruby 1.8.5 on
linux-x86_64). Simplified code example:

  irb:01> S = Struct.new(:a)
  => S
  irb:02> s = S.new(1)
  => #<struct S a=1>
  irb:03> s.a
  => 1
  irb:04> s.a = 2
  => 2
  irb:05> s 
  => #<struct S a=2>
  irb:06> s.instance_eval { puts a }
  2
  => nil
  irb:07> s.instance_eval { a = 3 }
  => 3
  irb:08> s
  => #<struct S a=2>
  irb:09> s.instance_eval { self.a = 3 }
  => 3
  irb:10> s
  => #<struct S a=3>

My problem is in line 07: it seems that the 'a' in the block is treated as a
local variable, therefore all the writer-accessor methods ('a=') are hidden
within the block. I get the same result when instance_eval-ing the
string "a = 3" instead of the block variant.

Is there a way to avoid this behaviour without using 'self.a' or passing the
struct as a parameter into the block (which quite obsoletes the meaning of
instance_eval)?

My real world use of this pattern is using a block with 'initialize()':

  class C
    def initialize(&blk)
      @format = Struct.new( ... many, many elements with default values ...)
      @format.instance_eval(&blk)
    end
  end

To explicitly override some default format-values I want to use

  c = C.new { elem8 = 8; elem17 = 'test'; ... }

This won't work as explained above. I know I can use

  c = C.new {|format| format.elem8 = 8; format.elem17 = 'test'; ... }

or

  c = C.new { self.elem8 = 8; self.elem17 = 'test'; ... }

but both seem somewhat redundant and need too much typing (yeah I know I'm
lazy ;-).

Thanks for any suggestions.

-Thomas

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