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copied!<p>You can think of blocks as closing over both the set of local variables and the current <code>self</code>. </p> <p>In Ruby, you will <strong>always</strong> have access to local variables, no matter what. The <code>self</code> encapsulates instance methods on the current object as well as instance variables.</p> <p>Consider the following code:</p> <pre><code>class Table def initialize(legs) @legs = legs end def with_legs yield @legs end end </code></pre> <p>And then:</p> <pre><code>def some_calling_method name = "Yehuda" Table.new(4) {|legs| puts "#{name} gnaws off one of the #{legs} legs" } end </code></pre> <p>By Ruby's block semantics, you can be assured that <code>name</code> will be available inside the block, even without looking at the method you're calling.</p> <p>However, consider the following:</p> <pre><code>class Person def initialize(name) @name = name end def gnaw Table.new(4).with_legs do |legs| puts "#{@name} gnaws off one of the #{legs} legs" end end end Person.new("Yehuda").gnaw </code></pre> <p>In this case, we are accessing the <code>@name</code> instance variable from inside the block. It works great in this case, but is not guaranteed. What if we implemented table a bit differently:</p> <pre><code>class Table def initialize(legs) @legs = legs end def with_legs(&block) self.instance_eval(&block) end end </code></pre> <p>Effectively, what we're saying is "evaluate the block in the context of a <strong>different</strong> self." In this case, we are evaluating the block in the context of the table. Why would you do that?</p> <pre><code>class Leg attr_accessor :number def initialize(number) @number = number end end class Table def initialize(legs) @legs = legs end def with_leg(&block) Leg.new(rand(@legs).instance_eval(&block) end end </code></pre> <p>Now, you could do:</p> <pre><code>class Person def initialize(name) @name = name end def gnaw Table.new(4).with_leg do puts "I'm gnawing off one of leg #{number}" end end end </code></pre> <p>If you wanted access to the person object inside of the block, you'd have to do:</p> <pre><code>class Person def initialize(name) @name = name end def gnaw my_name = name Table.new(4).with_leg do puts "#{my_name} is gnawing off one of leg #{number}" end end end </code></pre> <p>As you can see, the use of instance_eval can make it simpler and less bulky to access methods of a far-off object inside a block, but comes at the cost of making the <code>self</code> unaccessible. The technique is usually used in DSLs, where a number of methods are injected into the block, but the self doesn't matter that much.</p> <p>This is what's happening with Tk; they're using instance_eval to inject their own <code>self</code> into the block, which is wiping your <code>self</code> clean.</p>

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