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copied!<p>The issue arrises with the way that Ruby deals with instance and local variables. What is happening is that you are setting a local variable in your instance_eval block, rather than using the ruby accessor.</p> <p>This might help explain it:</p> <pre><code>class Foo attr_accessor :bar def input_local bar = "local" [bar, self.bar, @bar, bar()] end def input_instance self.bar = "instance" [bar, self.bar, @bar, bar()] end def input_both bar = "local" self.bar = "instance" [bar, self.bar, @bar, bar()] end end foo = Foo.new foo.input_local #["local", nil, nil, nil] foo.input_instance #["instance", "instance", "instance", "instance"] foo.input_both #["local", "instance", "instance", "instance"] </code></pre> <p>The way bocks work is that they distinguish between local and instance variables, but if a local variable is not defined when it's reader is called, the class defaults to the instance variable (as is the case with the call to input_instance in my example).</p> <p>There are three ways to get the behavior you want.</p> <p>Use instance variables:</p> <pre> class Foo attr_accessor :bar def evaluate(&block) instance_eval &block end end foo = Foo.new foo.evaluate do @bar = "instance" end foo.bar #"instance" </pre> <p>Use a self variable:</p> <pre> class Foo attr_accessor :bar def evaluate(&block) block.call(self) end end foo = Foo.new foo.evaluate do |c| c.bar = "instance" end foo.bar #"instance" </pre> <p>Use setter functions:</p> <pre> class Foo attr_reader :bar def set_bar value @bar = value end def evaluate(&block) instance_eval &block end end foo = Foo.new foo.evaluate do set_bar "instance" end foo.bar #"instance" </pre> <p>All of these examples set foo.bar to "instance".</p>

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