Evaluate instance variable in block passed to class_eval through wrapper

Question

I want to extend the functionality of a method asdf at runtime. A method append_asdf should allow modifying the method more easily.

I understand how to call the previous method but the private variable @b evaluates to nil inside the block used to specify the additional behaviour - if passed through the wrapper. It works as expected when passed to class_eval directly (which is not what I want).

Why?

class A
  def initialize
    @b = "144"
  end

  def asdf
    puts "12"
  end

  def self.append_asdf(&n)
    m = instance_method(:asdf)
    define_method(:asdf) { 
      m.bind(self).call
      n.call
    } 
  end
end

a = A.new

p = proc {
  puts @b
}

The proc p is used here to drive home the point that it doesn't depend on the block. It behaves the same as a literal.

This doesn't work:

A.append_asdf(&p)
a.asdf
=>
12

Note the empty line. The same proc used here evaluates as expected:

A.class_eval {
  define_method(:asdf, p)
}
a.asdf
=>
144

tadman · Accepted Answer · 2021-04-13 22:47:51Z

1

You need to evaluate that block in the proper context. In other words:

instance_eval(&n)

Instead of a regular call.

answered Apr 13, 2021 at 22:47

tadman

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1 Comment

yogo1212 Over a year ago

i thought i already did that - and now i feel very stupid... :-D thanks for your time

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